Voltage Definition in Physics

Voltage is a representation of the electric potential energy per unit charge. If a unit of electrical charge were placed in a location, the voltage indicates the potential energy of it at that point. In other words, it is a measurement of the energy contained within an electric field, or an electric circuit, at a given point. It is equal to the work that would have to be done per unit charge against the electric field to move the charge from one point to another.

Voltage is a scalar quantity; it does not have direction. Ohm's Law says voltage equals current times resistance.

Units of Voltage

The SI unit of voltage is the volt, such that 1 volt = 1 joule/coulomb. It is represented by V. The volt is named after Italian physicist Alessandro Volta who invented a chemical battery.

This means that one coulomb of charge will gain one joule of potential energy when it is moved between two locations where the electric potential difference is one volt. For a voltage of 12 between two locations, one coulomb of charge will gain 12 joules of potential energy.

A six-volt battery has a potential for one coulomb of charge to gain six joules of potential energy between two locations. A nine-volt battery has a potential for one coulomb of charge to gain nine joules of potential energy.

How Voltage Works

A more concrete example of voltage from real life is a water tank with a hose extending from the bottom. Water in the tank represents stored charge. It takes work to fill the tank with water. This creates a store of water, as separating charge does in a battery. The more water in the tank, the more pressure there is and the water can exit through the hose with more energy. If there were less water in the tank, it would exit with less energy.

This pressure potential is equivalent to voltage. The more water in the tank, the more pressure. The more charge stored in a battery, the more voltage.

When you open the hose, the current of water then flows. The pressure in the tank determines how fast it flows out of the hose. Electrical current is measured in Amperes or Amps. The more volts you have, the more amps for the current, same as the more water pressure you have, the faster the water will flow out of the tank.

However, the current is also affected by resistance. In the case of the hose, it is how wide the hose is. A wide hose allows more water to pass in less time, while a narrow hose resists the water flow. With an electrical current, there can also be resistance, measured in ohms.

Ohm's Law says voltage equals current times resistance. V = I * R. If you have a 12-volt battery but your resistance is two ohms, your current will be six amps. If the resistance were one ohm, your current would be 12 amps.