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.