Introduction to SAT II Physics - FreeExamPapers

Voltage
The batteries we use in flashlights and clock radios operate on chemical energy. This
chemical energy—which you may learn more about in chemistry class—separates charges,
creating a potential difference. To separate charges and create a positive and negative
terminal, the battery must do a certain amount of work on the charges. This work per unit
charge is called the voltage, V, or electromotive force, emf, and is measured in volts
(V). Remember, one volt is equal to one joule per coulomb.
You’ll notice that voltage is measured in the same units as potential difference. That’s
because they are essentially the same thing. The voltage of a battery is a measure of the
work that has been done to set up a potential difference between the two terminals. We
could draw an analogy to the amount of work required to lift an object in the air, giving it
a certain amount of gravitational potential energy: both work and gravitational potential
energy are measured in joules, and the amount of work done on the object is exactly equal
to the amount of gravitational potential energy it acquires.
When a current flows about a circuit, we say there is a certain “voltage drop” or “drop in
potential” across the circuit. An electric current converts potential energy into work: the
electric field in the circuit does work on the charges to bring them to a point of lower
potential. In a circuit connected to a 30 V battery, the current must drop 30 volts to send
the electrons from the negative terminal to the positive terminal.
Current
When a wire is connected between the terminals of a battery, the potential difference in
the battery creates an electric field in the wire. The electrons at the negative terminal
move through the wire to the positive terminal.
Although the electrons in the wire move quickly, they go in random directions and collide
with other electrons and the positive charges in the wire. Each electron moves toward the
positive terminal at a speed
, called the drift speed, which is only about one
millimeter per second. However, when we study circuits, we do not follow individual
electrons as they move along the wire, but rather we look at the current, I, that they
create. Current is the charge per unit time across an imaginary plane in the wire:
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