When a magnet is pushed into a coil whose ends are joined together, you feel a force against the…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
When a magnet is pushed into a coil whose ends are joined together, you feel a force against the push. When the ends are not joined, you feel almost none. Explain both, and say where the work you do goes.
Need a reminder? The key ideaShow
A current is induced in a coil while the magnetic field through it changes: when a magnet and the coil move relative to each other. It is larger for faster motion, a stronger magnet and more turns; it reverses when the motion reverses, and it opposes the change (Lenz's law). Fleming's right-hand rule: thumb for the motion, forefinger for the field, middle finger for the induced current.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Ends joined. A current is induced. It makes the coil a magnet whose near end repels the incoming magnet, so it pushes against you (Lenz's law). You must do work.
Step 2. Ends apart. There is no complete circuit, so no current flows, the coil is no magnet, and there is almost no force.
Step 3. The energy. The work you do becomes the electrical energy of the induced current, and then heat in the coil.
Answer: the induced current opposes the motion; with no current there is no opposing force; the work becomes electrical energy.
Check. If the induced current helped the push, the magnet would speed up by itself and make energy from nothing.
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