JEE Main 2025 · 7 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): Magnetic monopoles do not exist.
Reason (R): Magnetic field lines are continuous and form closed loops.
In the light of the 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
No monopole means no starting point for a field line, so every line has to come back and close on itself.
- Check A. Cut a bar magnet in half and each piece is again a full magnet with both N and S. No experiment has ever found an isolated N or S pole, so (A) is correct.
- Check R. Magnetic field lines leave N outside the magnet, enter S, and continue from S to N inside it. They have no start or end, so (R) is correct.
∮ B⃗· dA⃗ = 0
- Link them. This is Gauss's law for magnetism: the net magnetic flux through any closed surface is zero. A lone pole would be a source where lines begin or a sink where they end, giving non-zero net flux. Since every line closes on itself, net flux is always zero and no isolated pole can exist, so (R) explains (A).
- So. Both statements are correct and (R) is the correct explanation of (A), which is option (a).
The idea to keep. Closed magnetic field lines and the absence of monopoles are two faces of one law, ∮ B⃗· dA⃗ = 0. Electric lines can start and end on charges; magnetic lines never can.
Answer from the NTA final answer key. Solution by PrepWiser.