JEE Mains Model Paper 22

Initialy, the rod will be in equilibrium if 2T0 = Mg and T0 = kx0
Now, when the current I is passed through the rod it will experience a force F = BILvertically up. In equilibrium 2T + BIL = Mg with T = kx ... (ii)
So from eq (i) and (ii)

T

T0

=

Mg−BIL

Mg

i.e

x

x0

= 1 -

BIL

Mg

or
B =

Mg(x0−x)

ILx0

=

10×10−3×10×3×10−2

20×25×10−2×4×10−2

= 1.5 × 10−2 T