IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# Faraday's Experiment on Electromagnetic Induction

## Introduction

Faraday's law of electromagnetic induction is one of the most important discoveries as it allows us to produce electric power. In integral form it is $$e=-\frac{\mathrm{d}\phi}{\mathrm{d}t}$$ where $$e$$ is the emf induced in a circuit and $$\phi$$ is the magnetic flux through that circuit. Faraday did experiments on passing magnets through coils (and vice versa) to establish that change in magnetic flux produces emf. This is being done routinely as classroom demos and school science exhibitions. Here we do this experiment in a slightly modified way.

## Apparatus

A coil wrapped around the body of a syringe connected with two LEDs, 4 strong cylindrical magnets, a stopper to close the syringe at the wider end.

Coil: The coil is made by wrapping about 1000 turns of wire on the syringe body. Two LEDs, one red and one green are taken and joined to each other with longer leg of each LED joined to the shorter end of the other. The two joint ends are then joined to the ends of the coil. Thus depending on which end of the coil is at a higher potential than the other, one of the LEDs will be forward biased and the other will be reverse biased.

## Procedure

1. Put one magnet in the syringe. Close the (wider) open end by the stopper.
2. Slowly turn the syringe upside down keep repeating. Does any of the LEDs glow? If yes which one?
3. Increase the frequency and write your observation on which LED is glowing. From the sequence of the LEDs which glow with the increase in frequency of shaking, what can you say about the band gap of red and green LED?
4. Now put two magnets in the syringe and repeat steps 3 and 4. What do you observe?
5. Now put 4 magnet in the syringe and repeat steps 2 and 3. What do you observe?
6. With 4-magnets, insert the magnet and repeat steps 2 and 3. What do you observe?