Question

As shown in the figure two very long straight wires are kept parallel to each other and 2A current is passed through them in the same direction. In this condition the force between them is F. Now if the current in both of them is made 1 A and directions are reversed in both, then the force between them ______ .
(A) will be F/4 and attractive
(B) will be F/2 and repulsive
(C) will be F/2 and attractive
(D) will be F/4 and repulsive

Answer 1

use formula , $F=\frac{\mu_0l}{2\pi}\frac{i_1i_2}{d}$

Here , i1 and i2 are current through wires.
d is the distance between two parallel wires.
l is the Length of wire .

hence, $F\propto i_1i_2$
so, we can write $\frac{F_1}{F_2}=\frac{i_1i_2}{i_1'.i_2'}$

$\implies \frac{F}{F_2}=\frac{2A.2A}{1A.1A}$

$F_2=\frac{F}{4}$

also we know , two current carrying conductors attract each other when the current is in same direction and repel each other when the current are in opposite direction.
here current through two parallel wires is in same direction so, force is attractive.

hence, option (A) is correct.

Answer 2

Here as shown in attachment proceed.

Use force formula for wires

and observe it.