From Ideas to Implementation

HSC Physics Module 2: From Ideas to Implementation Summary

From Ideas to Implementation: 1. Cathode Rays x

Identify that moving charged particles in a magnetic field experience a force

Any moving charged particles (e.g. electrons and protons) in a magnetic field experience a force. Try placing a bar magnet near a CRT television screen. You should see a distortion of the image due to the magnetic field acting on the cathode ray (electrons). x

Perform an investigation and gather first-hand information to observe the occurrence of different striation patterns for different pressures in discharge tubes

Investigation: Discharge Tubes Aim: To investigate the effect of different gas pressures passing through a series of discharge tubes. Theory: When a high voltage from an induction coil is placed across a discharge tube, a discharge may occur. Electrons released from the cathode travel toward the anode, colliding with gas molecules to create different glowing patterns depending on the air pressure inside the tube. Equipment: -Power supply -discharge tubes of various pressures

-induction coil -connecting wires

Method: 1/ Set up the equipment as shown below: Cathode

Induction coil (approx. 50000 V)

Discharge tube

Anode

Robert Lee Chin

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HSC Physics Module 2: From Ideas to Implementation Summary The negative terminal induction coil was set across the cathode of the first discharge tube and the positive terminal connected to the bottom plug. Observations were made on different pressures by moving the top plug along to different tubes.

Results: Pressure in tube (mm Hg) 40 10 6. 0 3. 0 0.14 0.03

Observations - (may be able to observe in dark room) Violet glow appears at the cathode and lighter violet streamers appear from the anode, extending to the cathode Violet glow at cathode intensifies, and pink streamers extend from the anode to the cathode Violet glow at cathode ↑ in size and the pink streamers break into striations. There is a dark space between the striations and the cathode glow Cathode glow is less clear, and the dark spaces between the striations have ↑. The entire tube glows a faint purple. A faint green glow appears at the anode.

Discharge tube at 3.0 mm Hg:

_

+

50 000 V Crookes’ dark space Cathode

Cathode glow

Faraday’s dark space

The negative glow

To vacuum pump

Anode

Striated positive column

From left to right: 1) Cathode glow 2) Crookes’ dark space 3) The negative glow 4) Faraday’s dark space 5) Striated positive column

Robert Lee Chin

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HSC Physics Module 2: From Ideas to Implementation Summary x

Explain that cathode ray tubes allowed the manipulation of a stream of charged particles

The Cathode ray tube (CRT) is a variant of the discharge tube. A CRT consists of a highly evacuated glass tube containing two electrodes. The negative electrode is the cathode and the positive electrode is the anode. When a high DC voltage is applied across the CRT, a stream of electrons flows from the cathode to the anode. The electrons can do this because there is little obstruction from remaining air particles n the tube. Because cathode rays are simply a steam of electrons, structures built into the CRT, e.g. solid objects will block the flow of the cathode rays to the anode. Additional anodes can be built into the CRT to accelerate the cathode rays to have higher kinetic energies. Sets of electrodes can be placed in the CRT perpendicular to the beam to create an electric field that can change the path of the beam. Similarly, external magnetic fields can be applied to the cathode rays (e.g. using a magnet) - these will cause a deflection of the beam. Magnetic deflection of cathode rays:

_

+

Induction coil

Cathode ray

Cathode B

Anode

v

F

Because of the radial nature of the magnetic field from a bar magnet, the force on the cathode ray will be directed towards the North Pole of the magnet. It does not matter where the North Pole is, as long as it is facing towards the cathode ray. When the South Pole is placed near the CRT, the ray will deflect away from the South pole. End view of CRT:

F B

Robert Lee Chin

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HSC Physics Module 2: From Ideas to Implementation Summary

_

+

Induction coil

Cathode ray

Cathode F

Anode

v

B

x

Perform an investigation to demonstrate and identify properties of cathode rays using discharge tubes: -containing a Maltese cross -containing electric plates -with a fluorescent display screen -containing a glass wheel

and analyse the information gathered to determine the sign of the charge on cathode rays Investigation: The properties of cathode rays in discharge tubes Aim: To perform an investigation to demonstrate and identify the properties of discharge tubes containing a Maltese cross; electric plates; fluorescent display screen; glass wheel Equipment: -induction coil -power supply -connecting wires -Crooke’s tube containing Maltese cross -Crooke’s tube containing glass wheel -Crooke’s tube containing electric plates -Crooke’s tube containing a fluorescent display screen

Safety: Stay clear of the induction coil and bare conductors. The circuit has a high voltage across it that can cause electric shock. The tube also emits low levels of xrays. Sat at least 3m away from the tube if viewing for extended periods

Robert Lee Chin

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HSC Physics Module 2: From Ideas to Implementation Summary Method: Maltese Cross: 1/ Set up equipment as shown below with the Crooke’s tube containing the Maltese cross. Cathode rays

Maltese cross anode

Cathode

_ 2/

Induction coil

+

Turn on the electricity and observe what happens at the opposite end of the cathode.

Glass Wheel: 1/ Set up equipment as shown below with the Crooke’s tube containing the glass paddle wheel.

_

+

Induction coil

Cathode

Cathode ray

Anode

Glass runners on very slight incline towards anode

Robert Lee Chin

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HSC Physics Module 2: From Ideas to Implementation Summary 2/

Ensure the apparatus is set up on a level surface. Turn on the electricity and observe the effect of the cathode rays on the glass wheel.

Fluorescent screen: 1/ Set up equipment as shown below with the Crooke’s tube containing the fluorescent screen.

_

+

Induction coil

cathode

Fluorescent screen

anode

Cathode ray

2/

Turn on the electricity. Observe what happens to the fluorescent screen.

Electric Plates: 1/ Set up equipment as shown below with the Crooke’s tube containing the electric plates (and fluorescent screen).

_

+

Induction coil

cathode

Cathode ray -ve plate

anode

+ve plate

2/

Turn on the electricity. Observe what happens to the cathode ray.

Robert Lee Chin

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HSC Physics Module 2: From Ideas to Implementation Summary Results: Discharge tube type Maltese cross Glass paddle wheel Fluorescent screen Electric plates

Observation A shadow of the cross at the opposite end of the cathode

Property of cathode ray CR’s travel in straight lines and cast sharp shadows The wheel moves along the glass CR’s possess momentum runners from the cathode to and can do work (implying anode they have mass) The CR passes straight through CR’s cause fluorescence, to the anode. The entire like UV light waves fluorescent screen glows The CR is deflected towards the CR’s are deflected by positive plate electric fields and are negatively charged

In addition, the above observations (except for fluorescent screen) also demonstrate that cathode rays originate from the cathode. The results show that cathode rays travel in straight lines indicate that cathode rays must be composed of small particles that are x

Explain why the apparent inconsistent behaviour of cathode rays caused debate as to whether they were charged particles or electromagnetic waves

Early CR experiments provided inconsistent evidence as to the nature of CR’s, because they showed that CR’s possessed properties of both waves and particles. It must be noted that at this time, scientists had no knowledge of the structure of the atom or the type of radiation in discharge tubes and there were inadequacies in experimental design. A timeline of the events leading to the modern CRT is outlined below: 1858- Julius Plücker shows that cathode rays are deflected by a magnetic field 1865- H. Sprengel is able to ↓ the air pressure in discharge tubes. He discovers that the faraday dark space ↑ and a glow emanates from the cathode on the walls of the tube 1860- C F Varley suggests that cathode rays are composed of particles 1875- Sir William Crookes performs various experiments with discharge tubes including the Maltese Cross and glass paddle wheel. He showed that CR’s: -are identical, regardless of material used for cathode -travelled in straight lines -carried energy (and could do work) -are deflected by magnetic fields, suggesting CR’s and magnetism were related

Robert Lee Chin

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HSC Physics Module 2: From Ideas to Implementation Summary 1876- Eugene Goldstein proves that CR’s originate from the cathode. He coins the term “cathode ray” 1883- Heinrich Hertz finds that CR’s are NOT deflected by electric fields. He uses this to argue that CR’s are EM waves. This observation turns out to be false (the air pressure in the discharge tubes was not low enough, hence the CR became ionised) 1886- Eugene Goldstein 1890- Arthur Schuster calculates the q/m ratio of the CR particles using magnetic deflection. His answer was not accurate but showed that the q/m ratio was three orders of magnitude smaller than even hydrogen, the smallest atom. 1892- Heinrich Hertz shows that CR’s can penetrate thin metal foil, suggesting they are EM waves. 1894- J. J. Thompson calculates the velocity of CR’s to be 1.9 x 105 ms-1- MUCH lower than the value 3.0 x 108ms-1 for light. 1895- Jean-Baptiste Perrin shows that CR’s deposit a negative charge where they hit, suggesting CR’s are particles. 1897- J. J. Thompson performs his famous experiment to determine the q/m ratio of CR’s. By lowering the air pressure inside the discharge tube, he is able to show that CR’s are deflected by electric fields and have a negative charge, hence proving that CR’s are particles.

Evidence that CR’s were waves (before 1897)

Evidence that CR’s were particles (before 1897)

Travel in straight lines like light waves

Have kinetic energy, momentum and therefore must have mass

Caused fluorescence, like UV light waves

Deposit a negative charge on impact

Can expose photographic film, as light does

Have a velocity much lower than light waves

Are not deflected by an electric field (this was due to inadequate equipment) Can penetrate thin metal foil

x

Identify that charged plates produce an electric field

An electric field exists in ANY region in which an electrically charged object experiences a force. The observation that charged plates exert a force on other charged objects brought close to them indicates that an electric field is associated with charged plates. Robert Lee Chin

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HSC Physics Module 2: From Ideas to Implementation Summary x

Discuss qualitatively the electric field strength due to a point charge, positive and negative charges and oppositely charged parallel plates

Electric charges exert a force on eachother. Like charges repel while opposite charges attract. Charges act as if surrounded by a “force field”. Point charges are by definition, the direction a +ve charge would move in the field. Fields around point charges: +ve point charge

+

The strength of the electric field decreases with ↑ distance from the object. The direction of the field is defined as pointing radially away from a positive point charge

.

-ve point charge

-

Robert Lee Chin

The strength of the electric field due to a -ve point charge decreases with ↑ distance from the object. The direction of the field is defined as pointing radially towards a -ve point

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HSC Physics Module 2: From Ideas to Implementation Summary Repulsion Attraction

+

-

+

+

Attraction of –ve point charge and +ve plate

Attraction of +ve point charge and -ve plate

-

+

+++++++++++++++++++++

_______________________

The strength of the electric field due to oppositely charged plates is uniform in strength and direction (except at the edges). The direction of the field is defined as pointing at right angles from the +ve to the –ve plate. The more field lines, the stronger the field strength. +++++++++++++++++++++

_______________________ Field lines between a capacitor

Robert Lee Chin

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HSC Physics Module 2: From Ideas to Implementation Summary x

Describe quantitatively the electric field due to oppositely charged parallel plates

The electric field strength is proportional to the applied voltage and inversely proportional to the distance between the plates. The field strength is perpendicular to the plates and uniform in the region between the plates. Mathematically, V E where, d E V d

x

electric field, in NC -1 voltage applied to the plates, in volts distance between the plate, in metres

Describe quantitatively the force acting on a charge moving through a magnetic field

The force acting on a charged, moving particle in a magnetic field is proportional to: -the size of the charge, q -the magnetic flux density, B -the velocity of the charge, v -the sine of the angle between the field and the velocity of the particle, θ, (where max. force is at 90˚ and zero force at 0˚) Mathematically, F

qvB sin T where,

F magnetic force, in newtons q charge, in coulombs v velcoty of charge, in ms-1 B magnetic flux density, in teslas T the angle between the the magnetic field and the direction of the charge

θ q

Robert Lee Chin

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HSC Physics Module 2: From Ideas to Implementation Summary

E x

Solve problems and analyse information using F

F

V d qE qvB sin θ

To find the force acting on a charge, use the right-hand palm rule. Remember that this rule applies to positive charges, so the force on a negative charge is in the opposite direction as per the right-hand palm rule In a Thompson-type experiment, FMagnetic

FElectric i.e. qvB

qE .

mv 2 , r If the particle enters at 90˚ to the field, it will follow the path of a semicircle before leaving in the opposite direction. If it enters at: 0˚