The force per unit of length between two wires carrying current is
[tex] \frac{F}{L}= \frac{\mu_0 I_1 I_2}{2 \pi r} [/tex]
where I1 and I2 are the currents in the two wires, while r is the distance between them.
We can see from the formula that the force is proportional to the product between I1 and I2: [tex]F \sim I_1 I_2[/tex]
so, if we double both I1 and I2, we get a factor 4:
[tex]F' \sim (2I_1 )(2I_2)=4 I_1 I_2 =4 F[/tex]
so, the force between the wires will be 4 times the original value.
