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Illustration 28.2: Forces Between Wires


Please wait for the animation to completely load.

The wire in the center has a fixed current. You can change the current in the blue wire by using the slider (position is given in meters, current is given in amperes, and magnetic field strength is given in tesla). The animation shows the magnetic field vectors (you can also double click on the screen to draw the field lines). Restart.

Keep the current in the blue wire at zero. In what direction is the magnetic field at the point where the blue wire is located? When the current is turned on in the blue wire, the current will either come out of the page (positive) or go into the page (negative). If you put positive current in the blue wire, in what direction would the force be on those moving charges (the current in the wire)? This is the Lorentz force on the charge carriers, so you'll need to use the right-hand rule you used in the previous chapter. Use the slider to put positive current through the blue wire. The vector shown is the force on the wire. Using the right-hand rule, the direction of the force is q v x B. The positive charges are moving out of the screen and the direction of the magnetic field is in the plane of the screen, and perpendicular to the line separating the currents. Using the right-hand rule gives you a direction toward the red wire.

Move the blue wire to a new position. The force points in a different direction at this position, but it still points toward the other wire. What happens if you increase the current? The force gets bigger. What happens if you make the current negative? Now the direction of the current changes and so will the direction of the force from the right-hand rule. The currents will now repel instead of attract.

Why is there a force vector on the center (red) wire? Well, this wire also experiences a magnetic field due to the blue wire. We get the force on the red wire to be equal and opposite to that of the force on the blue wire from the right-hand rule, but we could have just as easily predicted this result from Newton's third law.

Illustration authored by Anne J. Cox.

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