Question

Topic: Magnetism and electromagnetic induction, faraday's law, lenz's law

Question: A magnet is held a little above the top of a coil. It is then dropped into and through the loop, north side facing down, and caught as it exits the loop. Assuming the magnet never touched the loop (zero friction), would its velocity after falling through the loop be greater, smaller, or the same as if it had been dropped freely (not through the loop)? Explain your answer using force concepts and using energy concepts.

Answer 1

Its speed would be smaller than that if it would had been dropped freely.

Explanation using Force concepts

As the magnet will start moving towards the coil flux passing through the coil will start increasing as a result a current would be induced in the coil which will oppose the movement of magnet towards the coil and thus magnet will be experiencing a force upwards and thus net downwards force will become lesser than force of gravity and resulting in lesser downward acceleration and hence smaller speed.

Similarly when the magnet will start leaving the coil the flux through the coil will start decreasing the direction of induced current will reverse and now the coil will oppose leaving of magnet and will start puling it up which again decreases net downward force and acceleration and hence the speed of the magnet will be slower in compared to when dropped freely.

Explanation using Energy concepts

During free fall of magnet the entire decrease in gravitational potential energy of the magnet is converted into Kinetic energy of the magnet but when it is dropped though the loop ,some part of decrease of Gravitational Potential energy is stored in the coils magnetic field = 0.5Li*i and remaining is converted into Kinetic energy of the magnet.

Thus Kinetic Energy of magnet falling through the coil is lesser than Kinetic Energy of magnet falling freely and thus speed of the magnet will be slower in compared to when dropped freely.