JEE Main 2023 · 8 April · morning · Single correct · +4 / −1
Certain galvanometers have a fixed core made of non magnetic metallic material. The function of this metallic material is
- (a) to bring the coil to rest quickly
- (b) to produce large deflecting torque on the coil
- (c) to oscillate the coil in magnetic field for longer period of time
- (d) to make the magnetic field radial
Answer and solution
Answer (a) to bring the coil to rest quickly
Solution
Physics sir · 4 steps
Moving metal in a magnetic field means eddy currents, and eddy currents always brake the motion.
- Core moves in field. The coil is wound on this metallic core, so the core turns with the coil inside the magnet's field. As it turns, the magnetic flux through the metal keeps changing.
- Eddy currents appear. A changing flux through a conductor induces circulating currents in it, called eddy currents. By Lenz's law these currents oppose the motion that causes them, so they act like a brake on the coil.
- Why non-magnetic. The material only needs to conduct, so that eddy currents can flow. Being non-magnetic, it does not change the field or the torque NIAB. It only adds damping, so the pointer settles quickly instead of swinging back and forth. Such a galvanometer is called dead-beat.
- So. The non-magnetic metallic core is there for electromagnetic damping: it brings the coil to rest quickly, which is option (a).
The idea to keep. A conducting but non-magnetic frame in a galvanometer is for eddy current damping. A soft iron core is for a strong, radial field. Do not mix up the two jobs.
Answer from the NTA final answer key. Solution by PrepWiser.