Q.

A 2 amp current is flowing through two different small circular copper coils having radii ratio 1:2. The ratio of their respective magnetic moments will be    [2025]

1 2 : 1  
2 4 : 1  
3 1 : 4  
4 1 : 2  

Ans.

(3)

Magnetic moment, M=IA

  M1M2=I×πr12I×πr22=r12r22  or  M1M2=(12)2=14