S4.13.5 For the adult
Electromagnetic induction
A circuit has no cell in it, only a coil and a meter. Yet the needle moves when a magnet is pushed into the coil. Where does the current come from?
What this chapter teaches
Moving a magnet in a coil, or changing the field through it, induces a voltage, and a current in a complete circuit. A still magnet gives none. The voltage is bigger for a faster change, a stronger magnet or more turns. By Lenz’s law the induced current opposes the change. This is the idea behind generators, transformers and microphones.
Three questions to ask
There is no one right wording. Listen for the idea.
- Why does the needle move only while the magnet is moving?
- What are three ways to make the induced voltage bigger?
- Why can you not get more energy out of induction than you put in?
After the chapter, the learner can
- Explain why moving a magnet in a coil makes a current
- Explain what makes the induced current bigger
What they copy into their notebook
A changing field through a coil induces a voltage. No change, no voltage. Bigger: faster, stronger magnet, more turns. Lenz: the current opposes the change. Right hand: Field, Current, Motion.