Passage V

For a demonstration on electric current, a teacher mounted 2 identical copper cables vertically and connected the cables to switch boxes as shown in Figure 1.

Each switch had 3 positions: X, Y, and OFF. The teacher flipped both switches from the OFF position to Position X, and the cables bent toward each other. Then, she flipped one switch to Position Y, and the cables bent away from each other. Finally, she flipped the other switch to Posi­tion Y, and the cables bent toward each other. See Figure 2.

The teacher asked each of 3 students to explain these observations.

Student 1

The switches controlled the direction of current flow through the cables: upward in Position X and downward in Position Y.

Each current generated an electric field. Because each current consisted of positive charges, its field pointed in the direction of current flow. Electric fields attract each other when parallel and repel each other when antiparallel (pointing in opposite directions). The field generated by one current attracted or repelled the field generated by the other current.

Student 2

The switches controlled the direction of current flow as Student 1 described: however, the currents consisted of negative charges.

Each current generated a magnetic field. A magnetic field exerts a force on any charge in motion, and this forces always perpendicular to the direction of the charges motion. When the currents were parallel, the field generated by one current attracted the charges in the other current. When the currents were antiparallel, the field generated by one current repelled the charges in the other current.

Student 3

The switches controlled the signs of charges in the currents: positive in Position X and negative in Position Y.

Regardless of whether they are at rest or in motion, charges of like sign attract each other and charges of opposite signs repel each other. The charges in one current attracted or repelled the charges in the other current.