What happens when the two ends of a conductor are held at different values of voltage? That is, what happens when a voltage difference is applied across a resistor, such as when a battery is connected across a thin wire?
Charges experience an electric field and move accordingly. In a typical metal, each atom in the lattice will allow one of its electrons to move freely. (See Periodic Table.) So the metal can be thought of as a rigid lattice of positively-charged ion cores surrounded by a swarm of free electrons.
Do the electrons accelerate indefinitely because of the applied voltage difference? NO, on average they reach a terminal speed, much as a skydiver reaches a terminal velocity of about 120 mph falling through air, a viscous medium. The electrons tend to drift slowly through the conductor, on the order of 0.01 mm/s. The "viscosity" that the electrons experience is called resistance. Many conductors are referred to simply as resistors.
But what about the speed of signal propagation? Many of you have heard that electronic signals propagate near the speed of light. For example, the bits coming to this computer via the Ethernet travel at about 0.8 times the speed of light. We will examine this more fully in the laboratory in a few weeks.
For the time being you may think of the "water in hose" analogy. When the valve to a garden hose is opened, water "immediately" leaves the end of the hose. However the water actually passing through the valve takes much longer to reach the end of the hose. Here the pressure wave that propagates at the speed of sound in water would represent the electronic signal.
Also please keep in mind that although electrons are flowing through the conductor, there is no charge accumulation anywhere. The electrons flow in a continuous loop, known as a circuit. The electrons are continuously flowing past the positively-charged ion cores, with no change in the net charge anywhere. See animation below:
So the result of a voltage difference across a resistor is that electric current flows. Precisely, current is the instantaneous rate at which charge passes through a cross-section of the conductor.
We will start with the study of steady currents, the so-called direct current, or dc for short. In two weeks, we will study alternating (or ac) circuits, followed by study of RC circuits and their decaying transient currents later in the semester.
Last updated Sept. 2, 1998.
Copyright George Watson, Univ. of Delaware, 1998.