Two long straight parallel current-carrying wires pass through the x-axis. Wire 1, carrying a current I, passes through the point x = -2 while wire 2 passes through the point x = +1. A third current-carrying wire, parallel to the other two, passes through the origin and feels no net force due to the other two wires.

In what direction is the current in wire 2?

1. The same direction as the current in wire 1
2. Opposite in direction to the current in wire 1

One way to look at it is that the third wire must be either repelled by both of the other wires or attracted to both, so the current must go the same way in wires 1 and 2. Another way to look at it is that the fields from wires 1 and 2 must cancel at the origin, and this can only happen in this situation if the currents in wires 1 and 2 go the same way.

What is the current in wire 2?

1. I/4
2. I/2
3. I
4. 2I
5. 4I
6. It depends on the current in wire 3

I/2. Field from a wire is inversely proportional to distance from the wire. Wire 2 is half the distance to the origin so the current must be half as large.

Two wires exert forces on a third wire, which carries a current out of the screen. The net force on the third wire is shown. In what direction is the current in each of the other two wires?

1. Out of the screen in both wires
2. Into the screen in both wires
3. Out of the screen in wire 1; into the screen in wire 2
4. Into the screen in wire 1; out of the screen in wire 2
5. Not enough information is given to answer this question

The third wire is repelled by wire 1 and attracted to wire 2. Wire 1 must have a current into the screen and wire 2 must have a current out of the screen.