QJEEM19Pha045: JEE Main 2019 Physics - Class 12 JEEM19Pha045 Solution: A circular coil having N turns and radius r carries a current…

According to the question, the situation can be drawn as

Let the current I is flowing in anti-clockwise direction, then the magnetic moment of the coil is

m = NIA

where, N = number of turns in coil

and A = area of each coil = $$\pi$$r².

Its direction is perpendicular to the area of coil and is along Y-axis.

Then, torque on the current coil is

$$\tau =m\times B=mB\sin {90}^{\circ }=NIAB$$

$$=NI\pi r^{2}B(N-m)$$