JEE Main 2024 · 9 April · evening · Numerical answer · +4 / −1
A straight magnetic strip has a magnetic moment of 44 Am². If the strip is bent in a semicircular shape, its magnetic moment will be ________ Am².
(given π=22/7)
Answer and solution
Answer 28
Solution
Physics sir · 4 steps
Bending does not change the pole strength. Only the straight-line gap between the poles shrinks.
- Straight strip. With pole strength m and length L, the moment is M = mL = 44 A m².
- Bend it. The strip becomes a semicircular arc, so L = π r and r = L/π. The poles now sit at the two ends of a diameter, so they are 2r = 2L/π apart.
M' = m × 2L/π = 2M/π
- Put in numbers. M' = (2 × 44)/(22/7) = (88 × 7)/22 = 28 A m².
- So. The magnetic moment of the semicircular strip is 28 A m².
The common slip: 14. Taking the pole separation as the radius L/π instead of the diameter 2L/π gives 14, not 28.
The idea to keep. When a magnet is bent, keep m the same and use the straight-line distance between its two ends as the new length. Arc length does not count.
Answer from the NTA final answer key. Solution by PrepWiser.