Verified Solution • 2025–26 Session moderate

QPages 230-234: Q14: HC Verma: Concepts of Physics - Volume 2 Magnetic Field Pages 230-234: Q14 Solution: A rectangular wire-loop of width is suspended from the…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often assume both horizontal sides experience the magnetic force and overlook that the field is confined to the lower region, so only the lower side contributes; reversing the current flips the force direction, resulting in a net tension change of $2 i a B$, not just $i a B$.

A rectangular wire-loop of width

a

is suspended from the insulated pan of a spring balance, as shown in the figure. A current

i

exists in the anti-clockwise direction in the loop. A magnetic field

B

exists in the lower region. Find the change in the tension of the spring if the current in the loop is reversed.

Step-by-Step Verified Solution

**Answer:** Change in tension =

2 i a B

. **Step-by-Step Explanation:** Initially, the force on the lower side of the loop is

F = i a B

downward. When the current is reversed, the force becomes

i a B

upward. The change in force is

2 i a B

. Hence, the change in tension is

2 i a B

.

All Questions in this Chapter

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