JEE Main 2023 · 31 January · morning · Single correct · +4 / −1
Spherical insulating ball and a spherical metallic ball of same size and mass are dropped from the same height. Choose the correct statement out of the following
{Assume negligible air friction}
- (a) Metal ball will reach the earth's surface earlier than the insulating ball
- (b) Both will reach the earth's surface simultaneously.
- (c) Insulating ball will reach the earth's surface earlier than the metal ball
- (d) Time taken by them to reach the earth's surface will be independent of the properties of their materials
Answer and solution
Answer (c) Insulating ball will reach the earth's surface earlier than the metal ball
Solution
Physics sir · 4 steps
The metal ball falls through the Earth's magnetic field, and the eddy currents in it brake its fall.
- Same gravity. Both balls have the same mass and size, and air friction is ignored. So gravity alone would give both the same acceleration g and the same time to fall.
- Earth's magnetic field. The Earth has a magnetic field everywhere near its surface. A ball moving through this field has a changing magnetic flux through it.
- Eddy currents. In the metal ball, free electrons respond to this changing flux and set up eddy currents. By Lenz's law these currents oppose the motion, so the metal ball feels a small upward retarding force. The insulating ball has no free charges, so no current flows and no such force acts.
- So. The metal ball's downward acceleration is slightly less than g, while the insulating ball falls with full g. The insulating ball reaches the ground first, which is option (c).
The idea to keep. Whenever a conductor moves through a magnetic field, even the Earth's weak one, think of eddy currents. By Lenz's law they always oppose the motion, so the conductor is slowed down.
Answer from the NTA final answer key. Solution by PrepWiser.