Conceptual Physics

28.21 - Two wires and their magnetic fields

To illustrate the nature of the fields and forces produced by currents, we use the

example of two wires placed parallel to one other and conducting currents flowing in the

same direction, as illustrated in Concept 1. The two wires attract each other.

Let’s determine why they attract each other by first considering the orientation of the

magnetic field created by the wire on the bottom. We use the right-hand rule for

currents to determine this orientation. This is shown in Concept 1.

Next, we determine the direction of the force exerted by the magnetic field on the

electrons moving in the upper wire. The direction of electron movement is shown with

velocity vectors in Concept 2. The right-hand rule for a charge moving in a magnetic

field is used to determine the direction of the magnetic force on the electron. To apply

the rule, point your fingers in the direction of the velocity vector and wrap them in the

direction of the magnetic field. As the diagram shows, when it intersects the upper wire,

the field is pointing away from you. Wrapping your fingers from the velocity vector to the

field vector will cause your thumb to point up. Since an electron is a negative charge,

your thumb will point in the direction opposite to the magnetic force on it. The result is

that the force points down, toward the lower wire.

The right-hand rule for force can be used again to determine the force on the moving

charges in the lower wire. If you apply it, you can confirm that they experience an

upward force. This analysis explains why the two wires attract each other. We show the

forces acting on both wires in Concept 3.

If the currents flow in opposite directions, the two wires repel each other. This scenario

is illustrated in Concept 4. Here, the current in the bottom wire now flows in the opposite

direction than in the prior discussion, which means its field has the opposite orientation.

The force on the electrons in the upper wire now pushes them and the wire that

contains them up, as the diagram reflects. The field of the upper wire pushes the

electrons in the lower wire down. The two wires repel each other.

Currents and their magnetic

fields

Each current creates a magnetic field

·Field intersects other wire

Currents in same direction

Field exerts force on moving charges

Currents in same direction

The wires attract

Currents in opposite directions

The wires repel

(^524) Copyright 2000-2007 Kinetic Books Co. Chapter 28