Electromagnetic Induction

• EM5.1 Four Turn Rectangular Coil

This demonstration shows the reality of theoretical calculations. A four turn rectangular coil attached to an ammeter is inserted slowly into the gap of an electromagnet. The dependence of induction on area and speed inserted can be shown. Knowing the strength of the field, the number of turns, and the area, one can calculate the voltage that should be created. Then perform the demonstration and prepare the meter's answer with the calculated one.

• EM5.2 Induction Coils

An inductor stores current. Three inductors in series are in parallel with a light bulb. As the switch is on, the inductors store current and the light is on. The current through the inductors is greater than that through the lamp. As the light is turned off, the current stored in the inductors is released and flows through the light only. Thus, when the power is turned off, the bulb flashes brightly for a split-second and then shuts off, after the inductors have discharged their current.

• EM5.3 Lenz's Law Tube

Drop a mass through the 1.5 meter tube. It takes about half a second to drop. Then drop a magnet with an identical mass. It takes over 10 times as long to fall.

• EM5.4 Coil, Magnet, and Demo Galvanometer

Simple, but effective. Two ends of a coil are connected to a demo galvanometer (Shelf C3-3), and a bar magnet is moved in and out of the coil, and varying speeds. One can pass a wooden ruler through the coil in the same way to show that there's no deflection with non-magnetic materials.

• EM5.5 Pendulums in Gap of Electromagnet

A large electromagnet is connected to a power supply. With the electromagnet off, a pendulum consisting of a circular plate will simply oscillate through the gap. When the magnet is turned on, the plate will stop once it enters the gap. Variations have plates of different material-conducting and non-conducting-and one plate that has cuts in the vertical direction. This last plate will oscillate through the gap with the magnet on due to the break of current in the plate. A plate with circular cuts does not.

• EM5.6 Transformer Demonstration (Jumping Rings)

This consists of metal rings lying in a solenoid. A metal bar is placed in the center of the rings to extend the field. When current is applied to the solenoid, an EMF is induced in the rings in such a way as to oppose the created B field. This causes the rings to be thrown vertically upward. One of the rings has a gap, so that no EMF is induced. Thus, it does not jump off. Variations include using a series of rings with and without gaps.

• EM5.7 Transformers

A large coil is wrapped by five, ten, and twenty turns of wire. Putting a known current through the large coil, the voltage in the secondary coil can be measured and compared with theoretical predictions.

• EM5.8 Ring Launcher

In this classic demonstration of electromagnetic induction, an aluminum ring is propelled straight up a maximum distance of 2 meters. The changing magnetic field from the AC powered coil causes a changing magnetic flux through the aluminum ring. The induced EMF in the ring sets up a current which produces a magnetic field. The induced magnetic field opposes the field of the coil, pushing the ring up. Accessories include a coil with a bulb that lights by induction when the coil is placed over the launcher core. Also includes three additional rings: one split aluminum ring that will not launch, one copper ring to show the effect of changing materials and one shorter aluminum ring with higher resistance to show that it will not go as high as because of decreased induced current.

• EM5.9 Hand Crank Generator

This generator produces up to 12 Volts with a sturdy handle, designed to be cranked by hand. When used to power light bulbs, you can feel the difference in effort required for lighting one bulb, two bulbs (series or parallel), or no load. (Shelf C1-3).

• EM5.10 Primary and Secondary Coils

The secondary coil slides over the primary coil, and the soft iron core slides into either or both, providing a look at magnetic induction and transformer theory. This rugged device is sensitive enough to be used with voltmeters instead of galvanometers. The coils are wound around hollow wooden cores, with a turns ratio of approximately 12 to 1. The primary coil is mounted on a wooden stand.