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
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.