# Current

We will explore the concept of current, the movement of charge.

Whenever charges of a like sign move, we say there is a current. This is in many ways like a water current. We define current as the rate that charge passes through a given area, (a wire for instance). The units of current are known as amperes (A) and they are equivalent to 1 coulomb per second. By convention, we give current the same direction (+ or -) as the flow of positive charge. Ordinarily, the direction of the flow of current is opposite the flow of electrons. Although we now realize that it is the electron flow that we should have used, it doesn't really matter, mathematically, it works out either way. Besides, we've been using the positive-based system for over two hundred years, and old habits die hard.

You probably know that the speed of light is incredibly fast. You may be wondering: "When I turn on my lights, how do all those little electrons make it to the end of the wire that quickly? Don't they bump into things?" The answer to this question is yes, they do bump into things. Quite often, actually. They zigzag wildly, smashing into the atoms in the conductor. This in turn raises the temperature of the conductor, as some of the energy of the electrons is converted into thermal energy. But back to your light switch, imagine the wires in your house are tubes, filled with electrons. (They are, by the way). When you push more electrons in one end, some fall out the other end. Although any given eletron may not move that far when you put it in the end closest to you, the electrons on the end will be pushed out long before you could blink.

We use a quantity known as drift speed when we want to know the actual average velocity of the electrons as they catapult themselves down that tube. Drift speed is given by the equation:

where Vd is drift speed in meters per second, I is current in amperes, n is the number of mobile charge carriers per unit volume, q is the charge on each carrier, and A is the area of a cross-section of the tube.