EM.2(1) - THERMOELECTRIC PAIR

This thermo-electric couple is made of nickel and copper. The two parallel stripes are mounted such that a magnetic compass needle stands on a pin between them. The compass needle is free to rotate. The apparatus stands about 15 cm high and the stripes are about 15 cm in length.

Hold a Bunsen burner under one of the junctions. The difference in temperature between the two junctions will result in a thermal current through the couple. The compass needle will be affected by this current and its magnetic field, and the needle will deflect in a given direction.

Do not overdo with the heating since the heat will be quickly transmitted to the other side. To increase the effect, a lump of ice may be laid on the cool end.

EM.2(2) - FARADAY'S ELECTROMAGNETIC INDUCTION EXPERIMENT

An induction coil and lecture galvanometer are used in this demonstration. A bar magnet plunged into the coil produces an electric current in the coil, that is indicated on the galvanometer. When the magnet is removed, a current in the opposite direction is produced. The needle deflection on the galvanometer is clearly seen by the whole class.

The Faraday Experiment is now tried with a single loop instead of a coil of wire. The deflection in the galvanometer is much less in this case. It can also be done with an increasing number of loops to show its dependence on the number of loops.

EM.2(3) - INDUCTION RAILS

Two conducting rails are connected to a lecture galvanometer. The rails are placed around the magnetic field of a large horseshoe magnet. When one slides a conducting rod quickly along the rails, cutting the magnetic field, an emf is induced. The induced emf is indicated by a deflection of the needle in the galvanometer. Move the rod in the opposite direction and the galvanometer needle will deflect in the opposite direction. The induced current is in such a direction as to produce a magnetic flux that opposes the change in the magnetic field caused by sliding the conductor.

EM.2(4) - CURRENT COUPLED COILS

Two induction coils are connected by long wires and set far apart in the lecture room. Tall stands are arranged close to them so that the bar magnets on springs oscillate in them. When one magnet is set oscillating, the induced current causes the other to oscillate also.

EM.2(5) - INDUCED CURRENT - TWO COILS

One induction coil is connected to a lecture galvanometers, as in EM.2(2) and the other to a DC power supply and a switch. One coil is set atop the other but they are not connected. Turn the power supply on. When the switch is opened or closed, the current induced in the other coil will be indicated at the galvanometer.

An iron core set through both coils will enhance the effect.

EM.2(6) - JUMPING RING EXPERIMENT

An induction coil with an extra long iron core is supported vertically with part of the iron core pushed up. A solid metal ring is set around the iron core, over the coil. The induction coil is connected to an alternating power source. When the alternating current is applied to the coil, the metal ring is thrown upwards into the air. Try it with a split ring and nothing happens.

The current induced in the metal ring produces a magnetic field that opposes the field generated by the induction coil

Click here to download the video clip of this demo

EM.2(7) - THE SUBMERGED LAMP

The same set-up of EM.2(6) is used, but now with the iron core lowered so that a beaker can be placed on top of the coil. Inside the beaker, there is a small coil of wire with a small lamp in the middle. When an alternating emf is applied to the induction coil, the small lamp will light up. Fill the beaker with water, and it lights up again with an alternating emf is applied.

EM.2(8) - FORCE BETWEEN A COIL AND A MAGNET

A coil is suspended as a pendulum and set in front of a strong magnet attached to a small cart. When the magnet moves towards the coil, the coil is repelled. When the magnet moves away from the coil, the coil is attracted to it. Faster movements produce larger effects. The coil can be connected to the large lecture galvanometer and the induced current is indicated as well as its reversal of direction when the magnet moves in the different direction. The effect on the coil is very small and more apparent with the galvanometer.

TOP

EM.2(9) - FORCE BETWEEN TWO COILS

Two coils are suspended as pendulums and set about 1 cm apart. Here a current carrying coil is used instead of a permanent magnet. When a current starts to flow through one coil, a current is induced in the other and the coils repel each other. A lecture galvanometer can be connected to the second coil to show the induced current. Again the effect is small and more visible with the galvanometer.

EM.2(10) ELECTROMAGNETIC DAMPING

The induction coil is laid on its side and supported so that the iron core is horizontal. The iron core is allowed to protrude about half of its lenght from the coil. A support rod clamped to the incutcion coil's end plate holds a copper or aluminum ring over the magnet core by a cord. The ring hangs freely about the core. The coil is connected to a 6 V battery or a DC power supply through a switch. Close the switch rapidly. The ring will be suddenly propelled outward, and then it will swing slowly back to its vertical position without oscillation. Open the switch and the ring will first swing toward the coil, and then oscillate about its free position.

When the switch is closed, the current induced in the ring creates an opposing field, which, by interaction with the field produced by the current in the induction coil, retards the motion of the ring. The energy supplied by the motion of the ring is absorbed by the induced current in the ring, thus providing an excellent demonstration of electromagnetic damping. When the switch is opened the magnetic field is nearly non-existent and no damping occurs.

A hand-cranked generator wired to a light bulb is also a useful demonstration of Lenz's law as it can be verified by a volunteer that it is much easier to turn when there is no load in the circuit, i.e. when the light bulb is disconnected.