# Demonstration of Electromagnetic Induction (Faraday's Laws)

Following simple demonstration can be performed at home or in the classroom.

## Demonstration of Faraday's Laws with Bar Magnet

A time varying magnetic flux through a coil generates electromagnetic force (emf) in it. The magnetic flux through a coil can be varied by moving a magnet relative to the magnet. The magnitude of generated emf is proportional to the time rate of change of magnetic flux. The direction of induced emf is such that it opposes the reason behind change in flux (Lenz's law).

### Procedure

Connect the ends of a coil to a galvanometer. Since there is no current in the coil, the needle of the galvanometer will be at the zero mark. Now, bring a bar or cylindrical magnet towards the coil. The galvanometer needle will get deflected in a particular direction. When the magnet comes to rest, the needle comes back to its zero position, showing that the current in the coil has stopped. Now, move the magnet away from the coil. The needle will get deflected again, but this time, in the opposite direction. This shows that the direction of the current has reversed. You will also find that the direction of the current depends on the pole of the magnet facing the coil while the magnet is moving.

What happens if we move the coil instead of the magnet? Fix the coil to a wooden block, and move it towards the magnet. You will find that as long as the coil moves, there is a current in the coil, as indicated by the deflection of the needle of the galvanometer. If you move the coil away from the magnet, the needle of the galvanometer gets deflected in the opposite direction. There is no deflection, i.e., no current, when both of them are at rest.

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.

### Procedure

You need 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.

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.

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?

## Faraday's laws with Mother Coil

The electromagnetic induction is used in many applications like motor, transformer, electromagnets etc. This demonstration show these concepts in an interesting and transparent manner.

### Procedure

The mother coil is constructed by giving a large number of turns of enamelled copper wire. The core of the coil is made from a bunch of bicycle spokes. A bell switch is given because coil should not be made on for a longer duration. The coil is usually connected to 220 V AC supply.

1. The first demonstration is to feel the magnetic field around the core when switch is made on. Take an iron nail and take it close to the core. Where is the magnetic field maximum and where it is minimum?
2. Put an aluminium ring over the coil. The ring jumps when switch is made on. Why?
3. Touch the aluminium ring with hand. Is it hot or cold? Now, put the ring over the coil and don't allow it to jump by pressing it with a wooden stick. Switch on the coil for few seconds. Touch the ring again. Is it hot or cold? Why?
4. Take an additional coil (secondary coil) with a torch bulb connected to it. Slide this coil over the bicycle spokes. What happened to the bulb? Why? Why does intensity varies with distance of this coil from the mother coil?
5. Take a cylindrical magnet and hold it in such a way that it is free to rotate. Bring this magnet close to the bottom of mother coil. The magnet will start rotating. Why?
6. We can ask students to measure the induced voltage in secondary coil with different number of turns in it. Also, how does induced voltage varies with separation between primary and secondary coils.

Extension: Connect a 100 $\Omega$ and 1000 $\Omega$ resistance in series and form a loop by using a connector of appropriate size. Place the loop on the mother coil. Use a multimeter to measure the AC voltage across one of the resistor. Now take multimeter to other side and measure the voltage again. Why there is a difference in measured voltage?

### Discussion

The house hold power supply is AC with rms voltage 220 V and frequency 50 Hz. Thus, current through the coil varies with time. The time varying current produces the time varying magnetic field.

Most of the demonstrations can be explained by Faraday's law of electromagnetic induction. In case of aluminium ring, the magnetic flux through the ring varies with time. This induces emf and hence current (called eddy current) in the ring. This, current carrying ring is placed in the magnetic field of the mother coil. The jumping of aluminium ring is caused by magnetic force acting on current carrying ring.

Heating of the ring is related to heat loss due to eddy current.

The lighting of bulb in secondary coil demonstrate the concept of transformer. The flux linkage between the primary and the secondary coils is more when two coils are close to each other.

The rotation of cylindrical magnet is related to the motors.

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