Voltages

Voltage, or electric potential difference, is a measure of the electric potential energy per unit charge between two points. It is what drives electric current through a circuit and is measured in volts (V). One volt equals one joule per coulomb. The term voltage is often used interchangeably with potential difference or electromotive force (emf) in certain contexts, though emf refers to the energy a source can provide, while terminal voltage is the delivered voltage under load.

In direct current (DC) systems, voltage is generally constant over time. In alternating current (AC) systems, voltage varies with time, typically in a sinusoidal pattern. The effective value of an AC voltage for heating and power calculations is the root-mean-square (RMS) value. Frequency for household AC is usually 50 or 60 hertz, depending on the country.

Voltage sources include batteries, power outlets, generators, and solar cells. A source’s emf represents its open-circuit voltage; under load, internal resistance reduces the terminal voltage. Ohm’s law relates voltage to current and resistance (V = I R) for DC, while AC circuits use impedance to generalize this relationship (V = I Z).

Common voltages vary by system: household supplies are typically about 120 V in some regions and 230 V in others; industrial transmission uses hundreds of kilovolts (kV). Higher voltages enable efficient long-distance power transmission, while lower voltages are used for local distribution and appliance operation.

Voltage must be measured with a voltmeter connected across two points. Safety considerations are paramount, as high voltages can cause severe injury; proper insulation, grounding, and isolation are essential in design and maintenance.