Forces on Wires



A cross section of three wires carrying unknown currents is shown above. You can click-drag the wires and the black arrow represents the force experienced by each wire. 


How many wires have a current that points out of the page?     


Instructor Resources

Reference: See Giancoli-PA: 20-6, Giancoli-SE: 28-2.
Answer: Cannot determine, not enough information. There are three wires, so how to begin? Students first need to realize that a systematic approach will serve them well in this exercise. Consider two wires at a time with the third as far from the two as possible. Given that like currents (currents pointing in the same direction) attract and opposite currents repel, students can figure out that two of the wires have currents pointing in the same direction. However, this is not enough to determine whether these currents are into or out of the page.
Script Author: Mario Belloni