Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
A bar magnet is pushed into a closed coil of resistance at a steady speed. The induced current is A, and it lasts for s while the magnet moves cm. Find the heat produced in the coil. What is the least work that the hand does? What is the average force that the hand must apply against the coil? What would the force be if the coil were open?
Lenz's law: the induced current has the direction whose own field opposes the change of flux. If the flux grows, the induced field is against the original field. If it falls, the induced field is along it. The heat in a closed circuit is , and it comes from the work that is done against the opposing force.
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. The heat. , that is mJ.
Step 2. The work. The magnet moves at a steady speed, so its kinetic energy does not change. The energy in the coil must come from the work of the hand. So mJ at least.
Step 3. The force. , so N.
Step 4. The open coil. An open coil has an emf but no current. There is no induced field and no force, so the hand needs no extra force and does no extra work.
Answer: mJ, N, and no force for an open coil.
Check. The force opposes the motion, as Lenz's law says. If it helped the motion, the energy balance would give energy from nothing.
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