Question SPM.1A.SL.TZ0.17

Two long parallel wires P and Q are a distance d apart. They each carry a current.

A magnetic force per unit length $F$ acts on P due to Q.

The distance between the wires is increased to 2d and the current in Q is decreased to $\frac{I}{2}$.

What is the magnetic force per unit length that acts on P due to Q after the changes?

A.  $\frac{F}{8}$

B.  $\frac{F}{4}$

C.  $\frac{F}{2}$

D.  $F$