Electromagnetism is often difficult for students to understand. The relationship between electricity and magnetism can be a complex and confusing subject, so we have put together some demonstrations to help students visualize and investigate it.
Faraday's Law
Faraday’s Law is the starting point to understand electromagnetism. Faraday's Law describes the interaction between moving magnets and coils. Moving magnets create forces on electrons, and moving electrons create magnetic fields. Another way of saying this is that a changing magnetic field through a coil induces a voltage in that coil. The faster the field changes, the larger the induced voltage.
Introduce the concept with a simple demo: a tube, copper collar, and neodymium magnet. Drop the magnet through the tube by itself to demonstrate it falls through with no issue. Then add the collar and ask “what do you think will happen when I drop the magnet through the copper collar?”
As the magnet passes through the collar, its moving magnetic field induces a current in the copper and some of its kinetic energy is transferred into electrical energy. Ultimately, this energy is transferred into heat. The magnet visibly slows down, and if you’re holding the collar, you can feel the change in this acceleration. This same effect,electromagnetic induction, is the operating principle of all electric generators and motors.
Now ask “what do you suppose happens if I flip the magnet?” Your students will see that nothing changes, but why is that?
Lenz's Law
Lenz's Law states that induced current flows in the direction that opposes the change in the magnetic flux inducing said current. The induced current's magnetic field is opposite to the falling magnetic field. Flip the magnet to show both the flux and the induced current reverse. That means the force on the magnet still opposes its own motion, and it still slows down.
You can repeat the demonstration a few times to really illustrate this concept. But wouldn't it be nice to see what's actually happening inside the copper collar? This is where the Faraday’s Law and Lenz’s Law demo set comes in. This apparatus helps students construct a conceptual understanding of Faraday's Law. This educator-developed tool consists of a plate with an exposed circuit that is designed to integrate a solenoid, so when a powerful magnet is dropped through the solenoid, the green led on the circuit board lights up to indicate the direction of induced current.
Using the Faraday's Law and Lenz's Law Demo Set
Start with only the green LED in the circuit. Because an LED passes current in one direction but not the other, a single flash tells students that the falling magnet pushed electrons one specific way around the solenoid. But suppose you repeat the experiment and this time reverse the poles on the magnet before dropping it.
The green led flashes again but why is that, since the current should only be going in one direction? Let's hang on to that question for a second, now let's include the red led into the circuit and repeat our experiment.
Now both LEDs are flashing but not at the same time . Flip the magnet again and see what happens. The LEDs are now flashing in the opposite direction. Each drop produces current in both directions: one way as the magnet approaches the solenoid and the flux through it increases, and the other way as the magnet exits and the flux decreases. Flipping the magnet doesn't eliminate a direction, it just swaps which one comes first.
To quantify what's happening inside the wires, you can connect a solenoid to a voltage probe and display a graph of voltage over time. Set the collection rate at 1000 times per second and a time interval of one second and drop the magnet. Repeat the drop and ask students to share their observations. You can see that the current flows first in one direction and then in the other. This is a really important confirmation of what students should be predicting from the flashing lights. If you reverse the magnet, it reverses the order of the LEDs and the peaks of the graph.
Not only is this a great way to demonstrate Faraday's Law but it also helps answer one of the most common questions in AP physics.
The Faraday's Law and Lenz's Law complete demo set comes with a plastic guide tube, a custom circuit plate, a thick copper collar, banana plugs, a giant neodymium magnet, an impact pillow to protect the magnet, and a 5 amp max air core solenoid. The ready to go construction and all-in-one nature of this set create a clear pathway for students to better understand a challenging topic.
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