### The rail gun uses electromagnetic induction to launch a metal ring into the air.

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Teachable Topics:

• Magnetic fields
• Induced current
• Circuits

Theory:

In this demo, an aluminum ring is placed on a post that can create a magnetic field at the flip of a switch. If the magnetic field through the ring is varied, an induced current flows. This induced current creates its own magnetic field. The direction of this induced magnetic field is given by Lenz's Law - induced magnetic fields are directed to oppose changes in magnetic flux through the ring. As an example, consider the figure below. Magnetic flux increasing up through the aluminum ring induces a magnetic field that points downward through the ring

When the switch is flipped, the suddenly-increasing magnetic field induces current to flow in the ring. The current creates an magnetic field opposing the original field. The field from the post and the field from the induced current in the ring repel each other just as north poles of two bar magnets would. It is this strong repulsion that launches the ring into the air.

 Figure 1: Lenz's Law of induced magnetic field in a ring

If the first aluminum ring is replaced by another one that has a split completely down one side, then can a steady current be induced in the ring? No, because the ring doesn't form a complete circuit. That means that when this ring is placed on the launcher and the magnetic field through it is increased, no current can be induced in the ring. Therefore, the ring can't produce its own magnetic field. No induced magnetic field ... no repulsion, and the ring never leaves the launcher.

 Figure 2: No current or magnetic field is induced in a split ring

Apparatus:

• Ring launcher
• Power source
• One complete aluminum ring
• One split aluminum ring

Procedure:

• Place the unbroken ring on the launching post and press the button on the launcher. The ring should fly into thr air.
• Place the split ring on the launching post and press the button. You should see that the ring doesn't move at all.
• Here's a neat trick: Hold a coin near the machine as you press the button... the strong magnetic field created will shake the coin in your hand.

SAFETY WARNINGS:

• Because the ring leaves the launcher at high speed, allow about seven feet of clearance above the launcher so that the ring doesn't hit the ceiling. Also, take care to keep your face out of the path of the ring and be prepared to catch it on the way back down.
• Press the switch and then release it immediately; holding it down can cause the device to over-heat.