JEE Main 2023 · 8 April · evening · Single correct · +4 / −1
Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R
Assertion A : Electromagnets are made of soft iron.
Reason R : Soft iron has high permeability and low retentivity.
In the light of above, statements, choose the most appropriate answer from the options given below.
- (a) Both A and R are correct and R is the correct explanation of A
- (b) A is not correct but R is correct
- (c) A is correct but R is not correct
- (d) Both A and R are correct but R is NOT the correct explanation of A
Answer and solution
Answer (a) Both A and R are correct and R is the correct explanation of A
Solution
Physics sir · 4 steps
An electromagnet must switch on strongly and switch off cleanly, and soft iron does both.
- What it needs. An electromagnet should become a strong magnet when current flows. It should lose that magnetism almost completely when the current stops.
- High permeability. Soft iron has a very high permeability, so a small current in the coil produces a large magnetic field B = μ H in the core. That gives the strong pull when the magnet is on.
- Low retentivity. Soft iron also has low retentivity and low coercivity, so its hysteresis loop is narrow. When the current is cut, very little magnetism stays behind, and the magnet really switches off.
- So. A is correct, R is correct, and these two properties are exactly the reason soft iron is chosen. The answer is (a).
The idea to keep. Match the magnet to the job: temporary magnets need high permeability and low retentivity (soft iron), while permanent magnets need high retentivity and high coercivity (steel).
Answer from the NTA final answer key. Solution by PrepWiser.