Potential difference

In electrical engineering, the potential difference between two points in an electrical circuit is equal to the difference in their electrical potentials. It is defined as the amount of work per charge needed to move electric charge from the second point to the first, or equivalently: the amount of work per charge that charge flowing from the first point to the second can perform.

A potential difference between two points gives rise to a "force" called an electromotive force or emf that pushes electrons from one point to the other. Common sources of emf are the battery, the electrical generator and the capacitor.

In the SI system of units, potential difference, electrical potential and electromotive force are measured in volts, leading to the commonly used term voltage and its abbreviation V. Named after Alessandro Volta, one volt is defined to be one joule of energy per coulomb of charge.

If one thinks of an electrical circuit in analogy to water circulating in a network of pipes, driven by pumps in the absence of gravity, then the potential difference corresponds to the pressure difference between two points. If there is a large pressure difference between two points, then water flowing from the first point to the second will be able to perform a large amount of work, such as driving a turbine.

The potential difference at the opposite ends of a resistor is (in most cases) proportional to the current flowing through the resistor; this is Ohm's Law. It can be used in a voltmeter to measure potential differences. Alternatively, oscilloscopes can be used, since the amount of deviation of an electron beam from the straight flight path is proportional to the applied potential difference.

Voltage is additive in the following sense: the voltage between A and C is the same as the voltage between A and B plus the voltage between B and C. Two points in an electric circuit which are connected by an (ideal) conductor without resistance will have a potential difference of zero. But other pairs of points may also have a potential difference of zero. If two such points are connected with a conductor, no current will flow through the connection. The various voltages in a circuit can be computed with Kirchhoff's Laws.

Since the current I is defined as the amount of charge per time, and potential difference V is energy per charge, the product VI equals energy per time, which is known as power.

In a capacitor, the potential difference between the two plates is proportional to the charge on the plates; the proportionality constant is the capacitor's capacity.

The voltage at the ends of a coil is proportional to the time rate of change of the current flowing through the coil, and is directed so as to slow down this change. The proportionality constant is the coil's inductivity. By using alternating current and combining two coils of different inductivities, it is possible to transform between high and low voltages. These transformers are important in electric power transmission since power losses are minimized if high voltages are used, which are however undesirable in the home.