The University of Sydney
School of Physics
Motors and Generators
Electromagnetic Induction and Lenz’s Law
Using the Predict, Observe, Explain model of scientific enquiry, examine these 5
experiments to synthesise the information into a description of Lenz’s Law.
Magnet and coil
The wire coil is connected to a galvanometer that measures small amounts of current.
Move the magnet near the coil and notice the current induced.
What happens when you move the magnet faster? What happens when you use the
weaker (smaller) magnet?
When you bring the magnet’s north pole towards the coil, can you tell which way the
induced current flows in the coil? How? Explain using a diagram.
Predict
Observe
Explain
The University of Sydney
School of Physics
Motors and Generators
Falling magnets
You have a very strong magnet — a neodymium magnet — and some tubes. Drop the
magnet down the tubes and observe how long it takes for the magnet to reach the
bottom of each tube.
The magnet falls slower in the copper tube than in the PVC pipe. Why?
Predict
Observe
Explain
Jumping rings
Why do the rings jump into the air? Which ones don’t work and why?
Predict
Observe
Explain
The University of Sydney
School of Physics
Motors and Generators
Pendulum
Three different pendulums can be swung between the poles of the permanent magnet.
Try each one in turn and observe how long they swing before stopping. Where is the
stopping force coming from? How can this be used in electromagnetic braking?
Predict
Observe
Explain
Arago’s Disk
Turn the magnets past the disk. The disk will move too. What is the disk made of? Is it
magnetic? Why does the disk spin? (This phenomenon is used in AC induction motors).
Predict
Observe
Explain
Lenz’s Law
There are 4 parts to Lenz’s Law.
1. A _________________________________________ induces a
2. ___________________________________________ that produces an
3. ___________________________________________ force that
4. ___________________________________________ the original force.