Lesson: Chapter - 14

Current

When a wire is connected between the terminals of a battery, the potential difference in the battery creates an electric field in the wire. The electrons at the negative terminal move through the wire to the positive terminal.

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Although the electrons in the wire move quickly, they go in random directions and collide with other electrons and the positive charges in the wire. Each electron moves toward the positive terminal at a speed V_d, called the drift speed, which is only about one millimeter per second. However, when we study circuits, we do not follow individual electrons as they move along the wire, but rather we look at the current, I, that they create. Current is the charge per unit time across an imaginary plane in the wire:

I = Q/t

The unit of current is the coulomb per second, which is called an ampere (A): 1 A = 1 C/s.

Direction of Current

Although the electrons are the charge carriers and move from the negative terminal to the positive terminal of the battery, the current flows in the opposite direction, from the positive terminal to the negative terminal. This may seem odd, but we can draw an analogous example from everyday life. Suppose you arrange 12 chairs in a circle, and get 11 people to sit down, leaving one chair empty. If each person in turn were to shift over in the clockwise direction to fill the vacant spot, the vacant spot would appear to move in the counterclockwise direction. If we think of the electrons in a circuit as the people, then the current moves in the direction of the vacant spot.

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