Physics Practical Project for Class 12 Boards

DELHI PUBLIC SCHOOL, FARIDABAD

PHYSICS DEMONSTRATION FILE

NAME

:

ROHAN SETHI

CLASS

:

XII – A

ROLL NO.

:

DEMONSTRATION 1

ELECTROMAGNETIC INDUCTION

ELECTROMAGNETIC INDUCTION

ELECTROMAGNETIC INDUCTION Defination: Electromagnetic induction (or sometimes just induction) is a process where a conductor placed in a changing magnetic field (or a conductor moving through a stationary magnetic field) causes the production of a voltage across the conductor. This process of electromagnetic induction, in turn, causes an electrical current – it is said to induce the current. Michael Faraday stated that electromotive force (EMF) produced around a closed path is proportional to the rate of change of the magnetic flux through any surface bounded by that path. In practice, this means that an electric current will be induced in any closed circuit when the magnetic flux through a surface bounded by the conductor changes. This applies whether the field itself changes in strength or the conductor is moved through it. In mathematical form, Faraday’s law states that:

Where Is the magnitude of electromotive force ΦB Is the magnetic flux. For the special case of a coil of wire, composed of N loops with the same area, the equation becomes

A corollary of Faraday’s Law, together with Ampere’s Law and Ohm’s Law is Lenz’s Law: The EMF induced in an electric circuit always acts in such a direction that the current it drives around the circuit opposes the change in magnetic flux which produces the EMF.

ELECTRIC GENERATOR

ELECTRIC MOTOR

APPLICATIONS OF ELECTROMAGNETIC INDUCTION The principles of electromagnetic induction are applied in many devices and systems. Induction is used in power gereration and power transmission. Electric generators and electric motors are based on electromagnetic induction.

1.An electric generator: An electric generator is a device for transforming mechanical energy into electrical energy. Generators have a wire coil in a magnetic field. When the device is used as a generator, the coil is spun which induces current in the coil. An AC (alternating current) generator utilizes Faraday’s law of induction, spinning a coil at a constant rate in a magnetic field to induce an oscillating emf. A coil turning in a magnetic field can also be used to generate DC power.

2.An electric Motor: An electric motor is a device which converts electric energy into mechanical energy. It also has a coil. When a current is passed through the coil, the interaction of the magnetic field with the current causes the coil to spin.

MUTUAL INDUCTANCE

SELF INDUCTANCE

3.Mutual inductance: Faraday’s law tells us that a changing magnetic flux will induce an emf in a coil. If two coils are put next to each other, end tom end, and the first coil has a current going through it, a magnetic field will be produced, and a magnetic flux will pass through the second coil. Changing the current in the first coil changes the flux through the second, inducing an emf in the second coil. This is known as mutual inductance, inducing an emf in one coil by changing the current through another. The induced emf can thus written as :

Where M is the mutual inductance of the secondary coil, is the electro motive force.

4. Self inductance: Coils can also induce emf’s in themselves. If a changing current is passed through a coil, a changing magnetic field will be produced, inducing an emf in the coil. As with mutual inductance, the induced emf is proportional to the change in current. The induced emf can be written as :

Where L is the self-inductance of the coil & v is the voltage in volts

5.Transformers: Electricity is often generated a long way from where it is used, and is transmitted long distances through power lines. Although the resistance of a short length of power line is relatively low, over a long distance the resistance can become substantial. A power line of resistance R causes a power loss of I R; this is wasted as heat. By reducing the current, therefore, the I R losses can be minimized. At the generating station, the power generated is given by P = VI. To reduce the current while keeping the power constant, the voltage can be increased. Using AC power, and Faraday’s law of induction, there is a very simple way to increase voltage and decrease current (or vice versa), and that is to use a transformer. A transformer is made up of two coils, each with a different number of loops, linked by an iron core so the magnetic flux from one passes through the other. When the flux generated by one coil changes ( as it does continually if the coil is connected to an AC power source), the flux passing through the other will change. Inducing a voltage in the second coil. It is used only for AC Cicuits. The relation between the primary & secondary voltages is follows:

Where Vs is the secondary voltage

Vp is the primary voltage Ns is the numbers of loops in secondary coil And Np is the number of loops in primary coil.

DEMONSTRATION 2

TOTAL INTERNAL REFLECTION

TOTAL INTERNAL REFLECTION Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is greater than the critical angle, no light can pass through and all of the light is reflected. When light crosses a boundary between materials with different refractive indices, the light beam will be partially refracted at the boundary surface, and partially reflected. However, if the angle of incidence is greater (i.e. the ray is closer to being parallel to the boundary) than the critical anglethe angle of incidence at which light is refracted such that it travels along the boundary – then the light will stop crossing the boundary altogether and instead be totally reflected back internally.

Two Requirements for Total Internal Reflection Total internal reflection (TIR) is the phenomenon that involves the reflection of all the incident light off the boundary. TIR only takes place when both of the following two conditions are met:

• the light is in the more dense medium and approaching the less dense medium. • the angle of incidence is greater than the so-called critical angle.

An experiment involving the use of a large jug filled with water and a laser beam also based on total internal reflection concept. The jug has a pea sized hole drilled in its side such that when the cork is removed from the top of the jug, water begins to stream out the jug’s side. The beam of laser light is then directed into the jug from the opposite side of the hole, through the water and into the falling stream. The laser light exists the jug through the hole but is still in the water. As the stream of water begins to fall as a projectile along a parabolic path to the ground, the laser light becomes trapped within the water due to total internal reflection. Being in the more dense medium (water) and heading towards a boundary with a less dense medium (air), and being at angles of incidence greater than the critical angle, the light never leaves the stream of water. In fact, the stream of water acts as a light pipe to pipe the laser beam along its trajectory.

What is the Critical Angle? The critical angle is the angle of incidence above which the total internal reflection occurs. It is the angle of incidence for which the angle of refraction is 90°. At this angle the refracted ray glances parallel to the boundary. The incident ray undergoes total internal reflection at any angle greater than the critical angle. If the incident angle is less than or equal to the critical angle, the refracted ray will be bent away from the normal (provided that n2 < n1 ). A high relative index of refraction (the ratio n2/n1 ) will result in a smaller critical angle. The critical angle ic can be determined from the general form of Snell’s Law. At the critical angle, R = 90°, so sinR = 1 and

sin ic ═ n2 sin 90° n1 sin 90° ═ 1, so

sin ic

═

n2 n1

Examples of Total Internal Reflection Following are some examples based on Total Internal Reflection phenomenon: 1. Fiber optic Cables use total internal reflection inside the optical Fibre. The light enters the optical fiber, and every time it strikes the edge of the fiber it experiences total internal reflection. This way the light travels down the length of the optical fiber. 2. Binoculars use prisms to reflect light. The light enters the prism in such a way that it will strike the other side of the prism and be totally internally reflected. The prism can in this way act as a mirror. 3. Rainbows from when light enters raindrops. The light is totally internally reflected inside the raindrop before leaving. In addition the light of different colors is refracted at different angles to separate the colors in an effect called dispersion.

4.The brilliance of diamonds results from light entering the diamond and being totally internally reflected from the opposite side before exiting in approximately the original direction. 5. Total internal reflection can be observed while swimming. if one opens one’s eyes just under the water’s surface. If the water is calm, its surface appears mirror-like. Total internal reflection of the green turtle can be seen at the air water boundary.