Parallel Currents Attract

While we are in the business of discussing mnemonic rules, there is another that makes it easy to remember whether a charge moving parallel to a current is attracted or repelled. In Figure (1) we had a beam of negative electrons moving parallel to a negative current -i, and the electrons were attracted to the current. In Figure (2) the current was reversed and the electrons were repelled. One can work out a thought experiment similar to the ones we have done in this chapter to show that a positive charge moving parallel to a positive current as shown in Figure (3) is attracted.

Figure 1c: For an experimental test of the results of the thought experiment, we replace the moving negative charge with a beam of electrons in an electron gun. The electrons are attracted to the wire as predicted.
The simple, yet general rule is that parallel currents attract, opposite currents repel. A positive charge moving in the direction of a positive current, or a negative charge moving along with a negative current are attracting parallel currents. When we have negative charges moving opposite to a negative current as in Figure (2) we have an example of opposite currents that repel.

Figure 2: If we reverse the direction of the current in the wire, the electrons in the beam are repelled.

Figure 3: A positive charge, moving parallel to a positive current, is attracted by the current. Thinking of the moving positive charge as a positive upward directed current, we have the rule that parallel currents attract, opposite currents repel.