Union University
Union University Department of Physics

The Science Guys

Science Guys > November 2001

November 2001

The lights turn on very quickly when I flip the switch. Just how fast does electricity flow in a wire?

To answer this question we need to look at matter itself at a most basic level. Matter is made up of small units called atoms. At this atomic level matter possesses two basic characteristics. Matter has mass and it may have an electrical charge, either positive, negative, or it could be neutral with no charge. Each atom contains three types of particles with different characteristics; positive protons, neutral neutrons, and negative electrons.

Electric current (electricity) is a flow or movement of electrical charge. The electricity that is conducted through copper wires in your home consists of moving electrons. The protons and neutrons of the copper atoms do not move. The actual progression of the individual electrons in a given direction through the wire is quite slow. The electrons have to work their way through the billions of atoms in the wire and this takes considerable time. In the case of a 12 gauge copper wire carrying 10 amperes of current (typical of home wiring), the individual electrons only move about 0.02 cm per sec or 1.2 inches per minute (in science this is called the drift velocity of the electrons.). If this is the situation in nature, why do the lights come on so quickly? At this speed it would take the electrons hours to get to the lights.

Atoms are very tiny, less than a billionth of a meter in diameter. The wire is "full" of atoms and free electrons and the electrons move among the atoms. In a typical copper wire there would be trillions of electrons flowing past any given point in the wire every second, but they would be passing that point very slowly. Think of the wire in comparison to a pipe full of marbles. If we push another marble into a filled pipe, then one marble would have to exit the other end. Electrons are like that in a wire. If one moves they all have to move. Thus when you turn on a switch an electrical potential difference (created by a generator) immediately causes a force that tries to move the electrons. If you make one electron move when you turn on a switch, the electrons throughout the wire move, even if the wire is miles long. Therefore when you turn on a switch, the electrons in the light start moving "instantly" as far as we are concerned, i.e. something starts to happen throughout the electrical system. Although the electrons are actually moving through the wire slowly, we say that the speed of electricity is near the speed of light (extremely fast). What we really mean is that the effects from the electricity occur "instantly." The light comes on the instant you flip a switch. You do not have to wait for electrons to flow from the switch to the light.