Q.

A current carrying rectangular loop PQRS is made of uniform wire. The length PR=QS=5 cm and PQ=RS=100 cm. If the ammeter current reading changes from I to 2I, the ratio of magnetic forces per unit length on the wire PQ due to wire RS in the two cases respectively fPQI:fPQ2I is              [2023]

1 1 : 2  
2 1 : 4  
3 1 : 5  
4 1 : 3  

Ans.

(2)

Force between two current-carrying wires=μ0I1I22πd×L

Here I1 and I2 are equal

F=μ0I22πd×L

FI2

FIF2I=I24I2=14