Current and Resistance

Electric Current

 

Consider charges moving through a conductor -- such as a wire.

If the charges that are moving are positive, we will describe this as a current I moving in the same direction as the moving charges.

If the charges that are moving are negative, we will describe this as a current I moving in the opposite direction of the moving charges.

Current is in the same direction as positive charge movement and in the opposite direction of negative charge movement. Electric current in a common wire is done by movement of (negative) electrons. In semiconductors, electric current is often done by movement of "holes" or electron vacancies that are positively charged. In an electrolytic solution, electric current is carried out by movement of both positive and negative ions.

How fast are these charges moving?

Q = (n A L) q

Q = (n A v_d t) q

I = Q/t = n q v_d A

n is enormous so the "drift velocity" v_d will be surprisingly small.

Homework Assignment			Resistance
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(c) Doug Davis, 2002; all rights reserved