SAT Physics Subject Test
Chapter 10 Direct Current Circuits
In Chapter 8, we learned that an electrostatic field cannot be sustained within a conductor: The source charges move to the surface and the conductor forms a single equipotential. We will now look at conductors within which an electric field can be sustained because a power source maintains a potential difference across the conductor, allowing charges to continually move through it. This ordered motion of charge through a conductor is called electric current.
Picture a piece of metal wire. Within the metal, electrons are zooming around at speeds of about a million m/s in random directions, colliding with other electrons and positive ions in the lattice. This is charge in motion, but it isn’t a net movement of charge because the electrons move randomly. If there’s no net motion of charge, there’s no current. However, if we were to create a potential difference between the ends of the wire, meaning if we set up an electric field
the electrons would experience an electric force, and they would start to drift through the wire. This is current. Although the electric field would travel through the wire at nearly the speed of light, the electrons themselves would still have to make their way through a crowd of atoms and other free electrons, so their drift speed, vd, would be relatively slow: about a millimeter per second.
Remember that current
is in the direction that
positive charges would
move (i.e. opposite the
direction of electron flow).
To measure the current, we have to measure how much charge crosses a plane per unit time. If an amount of charge of magnitude ∆Q crosses an imaginary plane in a time interval ∆t, then the current is:
Because current is charge per unit time, it’s expressed in coulombs per second. One coulomb per second is an ampere (abbreviated A), or amp. So 1 C/s = 1 A.
Although the charge carriers that constitute the current within a metal are electrons, the direction of the current is the direction that positive charge carriers would move. So, if the conduction electrons drift to the right, we’d say the current points toward the left.