Electrical tension (or voltage after its SI unit, the volt) is the difference of electrical potential between two points of an electrical or electronic circuit, expressed in volts. It is the measurement of the potential for an electric field to cause an electric current in an electrical conductor. Depending on the difference of electrical potential it is called extra low voltage, low voltage, high voltage or extra high voltage. Specifically, voltage is equal to energy per unit charge."To find the electric potential difference between two points A and B in an electric field, we move a test charge q₀ from A to B, always keeping it in equilibrium, and we measure the work W_AB that must be done by the agent moving the charge. The electric potential difference is defined from V_B − V_A = W_AB/q₀" Halliday, D. and Resnick, R. (1974). Fundamentals of Physics. New York: John Wiley & Sons. p. 465.

Explanation

Between two points in an electric field, such as exists in an electrical circuit, the difference in their electrical potentials is known as the electrical potential difference. This difference is proportional to the electrostatic force that tends to push electrons or other charge-carriers from one point to the other. Potential difference, electrical potential, and electromotive force are measured in volts, leading to the commonly used term voltage. Voltage is usually represented in equations by the symbols V, U, or E. (E is often preferred in academic writing, because it avoids the confusion between V and the SI symbol for the volt, which is also V.)

Electrical potential difference can be thought of as the ability to move electrical charge through a resistance. At a time in physics when the word force was used loosely, the potential difference was named the electromotive force or EMF—a term which is still used in certain contexts.

Voltage is a property of an electric field, not individual electrons. An electron moving across a voltage difference experiences a net change in energy, often measured in electron-volts. This effect is analogous to a mass falling through a given height difference in a gravitational field. ''' When using the term 'potential difference' or voltage, one must be clear about the two points between which the voltage is specified or measured. There are two ways in which the term is used. This can lead to some confusion.'''

Voltage with respect to a common point

One way in which the term voltage is used is when specifying the voltage of a point in a circuit. When this is done, it is understood that the voltage is usually being specified or measured with respect to a stable and unchanging point in the circuit that is known as ground or common. This voltage is really a voltage difference, one of the two points being the reference point, which is ground. A voltage can be positive or negative. "High" or "low" voltage may refer to the magnitude (the absolute value relative to the reference point). Thus, a large negative voltage may be referred to as a high voltage. Other authors may refer to a voltage that is more negative as being "lower".