It’s time to switch our attention from how magnetic fields are generated to how magnetic fields exert forces and torques. Oersted discovered that a current passing through a wire causes a magnetic torque to be exerted on a nearby compass needle. Upon hearing of Oersted’s discovery, the French scientist André-Marie Ampère, for whom the SI unit of current is named, reasoned that the current was acting like a magnet and, if this were true, that two current-carrying wires should exert magnetic forces on each other.

To find out, Ampère set up two parallel wires that could carry large currents either in the same direction or in opposite (or “antiparallel”) directions. FIGURE 29.33 shows the outcome of his experiment. Notice that, for currents, “likes” attract and “opposites” repel. This is the opposite of what would have happened had the wires been charged and thus exerting electric forces on each other. Ampère’s experiment showed that a magnetic field exerts a force on a current.

FIGURE 29.33 The forces between parallel current-carrying wires.

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