Two parallel current-carrying wires will exert forces on one another.

Let's say wire 1 carries a current I₁ out of the page. Wire 2, a distance d to the right of wire 1, carries a current I₂ out of the page. Assume that both wires are long.

Let's assume that I₁ is a lot larger than I₂. Which wire exerts more force on the other?

1. Wire 1 exerts more force on wire 2 than wire 2 exerts on wire 1
2. Wire 2 exerts more force on wire 1 than wire 1 exerts on wire 2
3. Wire 1 exerts the same magnitude force on wire 2 as wire 2 exerts on wire 1

If the two wires carry currents in the same direction, what is the direction of the force that wire 1 exerts on wire 2?

1. left
2. right
3. up
4. down
5. into the page
6. out of the page

Using the infinite wire equation, wire 1 sets up a magnetic field that wire 2 experiences. The magnitude of this field, at wire 2's location, is:

B1

=

mo I1 2pd

To find the force on wire 2, use:

F = I₂L ´ B₁

We don't have a length to use for wire 2, but at least we can get the force per unit length:

F L

=

I2 B1

=

mo I1 I2 2pd

By the right-hand rule, a current out of the page in a field up gives a force to the left. Two parallel wires carrying currents in the same direction attract one another; wires with currents going in opposite directions repel.