KISS Notes Ideas to Implementation

Coffs Harbour High School SL#703335

HSC Physics Topic 3

FROM IDEAS to IMPLEMENTATION

What is this topic about?

To keep it as simple as possible, (K.I.S.S.) this topic involves the study of: 1. FROM CATHODE RAYS to TELEVISION 2. FROM RADIO to PHOTOCELLS: QUANTUM THEORY 3. FROM ATOMS to COMPUTERS: SEMICONDUCTORS 4. FROM CRYSTALS to SUPERCONDUCTORS ...all in the context of how Physics has contributed to modern technology

but first, an introduction... The History of Physics

About the Same Time

is marked by a number of “landmark” discoveries that changed our understanding of the Universe...

as Cathode Rays were becoming understood, other scientists were studying electromagnetic radiation and obscure phenomena such as the “Photoelectric Effect”.

• Newton’s Laws of Motion, and Gravitation, and • Einstein’s Theory of Relativity have already been studied.

Photo: Oliver Ransom

No-one could have guessed that this led to, not only the radio and mobile phone, but to solar cells...

This topic covers a number of other great discoveries, experiments and scientists, so it is definitely a study of the History of Physics, from about 1850 into the 20th century.

Solar cells being used to make electricity on a remote outback property

However, it is not just history. Along the way, you will be studying some concepts, theories and facts that are vital to your overall understanding of this subject.

and Meanwhile,

In addition, as you learn both the history and some of the foundation ideas of modern Physics, you will see that much of our modern technology is a direct result these discoveries... When “Cathode Rays” were being studied between 1850-1900, people said “interesting, but what’s the use of it??” Little did they know...

the unravelling of atomic structure and study of electrical conductivity in “weird” substances like Germanium and Silicon, led to the discovery of “semiconductors”. The invention of the transistor followed... the basis of all modern electronics and computer systems.

Photo: John de Boer

...the study of Cathode Rays led directly to the invention of the TV set, so familiar today.

HSC Physics Topic 3

and the Study

of Crystal Structure

led to the discovery of Superconductors, the applications of which are only just beginning to be implemented. 1

copyright © 2005-2006

keep it simple science

Photo: Adam Page www.atomdriven.com

Photo: Peter Hamza

Coffs Harbour High School SL#703335

CONCEPT DIAGRAM (“Mind Map”) OF TOPIC Some students find that memorizing the OUTLINE of a topic helps them learn and remember the concepts and important facts. As you proceed through the topic, come back to this page regularly to see how each bit fits the whole. At the end of the notes you will find a blank version of this “Mind Map” to practise on. F = QE & E= V d

F = QvBsinθ θ

Behaviour of a Charged Particle in a Magnetic Field

Failure to follow-u up

The TV screen. Main parts and their role

Revision of Electric Fields

Revision of “Black Body Radiation”

Hertz’s Discovery of Radio Waves

Cathode Rays. Discovery & Properties

Plank’s Quantum Theory

From CATHODE RAYS to TELEVISION

From RADIO to PHOTOCELLS: Quantum Theory

From IDEAS to IMPLEMENTATION

From CRYSTALS to SUPERCONDUCTORS

Conductivity in Metals. Superconductivity

Current & Potential Applications of Superconductivity

Einstein’s Contribution

Particle-W Wave Duality of Light

Photoelectric Effect & Applications: • solar cells • photocells

E = hf and c=fλ

From ATOMS to COMPUTERS

The Braggs & X-rray Crystalography

HSC Physics Topic 3

Confirmation of EMR. Measurement of “c”

Discovery of the Electron... Thompson’s Experiment

“Band Theory” of Conductors, Insulators & Semiconductors

Electrons & “Holes” in Conductivity

Revision of Atomic Structure & Structures of Solid Lattices

“Doping”. n-ttype & p-ttype Semiconductors

2

Differing views on Science’s place in society

Valves to Transistors to Microprocessors... Impacts on Society

copyright © 2005-2006

keep it simple science

Coffs Harbour High School SL#703335

1. FROM CATHODE RAYS TO TELEVISION Experiments with CRT’s

Discovery of Cathode Rays By the 1850’s, scientists had developed the technology to produce quite high voltages of electricity and to make sealed glass tubes from which most of the air had been removed using a vacuum pump.

Maltese Cross Tube CATHODE ( -v ve)

It wasn’t long before these 2 things were combined, and some mysterious phenomena were discovered. You may have done some laboratory investigations with “Discharge Tubes” as follows... Each tube contains a different pressure of gas. (All are very low pressure, but some lower than others.) High voltage from an induction coil is applied to each tube in turn.

ANODE (+ve) in the shape of a Maltese Cross

Shadow of the cross in the glow at the end of the tube

What does this prove? Cathode Rays travel in straight lines, from the Cathode. Furthermore, Crookes tried this experiment with many different metals as his electrodes. The type of metal made no difference... Cathode Rays are identical, regardless of the materials used.

This tube is glowing and showing light and dark bands, or “striations”

Tube With a Fluorescent Screen Beam of Cathode Rays causing a fluorescent screen to glow

The result is that each tube shows glowing streamers, or light and dark bands, or glows at the end(s).

Fluorescence was known to be caused by certain waves, such as ultraviolet (UV) rays

The patterns change at different gas pressures. At the very lowest pressure, there is no glow from gas in the tube, but the glass itself glows at one end of the tube.

Tube With a Rotating Paddle-Wheel Wheel spins when cathode rays strike the paddles

It was soon established that whatever was causing these glows or “discharges” in the tubes was coming from the negative electrode, or “cathode”...so these emissions were called “Cathode Rays”.

This shows that the rays have momentum, and therefore have mass

Over the following 20 years these mysterious “rays” were studied by many scientists, most notably Sir William Crookes. He devised so many clever variations on these Cathode Ray Tubes (CRT’s) that they were known as “Crookes Tubes”.

This evidence from these various experiments was very inconsistent... some of the features of cathode rays suggested they are particles, other results suggested they are waves.

You will have seen, in the school laboratory, a number of different CRT’s and repeated many of Crookes’s famous experiments... HSC Physics Topic 3

3

Copyright © 2005-2006 keep it simple science

Coffs Harbour High School SL#703335

Revision of Electric Fields

Tube Containing Electric Plates

In a Preliminary Course topic you learned that: • Electric Charges exert force on each other... ...like charges REPEL each other. ...opposite charges ATTRACT each other • Charges act as if surrounded by a “Force Field”.

CRT with fluorescent screen Beam of cathode rays on screen Electric plates on either side of beam (no voltage applied yet)

When voltage is applied to the plates, the beam deflects

-ve

FIELDS AROUND “POINT” CHARGES

+ve +

-

FIELDS BETWEEN “POINT” CHARGES

What does this prove? Cathode Rays must be a stream of charged particles.

Repulsion

In fact, by considering the charge on the plates above, it follows that the particles must be negatively charged, because the beam is deflected by repulsion from the negative plate, and attraction towards the positive.

+

-

+

+

Attraction

Early Confusion About Cathode Rays Unfortunately, when the early experimenters tried something similar to the above, they did NOT detect a deflection of the beam. So, they concluded there was NO charge associated, and were confused about the nature of the Cathode Rays.

The strength of the field is defined as the force per unit of charge experienced by a charge in the field... E= F Q However, in this topic we are more interested in calculating forces, so

Evidence that CR’s were Waves: Cathode Rays: • Travel in straight lines like light waves. • Cause fluorescence, like ultra-violet waves. • Can “expose” photographic film, as light does.

F = Q.E

is more useful.

F = Force, in newtons (N), experience by the charge. Q = Electric charge in coulombs (C). E =Electric field strength, in newtons per coulomb (NC-1)

Evidence that CR’s were Particles Cathode Rays: • Carry kinetic energy and momentum, and therefore must have mass. • Carry negative electric charge. (but this vital clue was missed!)

Note: In this topic the most common charged particle we deal with is the electron. The value of its charge is Qe = ( -)1.602 x 10-19C. Get used to this very small value. Example Calculation: In the CRT shown at top left of this page, a stream of electrons passes between 2 electrically charge plates. The electric field strength is 400NC-1. What is the force acting on each electron?

This debate was finally settled by a famous experiment you will study soon... In 1897, J.J. Thomson showed that cathode rays had both mass and negative charge.

Solution:

F = Q.E = -1.602x10-19 x 400 = -6.41x10-17N. The negative sign simply means that the direction of the force will be in the opposite direction to the electric field.

He had discovered the electron. Note that all these investigations and discoveries involved the Cathode Ray Tube... a relatively simple device that allows the manipulation of a stream of charged particles. HSC Physics Topic 3

By definition, the direction of the field is the way a positive charge would move in the field

TRY THE WORKSHEET at the end of this section. 4

copyright © 2005-2006

keep it simple science

Coffs Harbour High School SL#703335

Electric Field Between Parallel Charged Plates

Force on a Moving Charge in a Magnetic Field

The field around and between point charges is irregular in direction, and varies in strength at every point. The field between parallel charge plates, however, is uniform in strength and direction at every point (except at the edges). The direction of the field is the way a positive charge would move.

In the previous topic you learned that when an electric current flows through a magnetic field, the wire experiences a force... the “Motor Effect”.

Positively (+ve) charged plate

Now you need to realise that the reason is that every electric charge, if moving through a magnetic field, will experience a force.

+

You may have seen the following experiment with a CRT in the laboratory: CRT with fluorescent screen. Cathode Ray beam goes straight across.

Negatively (-ve) charged plate

-

Uniform Field Between Plates

If a magnet is brought near, the beam deflects.

The strength of the field depends on the Voltage applied to the plates, and the distance between them: E= V d

S

A force is acting on the moving charged particles.

E = Electric Field strength, in NC-1. V = Voltage applied to the plates, in volts (V). d = distance between the plates, in metres (m).

The size of the force can be calculated as follows:

θ F = QvBsinθ Example Calculation: Two parallel plates are 1.25cm apart.(convert to metres) A voltage of 12.0V is applied across the plates. What is the magnitude of the field between the plates? Solution:

F = Force acting, in newtons (N). Q = Electric charge, in coulombs (C). v = velocity of the charged particle, in ms-1. B= Magnetic Field strength, in Tesla (T). θ = Angle between the velocity vector and magnetic Field vector lines.

E=V/d = 12.0 / 0.0125 = 960NC-1.

θ

B Since sin90o = 1, o and sin0 = 0, Mag. then maximum force occurs Field when the charge moves at right angles to the field.

TRY THE WORKSHEET at the end of the section.

Example Calculation: In the CRT above, the cathode rays (electrons; Qe=-1.602x10-19C) are moving at a velocity of 2.50x106ms-1. The magnet provides a field of 0.0235T. Held as shown, the field lines are at an angle of 70o to the beam. What force acts on each electron? Solution: θ F = QvBsinθ = -1.602x10-19x2.50x106x0.0235xsin70o = -8.84 x 10-15N. (negative sign simply refers to direction) HSC Physics Topic 3

5

How do you know the direction of the force? Remember the Right-Hand Palm Rule? Velocity vector,

v

Magnetic B Field

Force,

F

However, this applies to positive (+ve) charges. For negative charges ( -ve) the force is in the opposite direction... back of hand side. Can you verify the upward deflection in the photo above is consistent with theory? copyright © 2005-2006

keep it simple science

Coffs Harbour High School SL#703335

Discovery of the Electron... Thomson’s Experiment

How a TV Screen Works Thomson used a fluorescent screen at the end of his CRT to detect and measure the deflection of the cathode rays (electrons). Over the following 30 years, CRT technology evolved into the television screen. By the middle of the 20th century, TV was developing to become the major system for home entertainment and by the 1980’s the same screens became the vital display units for computers.

In 1897, the confusion and debate about Cathode rays was settled by one of the most famous, and critically important, experiments in the history of Science. The British physicist J.J. Thomson set up an experiment in which cathode rays could be passed through both an electric field, and through a magnetic field, at the same time. Electric Field Effect Cathode Rays

Photo: Peter Hamza

(charged plates)

+ve E down page -ve Variable voltage

Magnetic Field Effect

Fluorescent screen to measure deflection

(Adjustable Electromagnets)

A TV “picture-tube” is really just a more sophisticated version of Thomson’s CRT. The image on the screen is made up of thousands of spots of light, created as cathode rays strike a fluorescent screen on the inside of the glass.

Cathode Rays

The 3 main parts of a TV picture-tube are: B into page

The Electron Gun produces the beam of cathode rays (electrons).

Thomson was able to adjust the strengths of the 2 fields so that their opposite effects exactly cancelled out, and the beam went straight through to the centre of the screen. At this point,

The electrons leave a cathode, and are accelerated towards a series of anodes by the high voltage electric field between them, just like in the CRT’s of Crookes or Thompson.

Force due to = Force due to Electric Field Magnetic Field

The Deflection Plates are used to deflect the beam to create spots of light at different points on the screen.

Since the strengths of the fields could be calculated from the currents and voltages applied to the plates and electromagnets, Thomson was able to calculate the ratio between the charge and mass of the cathode rays. Charge to mass ratio = Q m This established beyond doubt that cathode rays were particles, not waves.

One set of charged plates are arranged so the field can deflect the beam up or down. Another set are arranged at right angles to cause deflection left or right. Between them, the sets of plates can “steer” the beam onto any point on the screen.

Furthermore, he repeated the experiment with many different cathode materials and always got the same result. This meant that the exact same cathode ray particles were coming from every type of atom.

The Fluorescent Screen glows with light when the electron beam strikes the fluorescent chemical coated on the inside of the glass.

Other experimenters had already determined the chargemass ratio for the hydrogen atom (the smallest atom). It was apparent that the cathode ray particle was much smaller than a hydrogen atom. The conclusion was that all atoms must be made of smaller parts, one of which was the “cathode ray particle”, soon re-named “ELECTRON”.

The total image is built from many thousands of light-spots (“pixels” = picture elements). The illusion of movement is achieved by replacing each full-screen picture many times per second. To produce colour TV there are actually 3 electron guns, and 3 sets of deflection plates. Three separate beams are steered onto separate spots of fluorescent chemicals which glow red, green or blue (RGB). The final colour is a combination of these 3 colours combined.

This was a vital piece of knowledge for better understanding of atoms and electricity, and the development of many new technologies. HSC Physics Topic 3

6

copyright © 2005-2006

keep it simple science

Coffs Harbour High School SL#703335

Worksheet 1 Part A Fill in the blanks. Check answers at the back. An electric u)............................. is created around anything with electric charge. The direction of the field is defined as v)........................ ............................................................................... Any charge within a field will experience a w)............................ The field between 2 x)........................... .................................. plates is uniform in both y)..................................... and ..........................................., and is determined by the z)................................... applied to the plates and the aa).................................. between them.

The discovery of a).............................. Rays was made with simple “b)................................... tubes” from which most of the air was removed with a c)....................................... pump. When high d)................................ was applied to electrodes at each end of the tube, it would produce a variety of e)........................, .................................... and .............................. The exact pattern changed as the f)..................................... in the tube was altered. It was discovered that the effects were due to mysterious emissions coming from the cathode (or g)............................. electrode).

Electric charges also experience a force if they are ab)....................................... through a ac)................................... field. This is easily observed by bringing a ad).............................. near a CRT with a fluorescent screen; the magnet causes the beam to ae)........................................... The direction of the force and the deflection of the CR beam is easily determined by the “af)..................................................... Rule.

About the 1870’s, Sir William h)............................ and others, built special CRT’s to study the cathode rays. The famous “i)................................... cross” tube showed that the rays travelled in straight lines. Tubes with j).................................... screens showed that the rays caused fluorescence, and tubes equipped with a “paddle-wheel” proved that the rays carried both k)......................... energy and l).....................................................

In 1897, J.J. ag)........................................... used the deflection of a CR beam by both ah)..................................... and ................................. fields to measure the ratio of ai)...................................................... of a cathode ray. This established, beyond doubt, that CR’s are aj)............................. and are a small part contained within all ak)........................... Thomson had discovered the al).............................................. The simple CRT was later used as the basis to invent the am).......................................... screen.

Unfortunately, attempts to detect deflection by applying an m)......................................... field were unsuccessful, so for many years there was confusion over whether CR’s were n)................................. or ............................................ Evidence they were waves: • CR’s travel in o)...................................... like light. • They cause p).......................................... like UV rays. • They can expose q)..................................................

The main parts of the “picture tube” are: • The an).................................. Gun, which produces a beam of ao)........................... from a ap).................................... and accelerates them towards a series of aq).................................. • The ar)..................................... plates, which use electric fields to as).................................... the beam onto the screen. • The at)..................................... screen, which forms the image when fluorescent chemicals au)............................ with spots of light when struck by av)...........................................

Evidence they were particles: • Carry r)................................ and ............................. and therefore must have s)........................... • Carry t)............................ electric charge

COMPLETED WORKSHEETS BECOME SECTION SUMMARIES

HSC Physics Topic 3

7

Copyright © 2005-2006 keep it simple science

Coffs Harbour High School SL#703335

Force on a Moving Charge in a Magnetic Field

Part B Practice Problems Field Between Charged Plates & Force on a Charge

6. An electron (Q=-1.60x10-19C) is travelling north at 3.00x107ms-1 in a cathode ray tube when it enters a magnetic field of strength 4.96x10-2T. The field is directed vertically upwards through the CRT. Find the magnitude and direction of the force experienced by the electron.

1. Two parallel plates are 4.00cm apart in a vacuum tube. A voltage of 50.0V is applied across the plates. An alpha particle with charge of (+)3.20x10-19C passes between the plates. a) What is the size of the electric field between the plates? b) What force will act on the alpha particle? c) Describe the direction of the i) field ii) force relative to the +ve and -ve plates.

7. In a nuclear accelerator, a charged ion has been accelerated up to a velocity of 2.90x108ms-1. As it enters a magnetic field of strength 8.05T (field is perpendicular to ion’s velocity vector) it experiences a force of magnitude 3.75x10-9N. What is the magnitude of the charge on the ion?

2. An electron (Q=-1.60x10-19C) experiences a force of -7.82x10-15N within an electric field created by parallel plates which are 2.50mm apart. a) Find the size of the electric field. b) Find the voltage applied to the plates.

8. A particle of the solar wind with charge of (+)1.60x10-19C (it is in fact a proton) encounters the Earth’s magnetic field at an angle of 25o to the field lines. At this point the field has a strength of 5.48x10-4T. The proton experiences a force of 7.40x10-15N. Find the velocity of the proton.

3. A speck of dust carrying a static electric charge, experiences a force of 2.29x10-12N in a field produced by 2 plates 5.00cm apart. A 200V potential difference is applied across the plates. a) Find the strength of the field between the plates. b) What charge does the speck of dust carry? c) The static charge was created when some electrons were either removed from, or added to, the speck of dust. How many electrons were added or removed? d) The speck of dust was observed to move toward the negative plate. Did the speck lose or gain electrons?

9. In an experiment similar to Thomson’s, a stream of electrons in a CRT are each experiencing a force of magnitude 4.06x10-15N when travelling through a perpendicular magnetic field at a velocity of 7.80x106ms-1. a) What is the strength of the magnetic field? The force on the electrons is exactly counteracted by an electric field produced by charged plates which are 8.00mm apart. b) What is the strength of the electric field? c) What is the voltage being applied across the plates?

4. Two parallel plates have a 40.0V potential difference between them. An electron between them experiences a force of (-)5.88x10-17N. How far apart are the plates? 5. In an inkjet printer, small droplets of ink are given an electric charge, then “steered” onto the paper by accelerating them in electric fields to achieve the desired velocities and directions.

FULLY WORKED SOLUTIONS IN THE ANSWERS SECTION

What force would be experienced by a droplet with charge of (+)9.75x10-10C, which is between parallel plates with potential difference of 100V, and separated by 5.00mm? HSC Physics Topic 3

Remember that for full marks in calculations, you need to show FORMULA, NUMERICAL SUBSTITUTION, APPROPRIATE PRECISION and UNITS

8

Copyright © 2005-2006 keep it simple science

Coffs Harbour High School SL#703335

2. FROM RADIO to PHOTOCELLS: QUANTUM THEORY The Radio Experiments of Hertz

Investigating Radio Waves

By the 1880’s, the theory of electromagnetic radiation (EMR) had been around for 20 years, but no-one had found proof that these waves existed. Until, that is, the famous experiment of Heinrich Hertz in 1887.

You may have done some simple studies in the laboratory, such as: Array of wire connected to induction coil acts as a transmitting antenna

Using the familiar “induction coil” to produce sparks across a gap, Hertz showed that some invisible waves were being produced... he had discovered radio waves. Radio waves Emitted from spark

Sparks produced in small gap in receiving loop

spark gap

High-voltage Induction coil

Wire loop acts as a receiving antenna. The radio waves induce currents in the wire, and sparks in the gap.

Induction coil & Power Pack

Radio receiver picks up loud bursts of noise, from some distance away

By adding a “tapping key” switch to the transmitter circuit, it is easy to send messages to the receiver in the form of “dots-and-dashes” of static noise.

Hertz went on to experiment with these invisible waves and showed that they could be reflected, refracted, polarized and diffracted just like light waves. The clincher was when he measured their velocity and got an answer of 3x108ms-1... the speed of light!

What Hertz Failed to Investigate In one of his many experiments with the new waves he had discovered, Hertz found that his “receiving loop” became more sensitive and sparked more if it was exposed to other radiations coming from his transmitter.

This was powerful evidence supporting the theory that light was just one of a whole spectrum of Electromagnetic waves that had been predicted earlier.

He didn’t realize the significance of this observation, and failed to follow up on it.

How did Hertz measure the speed of the radio waves?

We now know (with perfect hind-sight) that he had produced the “Photoelectric Effect”:

He reflected the radio waves (from metal sheets) so that they set up interference patterns. By moving his “receiving loop” around the lab. he could measure exactly where the peaks of interference occurred (where the waves added in amplitude). From this, the wavelengths of the waves were calculated.

Ultra-violet rays give their energy to electrons on the metal surface. Wire of receiving loop.

The frequency could be determined from the settings of his wave transmitter.

This can eject an electron from the surface so sparks are more likely. Spark gap

Later, this phenomenon was used by Einstein as proof of the new “Quantum Theory”... read on.

Then the wave equation was used: V = λ.f

This Photoelectric Effect was exploited in the 20th century to develop the technology of photocells and solar cells.

He found the radio waves travelled at the speed of light. In recognition of Hertz’s contribution to our knowledge of waves, the unit of wave frequency (Hz) is named in his honour.

Solar Cells

Within another 20 years, radio was being used for longdistance communications using morse code. Within 100 years the world was blanketed with radio transmissions for communication and entertainment. HSC Physics Topic 3

The induction coil’s high-voltage sparking produces all sorts of EMR, including radio, light, UV & even X-rays

9

Copyright © 2005-2006 keep it simple science

Coffs Harbour High School SL#703335

Black Body Radiation

Plank’s Quantum Theory proposed that the amount of energy carried by a “quantum” of light is related to the frequency of the light:

In a previous Preliminary topic (“Cosmic Engine”) you learned about the way that energy is radiated from hot objects. A “perfect” emitter of radiation had become known as a “black-body”...

E = h.f E = energy of a quantum, in joules ( J) h = “Plank’s constant”, which has a value of 6.63x10-34 f = frequency of the wave, in hertz (Hz)

Amount of Energy Radiated

BLACK BODY RADIATION CURVES

You are reminded also, of the wave equation:

very hot object

“peak” wavelength shorter

V = λ.f

c = λ.f

c = velocity of light (in vacuum) = 3.00x108ms-1. λ = wavelength, in metres (m). f = frequency, in hertz (Hz)

hot object

“peak” wavelength

(or, for light)

Example Calculation: A ray of red light has a wavelength of 6.50x10-7m.

warm object

a) What is its frequency? b) How much energy is carried by one quantum of this light?

“peak” wavelength longer

shorter

longer

Solution: a) c = λ.f 3.00x108 = 6.50x10-7x f ∴ f = 3.00x108/6.50x10-7 = 4.62x1014Hz. b) E = h.f = 6.63x10-34 x 4.62x1014 = 3.06x10-19 J.

Wavelength of Radiation

It was well known that as a “black body” became hotter, it not only emitted more energy as radiation, but that the wavelength of the peak of the radiation became shorter, and frequency became higher.

TRY THE WORKSHEET at the end of this section

The problem was that the standard Physics theories of the time could not explain the shape of these graphs, which were obtained from experiment.

Problems with Classical Physics At the same time that Plank was proposing his Quantum Theory to explain the Black Body radiation details, the “Photoelectric Effect” (that Hertz had observed but failed to study) was being investigated by others.

Plank’s Quantum Theory In 1900, Max Plank proposed a radical new theory to explain the black body radiation. He found that the only way to explain the exact details coming from the experiments, was that the energy was quantised: emitted or absorbed in “little packets” called “quanta” (singular “quantum”).

What IS the Photoelectric Effect? When metal surfaces are exposed to light waves (especially high frequency light or ultra-violet) some electrons are found to be ejected from the metal surface, as long as a certain critical energy level is exceeded.

The existing theories of “classical” Physics assumed that the amount of energy carried (say) by a light wave could have any value, on a continuous scale. Plank’s theory was that the energy could only take certain values, based on “units” or quanta of energy.

Experiments on the photoelectric effect were producing results that could NOT be explained by the existing theory of light. For a century or more, light had been accepted as a wave. This explained its reflection, refraction, interference, and many other phenomena.

It’s the same as with matter: The smallest amount of (say) carbon you can have is 1 atom. Then you can have 2 atoms, 3 atoms and so on, BUT you cannot have 1/2 atoms of carbon... the matter is quantised, with whole atoms as the minimum “quantum”. Well, says Plank, energy is the same! HSC Physics Topic 3

However, the photoelectric effect experiments were giving results that suggested light was best explained as a stream of particles... this could turn Science on its ear! Enter Albert Einstein... 10

copyright © 2005-2006

keep it simple science

Coffs Harbour High School SL#703335

Einstein and Quantum Theory

Applications of the Photoelectric Effect

It was Albert Einstein who came to the rescue and neatly combined Plank’s Quantum Theory with the classical wave theory of light, in a way that solved all the apparent conflicts, and explained the Photoelectric Effect as well!

Solar Cells Solar Cells (or “photovoltaic cells”) are devices which produce electricity directly from light energy. They are very familiar in the popular garden lights which need no wiring or battery replacements.

To keep it as simple as possible, (K.I.S.S. Principle) Einstein proposed that:

During the day, the solar cell(s) charge up a small rechargable battery.

• Light is a wave, but • the energy of the wave is concentrated in little “packets” or “bundles” of wave energy, now called “Photons”. • Each photon of light has an amount of energy given by E = h.f, according to Plank’s Quantum Theory. • When a photon interacts with matter, it can either transfer all its energy, or none of it... it cannot transfer part of its quantised energy.

At night, the battery provides electricity to a low-power garden lamp. Small array of solar cells powering a small electric motor and fan

Light is NOT a stream of particles Light is NOT a wave

More importantly, solar cells hold the promise of cheap, efficient, environmentally-friendly electricity production. Already they are used in remote areas (see photo on front page) and in special situations, such as power for orbiting satellites.

Light is a stream of “wave packets”... “ PHOTONS”. They have wave properties... refraction, interference, etc. They can also behave like a particle sometimes. Each photon is a Quantum of light energy.

Einstein’s model for light involves a “duality”... light must have a dual nature. Many of its properties are wave related; e.g. ability to reflect, refract and show interference patterns. In other cases, especially when energy transfers are occurring, the light photons are like little particles. This explained the Black Body Radiation curves, and the weird features of the Photoelectric Effect.

Solar cells produce electricity from the Photoelectric Effect: Light photons falling on the cell give up their quantum of energy to electrons in a sandwich of semiconductor material, called a “p-n junction”. The energy gained by electrons causes them to be emitted so that they travel through the semiconductor structure and create a potential difference across it. This voltage causes a current to flow in the electrical circuit.

Confirmation of the Einstein Model Einstein’s idea is very neat, but is it correct?

Photocells A photocell is a device which can detect and measure light. Photocells are used in light meters (photography), “electriceyes” and a variety of light-measuring scientific equipment, such as photometers.

Einstein was able to make certain mathematical predictions regarding further features of the Photoelectric Effect. (The exact details are complicated, and not required learning)

Once again, the photoelectric effect is involved. When a photon of light strikes the receiving surface, its energy causes emission of an electron, which is collected on a nearby anode.

In 1916, the experiments were done to test Einstein’s predictions, and the results agreed with his predictions precisely! This was confirmation that the photon theory of light, and the quantum theory of energy were both correct. Einstein was awarded the Nobel Prize for Physics in 1921, for his contribution to understanding the Photoelectric Effect.

HSC Physics Topic 3

A sensitive electric circuit is able to measure the level of electron emission, and this gives a measure of the amount of light being received. 11

copyright © 2005-2006

keep it simple science

Coffs Harbour High School SL#703335

Assessment of Einstein’s Contribution to Quantum Theory “Assess” means to measure or judge the value of something. The syllabus requires you to assess Einstein’s contribution to the Quantum Theory in relation to Black Body Radiation. To begin with, you might note that Einstein did NOT think up the Quantum Theory... Max Plank did that in 1900. However, it seems that Plank invented the quantum idea purely as a mathematical “trick” to explain the Black Body Radiation curves. Plank never proposed that the quanta might give light a particle-like nature. Plank never suggested that the old ideas of “classical” Physics might need changing. It was Einstein who did that! His “particle-wave” (photon) idea combined Plank’s Quantum Theory with the classical idea that light is a wave. This totally new way to look at things was one of the turning points of modern Physics, and set other scientists off into new and innovative directions of research. It should be noted that the other major turning point for Physics was Einstein’s Theory of Relativity, which he proposed in the same year (1905). No wonder we credit him as being one of the greatest!

Is Science Research Removed from Social & Political Forces? In World Wars I & II, Science and scientists played a major role in research and development of new weapons and war technologies. Some examples include:

Einstein was German-born, but became a Swiss citizen, and later American. In WW I he (and only 3 others) signed an anti-war declaration. He spent the war in neutral Switzerland, lobbying for peace and an end to war. In the 1930’s he was forced to flee Nazi Germany because he was of Jewish descent. In America, he fought against the development of the atomic bomb (developed directly from his own theories) and was appalled when it was used against Japan in 1945.

• radio communications and Radar. • nuclear weapons. • rockets. • new aircraft designs and jet engines. • chemical weapons such as poison gas systems. There are two contrasting views about the morality of weapons research, and the two great scientists of this section of the topic epitomise these different views.

Einstein believed that Science is a process that should work for peace and the good of all people, and not be involved in the political & social forces that come and go.

Max Plank was a patriotic German who believed that it was his duty to help his country fight a war. He gladly contributed to weapons research in WW I, and leading up to WW II he was the director of the main Scientific Institute in Nazi Germany. Plank’s outlook seems to have been that Science is part of the political & social structure, and must take an active role in it.

Who was right? There is no correct, nor simple, answer to that. You must form your own opinion... just be sure you have an informed opinion.

“A-bomb Dome”, Hiroshima, Japan by Kathy de la Cruz

HSC Physics Topic 3

12

copyright © 2005-2006

keep it simple science

Coffs Harbour High School SL#703335

Worksheet 2

Part B Practice Problems Quantum Theory (Plank’s Constant = 6.63x10-34) ( c = 3.00x108 ms-1) 1. A light wave has a wavelength of 4.25x10-7m. a) What is its frequency? b) How much energy is carried by one photon?

Part A Fill in the blanks. Check answers at the back. In 1887, Heinrich Hertz discovered a)............................. waves. His experiment involved high voltage from an b)..................................... coil which produced c)..................... across a gap. The sparking produced radio waves which he detected with a d)...................................................... in which a small gap also sparked. He was able to show that the new radiations showed typical wave properties such as e).................................. and ................................................ Hertz was also able to measure the f).................................... of the waves, and show it was equal to the speed of g)...................................... He also produced evidence of the h)................................................. Effect, but failed to investigate it further.

2. Compare the amount of quantum energy carried by a photon of i) infra-red (heat) radiation (λ = 5.45x10-6m) and ii) UV radiation (λ = 5.45x10-9m) 3. A photon of radiation is carrying 8.75x10-14J of energy. Calculate a) its frequency b) its wavelength

Meanwhile, other researchers had studied the way energy is emitted from hot objects. The “i)............................................. Radiation” curves showed a shape that could not be explained by the accepted theories. In 1900, j)........................ proposed the “k)....................................... Theory” to account for the problem. The basic idea of his theory is that the energy of light (or other EMR) is “l)........................” the same way that matter is. The minimum quantity of matter is one m)............................., and fractions cannot occur. Plank proposed that the energy of EMR is the same, and that the amount of energy carried by one “n)................................” is related to the o)................................ of the wave.

4. To cause emission of an electron from the surface of a certain metal requires the electron to gain a minimum of 2.38x10-20J of energy. a) Find the frequency and wavelength of the photon of EMR which carries this “threshold” amount of energy. b) What happens if the electron is struck by a photon with a longer wavelength than this? c) What will happen if the electron was struck by a photon of higher frequency than calculated in (a)? 5. An electron was emitted from a metal surface after being struck by a photon of EMR. The electron left the surface with energy of 6.22x10-17J. It firstly had to “use” 9.28x10-19J of energy to escape the metal surface. All of this energy was gained by interaction with a single photon. Find the frequency and wavelength of the photon.

The “Photoelectric Effect” occurs when p)............................ is absorbed at a metal surface. The energy is transferred to an q).................................... which may then be r)..................... from the surface. Experiments with this effect were producing results that could not be explained. In 1905, Einstein used Plank’s s)........................... Theory to explain all the difficulties. His idea was: • Light is a wave, but the energy is concentrated in “bundles” called “t)....................................” • Each bundle carries a u)............................ of energy, as described by Plank’s theory. • When a photon interacts with matter, it can either transfer v)............... of its energy, or w)....................... of it, but cannot transfer x)............................................................

FULLY WORKED SOLUTIONS IN THE ANSWERS SECTION

This idea allows light to have its “wave properties” such as y).........................................., ................................................. and ............................................., but to also sometimes show z)..............................-like properties when it transfers energy. Based on his theory, Einstein made certain mathematical aa)................................. regarding the ab).................................. Effect. These were confirmed by ac).................................. in 1916. This confirmed Plank’s ad)............................. Theory, and explained all the “problems” with ae).............................. ...................... radiation & the af).................................. Effect. HSC Physics Topic 3

Remember that for full marks in calculations, you need to show FORMULA, NUMERICAL SUBSTITUTION, APPROPRIATE PRECISION and UNITS

13

Copyright © 2005-2006 keep it simple science

Coffs Harbour High School SL#703335

3. FROM ATOMS to COMPUTERS: SEMICONDUCTORS Band Structure Theory

Revision of Atomic Structure

The explanation just given for conductors and insulators is OK, until you find out about “Semiconductors”. Elements such as Silicon and Germanium have a number of “strange” properties including being rather poor conductors of electricity until given a little jolt of energy. Then, suddenly they become quite good conductors.

After Thomson identified the electron as a particle present in all atoms, it didn’t take long for scientists to figure out the details of atomic structure. You are reminded of the basic model of a typical atom: Structure of an ATOM

Electrons in orbit at different “Energy Levels”

-

This ability, called “Semiconductivity”, allows these materials to act as electrical switches, turning electrical currents on and off, according to their energy state.

Electrons are relatively easy to remove from some atoms... this leads to electrical conductivity, Photoelectric Effect, etc

Atomic Nucleus

of protons & neutrons

This is the basis of all modern electronics & computer systems To understand semiconductivity, you need to learn about “Band Structures”. We have known since the early 20th century that the electrons around an atom can occupy different “orbits” or energy levels surrounding the nucleus. These energy levels are “quantised” (Quantum Theory applies) so there may be “forbidden energy zones” between them. An electron cannot exist in this “fobidden zone” because the energy level there does NOT correspond to a whole quantum.

Electrical Conductivity When millions and billions of atoms form a lattice structure (most strong solids are like this) they do so by forming chemical bonds with each other in a regular array. ATOMS in a SOLID ARRAY

Electrical Conduction occurs when electrons can “migrate” freely from one atom to the next

“Forbidden energy gap”. Electrons cannot exist there.

Migrating electron

Chemical Bonds

The highest energy level that has electrons in it, is called the valence band” “v

Electrons in quantised “energy bands”

In a conductor, electrons can “jump” from one atom to the next

Some bands overlap

Nucleus

Electrons can “jump” up and down through the different bands as they gain or lose energy. To jump up over a “forbidden zone” they must have enough energy to achieve the quantum energy level required to occupy the next band.

In a metal atom, the outer (“valence”) electrons are very loosely held by the atomic nucleus. They “feel” the force of attraction from other, surrounding atoms just as strongly as the attraction from their “own” atom. The result is that these outer electrons can easily move from atom to atom.

In any atom in its “rest state”, the highest band occupied by electrons is the “Valence Band”. If an electron has enough energy to get to the unoccupied levels above there, the electron is effectively free to “wander off ”. If an electric field is applied, the electron becomes part of a flowing current, and the substance is conducting electricity.

If an electric field is present (due to a voltage being applied) billions of electrons begin moving in the same direction... an electric current is flowing, and we say the metal is a good Conductor. In other solids such as plastic or glass, the outer valence electrons are more strongly attracted to their own atom, and cannot easily escape from it, to move from atom to atom. We say these things are poor conductors, or good Insulators. HSC Physics Topic 3

The unoccupied band above the valence band, is called the conduction band” “c

That’s why any energy band above the valence band is called a “Conduction Band”.

14

Copyright © 2005-2006 keep it simple science

Coffs Harbour High School SL#703335

Conduction of Electrons & Holes

Conductors, Insulators & Semiconductors

Normally we imagine that an electric current is composed of a flow of negative electrons. However, in a semiconductor, when an electron jumps out of the valence band and flows off somewhere, it leaves behind a “hole” in the valence band. This hole, is a space that an electron from elsewhere can jump into.

In terms of “Band Theory”, the difference in conductivity between different substances is simply the relationship between the Valence Band and the Conduction Band. In Conductors, these bands overlap.

In Insulators, In Semiconductors, the bands are there is a small gap separated by a between the bands. wide “forbidden energy gap”.

Imagine a line of atoms in a semiconductor lattice: Electron has enough energy to conduct away, leaving a hole behind.

Conduction Band Conduction Band

Valence Band

Forbidden Energy gap Valence Band

hole

Now imagine a sequence of movements in which the next electron in the valence band has enough energy to jump into the hole, leaving its own hole behind...

Conduction Band

Valence Band

In metals, electrons can move into the conduction band at any time, so the solid array of atoms is a good conductor at all times.

Electrons are jumping to the right

In an insulator, such as plastic, the electrons can never achieve the conduction band unless they are given a huge boost of energy. At normal temperatures and voltage levels, the substance will not carry a current. ...and the hole is jumping left.

A semiconductor, like Silicon, will not normally carry current, because electrons lack the energy to jump the “forbidden energy gap”. However, if the temperature is increased, and a voltage applied, there comes a point when electrons jump the gap in great numbers, and the substance suddenly conducts very well indeed.

If you can imagine this sequence like the pictures making a motion cartoon, you can imagine that an electron flows to the right and the hole flows to the left.

This effect does not occur at room temperature unless the semiconductor substance is “Doped”.

In fact, in terms of electrical energy, it makes no difference whether the current really is negative electrons going one way, or “holes” going the other way... either way, it constitutes an electric current. The holes are considered as positively charged spaces (relative to the electrons) and so the flow of positive holes may be thought of as genuine “Conventional Current”.

Doping a Semiconductor

“Doping” means to add a very small quantity of a different type of atom to an otherwise pure solid lattice of semiconductor atoms. Atoms of Semiconductor substance e.g. Silicon, normally have 4 valence electrons

Each chemical bond is formed by atoms

So, there is another way to “Dope” a semiconductor. The diagram on the left shows the use of atoms with an “extra” valence electron. The other way to do it is to use atoms with only 3 valence electrons, creating extra “holes” in the lattice.

extra valence electron

sharing 2 electrons.

Atom with 3 valence electrons used to “Dope” the

These electrons are in the valence energy band

extra hole in the lattice

lattice. Atom with 5 valence electrons used to “Dope” the lattice.

DOPING increases the conductivity of the lattice HSC Physics Topic 3

15

copyright © 2005-2006

keep it simple science

Coffs Harbour High School SL#703335

p-Type and n-Type Semiconductors

Invention of the Transistor

The two different ways to “dope” the lattice result in two different types of semiconductor material:

Thermionic valves had been widely used in radios for some years and were vital components of the new industry of television.

n-Type Semiconductors are doped with atoms with 5 valence electrons, such as arsenic or antimony. This adds extra valence electrons to the lattice. Electrical current is carried mainly by this flow of negative charges (hence “n”-type).

Valves were also important in the switching of connections in telephone exchanges, where the growing communication demands required automatic dialing and connection technology. (The original system involved human “operators” manually plugging wires into sockets to connect phone calls.)

p-Type Semiconductors are doped with atoms with 3 valence electrons, such as aluminium or gallium. This adds extra “holes” to the lattice. Electrical current is carried mainly by this flow of positive holes (hence “p”-type).

However, the valve-based technology was proving too slow, too unreliable and too expensive for the booming telephone industry. The major U.S. phone company “Bell Telephone” set its scientists the task of researching new materials and processes to replace the valves.

Some History: Electronics & Computers The concept of a machine to carry out high speed calculations and “logical” operations has been around for centuries. Prior to the 20th century, any such device had to be mechanical, using “clockwork” gears and so on. There were some notable successes with control devices for weaving looms, and mechanical “adding machines”, but applications were very limited.

In 1947, 3 scientists at Bell Laboratories, invented the transistor, using a “sandwich” of p-type and n-type doped semiconductor material. Transistors

2 cm

During World War II the first electronic computers were built (in tight secrecy) to help decode enemy radio messages. Instead of gears and dials, the “Collosus” computer used thermionic valves to electronically switch circuits on and off, to store and manipulate data. Thermionic Valves are Cathode Ray Tubes “Thermionic” refers to the way these CRT’s would emit many electrons from the cathode (and thereby carry a current) when the cathode became hot. Once “warmed up” the valve can act as an electronic “switch” in a circuit, when the voltage to the anode is varied.

Because of the properties of the semiconductor (conductivity that can be switched on and off) the transistor can do the same job as the thermionic valve, but

Characteristics: • relatively large & expensive

• is only a fraction of the size and costs much less to make.

• consume relatively large amounts of electricity

• consumes only tiny amounts of electricical power.

• produce large amounts of “waste” heat

• produces virtually no waste heat.

Photo by Ben Merghart

• operates much faster than a valve.

10 cm

• although faster than mechanical switches, valves are slow-acting by modern standards

• does not need to “warm-up”. • is highly reliable, and rarely needs maintenance.

• require time to “warm up” • have a limited lifetime, and can “burn out” like a light bulb. Therefore their reliability is low, and maintenance needs are high.

Photo by Don Jolley

HSC Physics Topic 3

The comparison is a “no-brainer”... The transistor replaced Thermionic valves as rapidly as electronics industries could re-design their products, and begin mass production

Despite these limitations, “Collosus” was very important in helping to win the war.

16

copyright © 2005-2006

keep it simple science

Coffs Harbour High School SL#703335

Assessment of Impacts of the Transistor on Society

Silicon v Germanium To make semiconductor material with the desired conductivity properties, it is necessary to firstly prepare extremely pure samples, then add minute amounts of the “doping” chemical, and finally grow crystals of the semiconductor from the molten material in a furnace.

It could be argued that the invention of the transistor was one of the most profound technological developments in history. It ranks right up there beside the developments such as: • fire, by ancient humans around 500,000 years ago. Fire transformed human society because of its power to warm people, cook food and protect from predators. • agriculture, about 10,000 years ago. This transformed society from nomadic hunting-gathering to settled communities that invented law, commerce, government and “civilization”. • metallurgy and the Industrial Revolution, which led to new tools, machinery, mass production, urbanization, and mass transport systems.

The original transistors were made from Germanium because the technology to produce crystals of the pure element was already known. However, Germanium is a rare element, whereas its close “sister element” Silicon, is one of the most abundant elements on Earth. By the 1960’s, the technology to obtain pure crystals of Silicon had been developed, and because Silicon is so abundant and therefore cheaper, it quickly replaced Germanium. Silicon’s electrical properties turned out to be better too. For example, it held its semiconductive properties constant over a wider range of temperatures.

The transistor ushered in the “Information & Communication Revolution”, which is still developing today. Electronic circuits, using microchips, are the basis of all the computers which allow:

Also in the 1960’s, the technology of the computer began to emerge for financial and communication uses. The “solid-state” transistor technology allowed a computer to be built to fit a table-top, rather than fill a room. Every teenager had a brick-size “transistor radio”, in the same way that in this decade everyone has an MP3 and a mobile phone the size of a matchbox.

• instant access to (virtually) all the information on the planet via the internet. • instant access to money from your bank account from (virtually) any town or city on Earth. • instant communication via your mobile phone to and from (virtually) anywhere. Computers are the key to the global economy and mass consumerism which keeps thing cheap through mass production & distribution. Computers keep track of the billions of business transactions that feed us, clothe us, entertain us, transport us and service all our needs.

Photo by John de Boer

Silicon Chip

Like it or hate it, (some people think we should have stayed in the trees) the modern world could not exist without the invention of the transistor!

Photo pipp

The miniature “integrated circuit board” led to the technology of the “silicon chip” where thousands, and now millions of transistor-equivalents can be printed microscopically in the space of a postage stamp... a “microchip”. In the 1980’s the first cheap PC’s (personal computers) could process a magnificent 2x103 “bytes” of information. Twenty years later, these notes are being typed on an even cheaper PC which can process 2x109 bytes, (2 GB). The computers have become a million times more powerful! Photo: Martin Boulanger

HSC Physics Topic 3

17

copyright © 2005-2006

keep it simple science

Coffs Harbour High School SL#703335

In a semiconductor, the valence and conduction bands are separately by a x).............................. gap. In the “rest” state, electrons cannot get across, and the substance does not y).......................................... However, it only requires a slight increase in energy and suddenly many electrons z)................. the gap and the substance begins aa).......................................

Worksheet 3 Fill in the blanks. Check your answers at the back. a)........................... orbit around the nucleus of atoms at various b)............................... levels. Basically, a substance will be an electrical conductor if c)................................ can move from d)............................................................ freely. If electrons cannot do this at all, the substance is an e)..................................................

The semiconductor material can be made more sensitive and conductive if ab).......................... quantities of other elements are added to the atomic lattice. This is called “ac..............................” the semiconductor.

A “semiconductor” is a substance which has very low f)............................................... until its electrons are given just a little energy. Then, at a certain point, it suddenly becomes g)......................................... This allows electrical circuits to be h)................................. on and off, and is the basis of modern i)........................... and j).................................

Semiconductors can carry electricity in 2 ways: by the flow of ad).................................... which have reached the conduction band, or by the flow of “ae)............................” left behind by departing electrons. If a af)....................................... is doped with atoms with 5 valence electrons, this results in ag)........................................... in the lattice to carry the current. This is an “ah).......-Type” semiconductor.

The best explanation of semiconductivity involves “k)....................................................... Theory”, summarized as follows: • the highest energy level in an atom that has electrons in it, is called the l).................................. band. • any further (unoccupied) levels above this are called m).......................................... bands. • If an electron has enough energy to get to a m)...................................... band, then it is free to flow, and form an electric n).......................................

If it is ai)................................... with atoms with only aj)................. valence electrons, this creates extra ak)..................................... in the lattice to carry current. This is a “al)........-Type semiconductor. Before semiconductors, electronic switching was done by am).................................... valves. These were an)................................................. tubes. The ao)............................................ was invented to replace these valves. Compared to a valve, a transistor is • ap) ........................ (size) and aq)................... (cost) • consumes ar)...................... electricity and produces almost no as)........................................ • operates at a at)............................ rate • does not need to au)............................................... • is highly av)..........................................................

However, between the bands there may be “forbidden” o)..................................................... The energy levels are quantised, so a “forbidden” level is where the energy is not equal to a whole p)............................... In a conductor, the q)...................... band and r)...................... bands s)................................... each other. This means electrons can freely enter the conduction band and t)......................................... can flow through the substance.

The early transistors were made from aw)..............................., but this was later replaced by ax).................................. because it is more ay)................................ and a lot az)............................ Miniaturization of electronics on “silicon ba)........................” has led to the development of “bb)...................................” which are at the heart of all modern computers.

In an u)....................................., these bands are separated by a wide v).............................................. so that electrons can never reach the w)................................... band. COMPLETED WORKSHEETS BECOME SECTION SUMMARIES HSC Physics Topic 3

18

Copyright © 2005-2006 keep it simple science

Coffs Harbour High School SL#703335

4. FROM CRYSTALS TO SUPERCONDUCTORS Crystal Structure of Metals

Investigating Crystal Structures... Bragg and Son

Unlike silicon, salt and other crystals, metal atoms are not chemically bonded to each other by the sharing or exchanging of electrons.

The regular shapes of crystals (such as salt) had long been assumed to be due to a regular arrangement of the atoms or ions in a lattice-like structure. However, until the early 20th century, there was no way to prove or confirm this idea.

You will remember that the outer “valence” electrons in metals are weakly held, and can access the “conduction band” at any time. The result is that the valence electrons on each atom are NOT confined to that atom, but freely wander around from atom to atom.

The discovery of high frequency EMR in the form of Xrays opened up a new line of investigation. Sir William Bragg and his son Lawrence, beamed X-rays through crystals and studied the diffraction patterns which were formed as the crystal lattice scattered the X-rays.

Each metal atom is, therefore, ionized because its valence electron(s) are on the loose. The metal lattice is often described as “an array of ions, embedded in a sea of electrons”.

Photographic film sensitive to x-rays

+

+ +

Crystal

x-ray beam

+

+

The Braggs were able to analyse the interference pattern in order to deduce the arrangement of the atoms within the crystal. For this, they were jointly awarded the Nobel Prize for Physics in 1915.

Si

Each chemical bond is formed by atoms sharing 2 electrons with each neighbour

Si

HSC Physics Topic 3

Si

Si

Si

Si

Si

+ +

+

+

Any impurities in the metal distort the shape of the lattice and impede the electron flow. Also, as the ions vibrate due to thermal energy, the vibration causes more collisions among electrons, so their flow is resisted. As temperature increases, the vibrations increase too, and that’s why resistance in metals increases with temperature.

Si

Si

+

+

So why is there resistance in a metal wire? Although the electrons can flow quite easily, their movement is not totally free.

Thanks to scientists like the Braggs, we now understand the atomic-level structure of most substances. You learned previously how a substance like the semiconductor Silicon is a lattice of atoms chemically bonded together: Si

+

+

Resistance in Metals

Crystal Structures

Si

+

This “sea of electrons” shifts and flows freely. If an electric field is present, the electrons will all flow in the same direction as an electric current. That’s why metals are all good conductors.

This opened up a whole new investigative technique, allowing scientists to probe the structure of matter as never before. It was X-ray diffraction crystallography, for example, that allowed the structure of DNA to be determined in the 1950’s.

Si

+

+

X-rays diffracted by the crystal lattice, form Interference patterns which are captured on the film.

+

+

Logically, if you re-read the previous paragraph and think backwards, you might infer that if you had a really pure metal, and cooled it right down so that all lattice vibrations stopped, then it would become a perfect conductor.

Superconductivity! 19

Copyright © 2005-2006 keep it simple science

Coffs Harbour High School SL#703335

Superconductivity in Metals and Ceramics

How Superconductivity Occurs... BCS Theory

In 1911, a Dutch physicist managed to cool mercury down to about 4oK (-269oC) and found that its electrical resistance dropped to zero.

How do we explain the phenomenon of superconductivity? The accepted explanation is known as “BCS Theory”, where “BCS” are the initials of the 3 scientists who developed the theory in the 1950’s.

Over the following years, various other metals were found to become superconducting at very low temperatures. The potential to build electrical generators and equipment with zero resistance was a very attractive idea, but the temperatures involved (no higher than about 20oK) were so low that there seemed no practical way to take advantage.

Imagine part of the solid lattice of positive ions in a conducting metal or ceramic. As an electron (part of an electric current) approaches, it attracts the positive ions and distorts the crystal structure slightly:

Then in 1986, Swiss scientists discovered some ceramic materials containing rare elements like Yttrium and Lanthanum, which became superconductors at much higher temperatures. Still cold by human standards, but 100o higher than the metal superconductors, these ceramics had zero resistance at temperatures as high as 130oK (around -150oC). This is a temperature that is much more practical to achieve.

+

+

+

+

+

+

+

This distortion concentrates the positive charge in this part of the lattice, and attracts other electrons. In a normal conductor, this distortion leads to collisions and loss of energy by the flowing electrons which repel each other... this is the normal electrical resistance within the conductor. But in a superconductor below its “transition temperature”, something very strange occurs; due to Quantum Energy Effects, 2 nearby electrons “pair up” to form what is called a “Cooper Pair”: (Cooper is the “C” in “BCS Theory”)

The Meissner Effect

You may have seen a practical demonstration of a superconductor in action, in class. The “Meissner Effect” is named after the scientist who discovered it.

+

If a disk of superconductor ceramic is chilled below its “transition temperature”, a small magnet placed close above it will “levitate”; spinning freely if prodded, but held up against gravity by unseen forces.

+

+

+

+

Cooper Pair of electrons forms +

Small Levitating magnet

+

Approaching electron

The syllabus requires that you identify some of the superconducting metals and compounds. Here is a very short list... Temperature Superconductor of Transition (oK) Metals to Superconductivity Mercury 4 Lead 9 Alloy Niobium-Germanium 23 Ceramics Yttrium-Barium-Copper oxide 92 Thallium-Barium-Calcium-Copper oxide 125 (-148oC)

Disk of Superconducting Ceramic

+

+

Liquid Nitrogen

+

+

+

+

Due to quantum effects (which are beyond the scope of this Course... KISS Principle) each electron of the Cooper Pair helps the other to pass through the lattice without any loss of energy. This means there is ZERO resistance.

dish

However, at a temperature above the “transition”, the thermal vibrations in the lattice keep breaking up the Cooper Pairs as fast as they can form. This destroys the superconductivity, and the normal electrical resistance of the substance returns.

Explanation: As the magnet is brought near, its magnetic field induces currents in the ceramic. Since there is NO electrical resistance, the currents flow freely, non-stop and generate a magnetic field that repels the approaching magnet. Superconductors will never allow an external magnetic field to penetrate them. HSC Physics Topic 3

20

copyright © 2005-2006

keep it simple science

Coffs Harbour High School SL#703335

Using Superconductor Technology

Advantages Superconductor technology offers Possible Future Applications • high efficiency in any electrical situation, because there is no energy loss due to resistance. • the ability to generate extremely strong magnetic fields from superconducting electromagnets. • faster operation of computers, since superconducting switching devices could be 10 times faster than a semiconductor transistor

Current computer technology is based on semiconductor microchips. Although these become faster and more powerful every year, there is a limit to how far they can go. A superconductor computer could open a whole new level of enhanced performance due the possible high speed switching of circuits. Electricity generation & distribution could be made much more efficient with superconductor technology.

Limitations • Superconducting metals must be chilled with liquid helium. This is impractical and expensive. • New, superconducting ceramics can be chilled with liquid nitrogen, which is cheaper and much more practical, BUT these ceramics: • are fragile and brittle and difficult to make into wires. • can be chemically unstable and have a limited life span.

A lot of energy is lost due to resistance heating in transmission lines. This could be eliminated if power lines were superconductors. Generators lose energy by resistance heating in the coils needed to produce magnetic fields, and are limited in the strength of the fields they can produce. Superconducting coils would allow generators to be much more powerful and efficient. Greater efficiency generally in electrical technology would reduce associated environmental problems, such as Greenhouse gas emissions.

The Maglev Train The idea of using superconducting electromagnets to “levitate” a train above a magnetic guide-rail has been around for many years and experiments have been going on for decades.

MAGLEV = MAGnetic LEVitation Shanghai Maglev Train Photo © 2004 Matthew Hillier used with permission

The guiderail(s) under the train contain conventional electromagnets. On board, helium-chilled superconducting electromagnets produce powerful magnetic fields. The fields in the rail and the train repel each other so that the entire train is levitated 1-2cm above the track. Propulsion and braking is also done magnetically, by the fields in front and behind the train attracting and repelling it. The actual motive power is supplied from the rail, not from onboard the train.

Experiments have been going on for years in Germany and in Japan. The first truly operational Maglev now connects the city of Shanghai in China, with its airport 30km away. German built, it cost US$1.2 billion, and reaches speeds of around 430km/hr.

The big advantage is the high speed possible without any rail friction, and the low maintenance and low noise that goes with this. A disadvantage is the very high cost of building the guide rail track. HSC Physics Topic 3

21

copyright © 2005-2006

keep it simple science

Coffs Harbour High School SL#703335

Worksheet 4 Fill in the blanks. Check your answers at the back. The explanation of superconductivity is “v)...................... Theory”, which states: • an approaching electron causes a slight w)............................ of the ion lattice. • this concentrates the “density” of x)............................ charge, which attracts more electrons. • 2 electrons can form a “y).....................................................” which results in both of them z)............................................... the lattice without energy loss, due to aa)....................................... energy effects.

Sir William Bragg, and his son Lawrence beamed a).................................. through crystals. The waves were b)......................................... by the atom/ion array, and formed c)......................................... patterns, which were recorded on d).................................... film. By measurements of these images, they could deduce the exact structure and geometry within the e).................................................. Unlike other crystals, metals have a structure described as “an array of f).........................., enbedded in a sea of g)......................................” The electrons have free access to the h)..................................... band, so the metal is a good i).................................................... of electricity. There is some j)........................................ because of collisions caused by thermal k).................................. of the lattice.

The advantages and possible applications offered by superconductor technology include high ab)............................................. of electrical generation and ac)............................................., because it could eliminate energy losses due to ad).......................................... Another possiblity is in computers, which could operate ae)............................................ because a superconducting af)...................................... can work ag)............. times faster than a ah)................................................

l).......................................... was first discovered in mercury metal which had been m)............................. to a temperature of about n).......................... In the 1980’s, a new class of superconducting o)....................................... were discovered, with “transition” temperatures up around p)..........................

A limitations of superconductor technology is the need to ai)............................... a metal using aj)......................................, which is very ak).................................. and ............................... The “higher temperature” al)................................ superconductors solve part of this problem, but they are am).................................. and ................................... and difficult to make into an)........................................ They may also be ao)............................................................................ and have a short life-span.

If a magnet is placed above a superconductor, it will q)......................................, being held up by r)........................... forces. The field is created by s)......................................... in the superconductor, induced by the external t)........................................ Superconductors will never allow an external field to u)...................................................... them.

One superconductor technology that has been implemented is the ap)................................ train, which uses superconductor magnets to aq)......................................... the train above its guide rail.

COMPLETED WORKSHEETS BECOME SECTION SUMMARIES

HSC Physics Topic 3

22

Copyright © 2005-2006 keep it simple science

Coffs Harbour High School SL#703335

CONCEPT DIAGRAM (“Mind Map”) OF TOPIC Some students find that memorizing the OUTLINE of a topic helps them learn and remember the concepts and important facts. Practise on this blank version.

From CATHODE RAYS to TELEVISION

From RADIO to PHOTOCELLS: Quantum Theory

From IDEAS to IMPLEMENTATION

From CRYSTALS to SUPERCONDUCTORS

HSC Physics Topic 3

From ATOMS to COMPUTERS

23

copyright © 2005-2006

keep it simple science

Coffs Harbour High School SL#703335

5. If you were to alter the voltage to the anode in the “electron gun” part of a TV picture tube, this would alter: A. the position of the image on the screen. B. the speed of the cathode ray beam. C. the brightness & colours of the fluorescent image. D. the size of the image.

Practice Questions

These are not intended to be "HSC style" questions, but to challenge your basic knowledge and understanding of the topic, and remind you of what you NEED to know at the K.I.S.S. principle level. When you have confidently mastered this level, it is strongly recommended you work on questions from past exam papers.

6. Which of the following best describes the outcome of Hertz’s famous experiments of 1887? A. His discoveries led to the Quantum Theory of light. B. He showed that light gives interference patterns. C. He confirmed that light is an electromagnetic wave. D. He determined a more accurate value for the speed of light.

Part A Multiple Choice 1. The “Maltese Cross” cathode ray tube proves that cathode rays: A. travel from anode to cathode. B. travel in straight lines. C. are particles with mass. D. are electrically charged.

7. According to “Quantum Theory”, if you compared the energy of 2 photons of light and found that one had more energy than the other, then the one with more energy must have: A. more mass. B. longer wavelength. C. higher frequency. D. a higher velocity.

2. A cathode ray beam is passing through an electric field directed as shown in the diagram. E field This is part of an experiment in which the beam will simultaneously cathode rays pass through a magnetic field. The arrangement and strengths of the 2 fields is such that the effects will cancel out so the beam travels through without deflection.

8. The “Photoelectric Effect” involves: A. emission of electrons that have absorbed a quantum of energy from a photon. B. emission of a photon of light that has absorbed the excess energy from a falling electron. C. using photographic film to get an image of x-ray interference patterns. D. using an electrical induction coil to cause sparks in a separate receiving coil or antenna.

In which direction must the magnetic field be directed in order to achieve this? A. into the page B. up the page C. to the left D. out of the page 3. Which of the following diagrams correctly shows the electric field between point charges and/or charged plates? A.

-

D. +

+

+

+

C. -

9. According to Einstein, light often behaves like a wave, but sometimes acts like a particle. A phenomenon in which the particle nature of a photon is noticeable, is: A. interference of photons scattered by crystals. B. refraction of light by a glass lens. C. photoelectric effect occurring in a solar cell. D. polarization of light by sunglasses.

B.

-

10. According to “Band Structure Theory” of electrical conductivity, the “valence band” and the “conduction band” in a semiconductor: A. overlap each other. B. are sparated by a very wide “forbidden energy gap”. C. are inverted in reverse order to normal. D. are separated by a narrow energy gap.

-

4. Thomson’s famous cathode ray experiment was able to get a value for: A. the charge to mass ratio, of cathode rays. B. the mass of the electron. C. the strength of crossed electric and magnetic fields. D. the electric charge of an electron. HSC Physics Topic 3

24

Copyright © 2005-2006 keep it simple science

Coffs Harbour High School SL#703335

17. In a superconductor above its transition temperature: A. lattice vibrations break up the Cooper Pairs as fast as they can form. B. lattice distortions attract electrons to form Cooper Pairs. C. the Meissner Effect can levitate a magnet. D. the “holes” in a doped lattice allow electrons to “tunnel”.

11. Which line of information below, best describes a “p-type” semiconductor? Valence of atoms Current mainly used to dope lattice carried by A. 5 electrons B. 3 holes C. 5 holes D. 3 electrons

Longer Response Questions Mark values shown are suggestions only, and are to give you an idea of how detailed an answer is appropriate.

12. Which of the following is NOT an advantage of a transistor, compared to a thermionic valve? A. consumes less power. B. needs time to warm up. C. operates faster. D. smaller and more reliable.

18. (5 marks) Explain why the apparent behaviour of cathode rays caused debate as to whether they were charged particles or electromagnetic waves. 19. (6 marks) Two parallel charged plates + are 6.00cm apart, in vacuum, + and have a potential + difference of 30.0V between + them. An electron (Qe = -1.60x10-19C) is located between the plates. a) Find the magnitude of the electric field between the plates. b) Calculate the force that will act on the electron due to this field. c) At what rate will the electron accelerate? (electron mass = 9.11x10-31kg)

13. The original transistors were made from Germanium, but the technology later switched to use Silicon, because: A. Silicon crystals are easier to grow. B. Germanium supplies were running out. C. Silicon is more abundant and cheaper. D. Germanium crystals couldn’t be miniaturized as well. The following diagram describes a famous experiment carried out by Sir William & Lawrence Bragg. The diagram refers to Q 14 & Q15. Photographic film

20. (8 marks) An alpha particle (Qα = + 3.20x10-19C) is about to enter a magnetic field of strength 5.22T at a velocity of 2.95x103ms-1. a) Find the magnitude and (initial) direction of the force due to the magnetic field it will experience. b) A pair of charged plates (not shown in the diagram) are arranged so that the force due to the magnetic field will be exactly cancelled out by the force due to the electric field. Sketch where the plates need to be to do this, and indicate the type of charge on each plate. c) If these electric plates are 10.0cm apart, what voltage must be applied to exactly cancel the magnetic deflection?

Crystal

14. The radiation used by the Braggs was: A. x-rays B. radio waves C. ultra-violet D. visible light 15. The pattern captured on the photographic film was due to the phenomenon of: A. refraction. B. photoelectric effect. C. polarization. D. interference.

21. (6 marks) A TV picture tube is made up of several main components. Outline the role of the a) electrodes of the “electron gun”. b) deflection plates or coils. c) fluorescent screen. 22. (4 marks) As part of your studies you have carried out an investigation to demonstrate the production and reception of radio waves. Describe briefly how you did this.

16. Superconductor technology is currently limited by: A. lack of suitable applications to apply it to. B. superconducting chemicals being fragile and brittle. C. the operating temperatures being too low to achieve. D. semiconductors do the same job more efficiently. HSC Physics Topic 3

25

Copyright © 2005-2006 keep it simple science

Coffs Harbour High School SL#703335

23. (6 marks) Two different photons of light have wavelengths of 5.00x10-7m (photon P) and 2.40x10-8m (photon Q). Qualitatively (no calculation required) compare P & Q’s: a) speed b) frequency c) energy Explain your answers in each case.

28. (5 marks) Compare and contrast a “p-type” semiconductor and an “n-type” semiconductor.

24. (4 marks) For an electron to escape from the surface of a particular metal, it needs to absorb a minimum of 6.75x10-19J of energy. Calculate the a) frequency b) wavelength of a photon with just enough energy to cause this.

30. (4 marks) Assess the impact of the invention of the transistor on society, with particular reference to their use in microchips.

25. (3 marks) Identify the contribution made by Einstein to Quantum Theory.

32. (3 marks) Discuss the BCS Theory of superconductivity.

29. (4 marks) Describe the differences between a solid state and thermionic device in terms of structure and discuss why solid state devices replaced thermionic devices.

31. (3 marks) Outline the methods used by the Braggs to determine crystal structure.

33. (3 marks) Outline the possible benefits from applying superconductor technology to computers, generators and electrical transmission systems.

26. (4 marks) a) What is the “photoelectric effect”? b) Summarize how this effect is used in a “solar cell”. 27. (5 marks) In relation to the “Band Structure Theory” of conductivity, a) what is meant by the “valence band” of an atom? b) what is meant by the “conduction band” of an atom? c) explain the difference between conductors insulators semiconductors

Remember that for full marks in calculations, you need to show FORMULA, NUMERICAL SUBSTITUTION, APPROPRIATE PRECISION and UNITS

HSC Physics Topic 3

26

Copyright © 2005-2006 keep it simple science

Coffs Harbour High School SL#703335

Answer Section

Moving Charges in Magnetic Fields 6. F = QvBsinθ = -1.60x10-19x3.00x107x4.96x10-2xSin90o = -2.38x10-13N. (Negative sign indicates direction is opposite to whatever the RH Palm rule tells us) RH Palm rule: if v vector is north, and B vector vertically up, then F vector is east for a +ve charge. Therefore, for -ve electron, is west. Force = 2.38x10-13 N, west. 7. F = QvBsinθ, so Q = F/vBsinθ = 3.75x10-9/(2.90x108x8.05xsin90o) = 1.61x10-18C. 8. F = QvBsinθ, so v = F/QBsinθ = 7.40x10-15/(1.60x10-19x5.48x10-4xSin25o) = 2.00x108ms-1. (2/3 light speed!) 9. a) F = QvBsinθ, so B = F/QvSinθ = 4.06x10-15/(1.60x10-19x7.80x106xsin90o) = 3.25x10-3T. b) The force due to the electric field must be equal, so F = 4.06x10-15N. E = F/Q = 4.06x10-15/1.60x10-19 = 2.54x104NC-1. c) E = V/d, so V = E.d = 2.54x104x0.00800 = 203 V.

Worksheet 1 Part A a) cathode b) discharge c) vacuum d) voltage e) glows, streamers and striations f) gas pressure g) negative h) Crookes i) Maltese j) fluorescent k) kinetic l) momentum m) electric n) waves or particles o) straight lines p) fluorescence q) photographic film r) kinetic energy & momentum s) mass t) negative u) field v) the direction a positive “test” charge would move w) force x) parallel, charged y) strength & direction z) voltage aa) distance ab) moving ac) magnetic ad) magnet ae) deflect af) Right-Hand Palm ag) Thomson ah) electric & magnetic ai) charge to mass aj) particles ak) atoms al) electron am) TV an) electron ao) electrons ap) cathode aq) anodes ar) deflection as) steer/direct at) fluorescent au) glow av) electrons

Worksheet 2 Part A a) radio b) induction c) sparks d) wire loop antenna e) reflection & diffracted f) velocity g) light h) Photoelectric i) Black Body j) Max Plank k) Quantum l) quantised m) atom n) quantum o) frequency p) light energy q) electron r) emitted s) Quantum t) photons u) quantum v) all w) none x) part of its energy. y) reflection, refraction and diffraction (plus others) z) particle aa) predictions ab) Photoelectric ac) experiment ad) Quantum ae) Black Body af) Photoelectric

Part B Electric Fields & Forces 1. a) E = V/d = 50.0/0.0400 = 1250 = 1.25x103NC-1. b) F = Q.E = 3.20x10-19x1.25x103 = 4.00x10-16N. c) i) Field is directed from +ve plate to -ve plate. ii) Force is also directed towards -ve plate. 2. a) F = Q.E, so E = F/Q = -7.82x10-15/-1.60x10-19 = 48.9x104 NC-1. b) E = V/d, so V = E.d = 48.9x104 x 0.00250 = 122V. 3. a) E = V/d = 200/0.0500 = 4.00x103NC-1. b) F = Q.E, so Q = F/E = 2.29x10-12/4.00x103 = 5.73x10-16C. c) No. electrons = charge on speck/ Qe = 5.73x10-16/1.60x10-19 = 3.58x103 electrons. d) Attracted to -ve plate, therefore speck must have +ve charge, therefore must have lost electrons. 4. First find field, from force on electron: E = F/Q = -5.88x10-17/-1.60x10-19 = 368NC-1. Now use E = V/d, d = V/E = 40.0/368 = 0.109m. (10.9cm) 5. Find E: E = V/d = 100/0.00500 = 20,000NC-1 Next use: F = Q.E = 9.75x10-10x20,000 = 1.95x10-5N.

HSC Physics Topic 3

Part B Quantum Theory Problems 1. a) c = λ.f, so f = c/λ = 3.00x108/4.25x10-7 = 7.06x1014Hz. b) E = h.f = 6.63x10-34x7.06x1014 = 4.68x10-19 J. ii) UV 2. i) infra-red c = λ.f, so f = c/λ c = λ.f, so f = c/λ = 3.00x108/5.45x10-9 = 3.00x108/5.45x10-6 13 = 5.50x10 Hz = 5.50x1016 Hz E = h.f E = h.f = 6.63x10-34x5.5x1016 = 6.63x10-34x5.5x1013 = 3.65x10-20 J. = 3.65x10-17 J. Comparison: UV photon carries 1,000 times more energy

27

copyright © 2005-2006

keep it simple science

Coffs Harbour High School SL#703335 k) vibrations m) cooled o) ceramics q) levitate s) currents u) penetrate w) distortion y) Cooper Pair aa) quantum ac) distribution/ transmission ae) much faster ag) 10 ai) cool/chill ak) expensive & impractical am) brittle & fragile ao) chemically unstable aq) levitate

Worksheet 2 Part B (continued) 3. a) E = h.f, so f = E/h = 8.75x10-14/6.63x10-34 = 1.32x1020 Hz. b) c = λ.f, so λ = c/f = 3.00x108/1.23x1020 = 2.44x10-12m. (this is extremely high energy, high frequency, short wavelength EMR in the range of “hard” x-ray or gamma radiation) 4. a) E = h.f, so f = E/h = 2.38x10-20/6.63x10-34 = 3.59x1013 Hz. c = λ.f, so λ = c/f = 3.00x108/3.59x1013 = 8.36x10-6 m. b) A longer wavelength photon would have lower frequency, and therefore less energy. Since this would be below the “threshold” energy for emission of an electron, no emission would occur. c) A higher frequency photon will transfer all its energy to an electron. The “threshold” energy is used to “escape” the metal surface, while any excess becomes the kinetic energy of the electron. 5. Total energy of the photon = 6.22x10-17 + 9.28x10-19 = 6.31x10-17J. Frequency, f = E/h = 6.31x10-17/6.63x10-34 = 9.52x1016Hz. Wavelength, λ = c/f = 3.00x108/9.52x1016 = 3.15x10-9m.

Practice Questions Part A Multiple Choice 1. B 5. B 9. C 2. D 6. C 10. D 3. D 7. C 11. B 4. A 8. A 12. B

a) Electrons b) energy c) electrons d) atom to atom e) insulator f) conductivity g) conductive h) switched i) electronics j) computers k) Band Structure l) valence m) conduction n) current o) energy gaps p) quantum q & r) conduction & valence s) overlap t) currents/electricity u) insulator v) forbidden energy gap w) conduction x) narrow y) conduct z) cross aa) to conduct ab) minute / very small ac) doping ad) electrons ae) holes af) semiconductor ag) extra electrons ah) n-type ai) doped aj) 3 ak) holes al) p-type am) thermionic an) cathode ray ao) transistor ap) much smaller aq) cheaper ar) less as) waste heat at) faster au) warm up av) reliable aw) Germanium ax) Silicon ay) abundant/ common az) cheaper ba) chips bb) microchips / microprocessors

17. A

c) Force on particle must be equal to (a) F = 4.93x10-15N.

-v ve

E = F/Q = 4.93x10-15/3.20x10-19 = 1.54x104NC-1 and E = V/d, so V = E.d = 1.54x104x0.100 = 1.54x103V. 21. a) Electron gun has a cathode to act as a source of cathode rays (electrons), and a series of anodes to accelerate the electrons up to the desired velocity. b) The deflection plates are parallel charged plates (or magnetic coils) which deflect the electron beam with the electric (or magnetic) field, to steer the beam to any point on the screen. One set of plates/coils deflect left/right, another set deflect up/down. c) Fluorescent screen glows when struck by electrons. The image is formed by many glowing spots of fluorescence.

Worksheet 4

HSC Physics Topic 3

13. C 14. A 15. D 16. B

Part B Longer Response Questions In some cases there may be more than one correct answer. The following “model” answers are correct but not necessarily perfect. 18. Cathode rays were found to have some waves properties (e.g. travel in straight lines, fluorescence, expose photo film) and also to have some particle properties (e.g. carry kinetic energy and momentum). This caused confusion and debate about their nature, finally resolved when Thomson measured the charge/mass ratio, proving them to be particles. 19. a) E = V/d = 30.0/.0600 = 500NC-1. b) F = Q.E = -1.60x10-19x500 = -8.00x10-17N. (left in diag.) c) F = ma, so a = F/m = -8.00x10-17/9.11x10-31 = 9.78x1013ms-2. 20. a) F = QvBsinθ = 3.20x10-19x2.95x103x5.22xSin90o = 4.93x10-15N. Initial direction up the page. (RH Palm rule) b) Plates need to be as +ve shown in diagram.

Worksheet 3

a) x-rays c) interference e) crystal lattice g) electrons i) conductor

l) Superconductivity n) 4oK (-269oC) p) 125oK (approx -150C) r) magnetic t) field v) BCS x) positive z) pass through ab) efficiency ad) resistance heating af) switch ah) semiconductor/ transistor aj) liquid helium al) ceramic an) wires ap) Maglev

b) diffracted d) photographic f) ions h) conduction j) resistance

28

copyright © 2005-2006

keep it simple science

Coffs Harbour High School SL#703335 28. “Compare and contrast” means to point out similarities and differences... be sure to shown both. Similarities Both types of semiconductor are solid crystals of silicon with a lattice structure made up of atoms chemically bonded to 4 neighbours. The atoms have a narrow “forbidden gap” between valence and conduction bands, and can switch from being a nonconductor, to a good conductor with a very small change in energy. The sensitivity to this “switching on” can be increased by “doping” the lattice with other atoms.

22. (many possible answers) A “spider web” of wire was connected to an induction coil. When switched on, the fluctuating, high voltage from the coil caused the wire to act as a tranmitting antenna, giving off radio frequency waves. This was proven by placing a modern radio receiver on the other side of the room. It picked up loud bursts of “static” noise whenever the coil was on. 23. a) both travel at the same velocity (= 3x108ms-1 in vacuum) because ALL EMR waves travel at this “speed of light”. b) Photon Q has a shorter wavelength, and therefore must have higher frequency. c) Photon Q carries more energy, because quantum energy is proportional to frequency. 24. a) E = h.f, so f = E/h = 6.75x10-19/6.63x10-34 = 1.02x1015 Hz. b) c = λ.f, so λ = c/f = 3.00x108/1.02x1015 = 2.94x10-7m. 25. Quantum Theory was proposed by Max Plank as a “mathematical convenience” to explain the shape of the “Black Body Radiation” curves. However, it was Einstein who used quantum theory to cause a major change in the direction of Physics. His “photon” idea changed “classical” Physics, and caused physicists to look at the things quite differently.

Differences In “n-type” semiconductors, the lattice has been doped with atoms with 5 valence electrons. This places “extra” valence electrons in the lattice and increases the sensitivity of the substance to carrying currents by the flow of negative electrons. In “p-type” semiconductors, the lattice has been doped with atoms with only 3 valence electrons. This leaves extra “holes” in the lattice and increases the sensitivity of the substance to carrying currents by the flow of positive holes. 29. A Thermionic valve is a cathode ray tube: a glass tube containing metal electrodes in a vacuum. Typically the valve is 10-20cm in size. A solid state transistor is a “sandwich” of n-type and p-type semiconductor material (i.e. doped silicon crystals). A transistor can range in size from 1-2 cm, down to microscopic layers etched into the crystal in a “microchip”. Transistors can do exactly the same job as valves, but • are much smaller and cheaper. • use much less electricity, and produce hardly any waste heat. • are faster, more reliable, and do not need time to “warm up”. For all these reasons, the transistor replaced the valve in electronics. 30. The invention of the transistor has had an enormous impact on society by making possible the development of electronics, especially computers, operated by cheap, efficient and miniature “microchips”. This has allowed the development of: • business and financial systems for cheap, efficient operation of a global economy. • instant access to information, communication and entertainment via the internet, TV, DVD technology, mobile phones, etc. 31. Sir William & Lawrence Bragg beamed x-rays through crystals. The atomic or ionic lattice in the crystal diffracted the x-rays, which then formed interference patterns. These were captured as geometric patterns on photographic film. Analysis of the geometry of the “x-ray diffraction pattern” allowed them to calculate the spacing and geometry of the lattice. 32. BCS theory states that: • an electron (in the conduction band of a conductor) causes a slight distortion of the ionic lattice. • This increases the density of +ve charge in this area, which attracts more electrons. • Normally this causes electron collisions and heating in a conductor, resulting in “electrical resistance”. • In a superconductor below its “transition temperature”, the electrons can form “cooper pairs” which use quantum effects to “tunnel” through the lattice with zero resistance. 33. Possible benefits: • faster computers, because superconducting “switches” are 10 times faster than transistors. • more efficient generation of electricity from superconducting coils producing more powerful magnetic fields in generators. • Elimination of restistance heating losses in transmission lines could save energy, and reduce costs and environmental impacts.

Einstein’s contribution was to combine Plank’s theory with classical wave ideas so that phenomena (like the photoelectric effect) could be explained and understood. 26. a) The photoelectric effect occurs when light waves are absorbed by a metal surface so that the energy of the light causes electrons to be emitted from the surface. b) In a solar cell (or “photovoltaic cell”) the photoelectric effect occurs in a sandwich of semiconductor materials called a “p-n junction”. The light energy promotes valence electrons to the conduction band in such a way that a potential difference is set up across the junction. This can cause a flow of current in an external circuit, so the device is a way to generate electricity directly from light energy. 27. a) The valence band is the highest “orbit” or energy level of an atom that has electrons in it (when the atom is in its “ground state”) b) The conduction band is any energy level above the valence band. In an atom’s ground state, the conduction bands would normally be empty. If an electron can reach a conduction band it is effectively free to flow through the lattice of the substance. c) In a conductor, the valence & conduction bands overlap with each other, so that valence electrons can access the conduction band at any time, and thereby flow as a current. In an insulator the valence and conduction bands are separated by a very wide “forbidden energy gap” in which an electron cannot exist because the energy required does not correspond to a whole quantum of energy. To get to the conduction band, an electron needs a huge amount of energy, and at normal temperatures and voltages, this does not happen. In a semiconductor, the gap between valence and conduction bands is narrow. A small energy input can “kick” and electron up so the substance goes from non-conducting to conductor. HSC Physics Topic 3

29

copyright © 2005-2006

keep it simple science

Need to contact us? PO Box 2575 PORT MACQUARIE NSW 2444 ABN 54 406 994 557

(02) 6583 4333

FAX (02) 6583 9467