Demo overview:
The jumping ring demonstration is a classic physics experiment that shows electromagnetic induction and Lenz’s Law in action. It usually involves a metal ring (often aluminum) and a solenoid (a coil of wire with a core) connected to an AC power source. In the experiment, a metal ring is placed over a vertical solenoid. When a high-voltage AC current is switched on in the coil, it creates a strong alternating magnetic field. This changing magnetic field induces an electric current in the metal ring due to Faraday’s Law of Induction. According to Lenz’s Law, the induced current creates its own magnetic field in the opposite direction to oppose the change that caused it (in this case, the magnetic field of the solenoid). The magnetic field generated by the ring pushes against the solenoid’s magnetic field, creating a repulsive force. If the current is strong enough, this force can lift the ring off the solenoid, causing it to jump.
Instructor Notes:
- Before starting with the experiment, explain the setup and have kids vote on whether they think the ring will jump, move to the side, or stay still. They’ll be more engaged when they’re invested in the outcome!
- Make sure no one is too close to the coil, as the ring can jump up quickly.
- Always turn off the power when not actively demonstrating.
- If using liquid nitrogen for cooling, only adults should handle it, with kids kept at a safe distance.
Materials needed for the demo:
- Iron-core solenoid with push-button
- 2 Aluminum rings: 1 complete ring, and 1 has a slit
- Liquid Nitrogen
- Container, wide enough to fit the rings
- Optional, but recommended: Styrofoam, or any material, to put the cool container on top
- Safety gear: thick gloves, tongs, goggles
Steps to run the demonstration:
- Place the two aluminum rings around the iron core, one at a time. Plug the apparatus in and press the button to send current through the coil. The solid ring will eject into the air, while the slitted ring will remain unaffected.
- Pour the liquid nitrogen into the container and put the two rings inside. Allow the rings to cool for a few minutes. You may use this time to discuss with students about what is going on and gather guesses on the second round of the experiment.
- Now, perform the experiment again. Note, this time the complete ring is going to jump higher, so make sure the students are standing at a safe distance away from the experiment.
Tips to interact with kids:
Lenz’s law can be tricky to explain to kids especially that it is very unintuitive and includes a few concepts that kids may not be able to comprehend very well. For this reason, you might need to find analogies to assist in helping them visualize the process. Here are some ideas that may help:
- Use relatable analogies like, “Imagine there’s an invisible force giving the ring a strong push up!” Have them try to guess what the “invisible pusher” is (the magnetic field) and how it makes things move without touching them.
- Describe how electricity can create an invisible force field. You might say, “When the coil is turned on, it’s like creating a small ‘magnet’ inside, but this magnet doesn’t want the ring to be near it, so it pushes the ring away.”
- Explain Lenz’s Law as the ring’s way of “saying no” to the magnetic field. Describe how, just like when someone tries to push you, you can push back; the ring pushes back onto the coil’s field, which makes it jump away.