Voltage is the electrical potential difference between two points and represents the difference in electric potential energy per unit charge between those points. Its SI unit is the volt, represented by V, where one volt corresponds to one joule per coulomb. Voltage is not an amount of charge and is not itself a rate of charge flow. It is a relational quantity: a voltage exists between two points rather than independently at one isolated point.
In electrostatics, electric potential can be defined relative to a chosen reference, and voltage is the difference between two potential values. In circuits, one point is often designated as ground or zero volts purely as a reference. The physical behaviour of a circuit depends on potential differences, not on the absolute numerical value assigned to an arbitrary reference.
Voltage can be understood through electrical potential energy. If a charge moves between two points with a potential difference, its electrical potential energy changes according to the charge multiplied by the potential difference, with the sign determined by the direction of movement and the charge convention. A positive charge naturally accelerates in the direction of decreasing electric potential under appropriate electrostatic conditions, while a negative charge responds oppositely.
Voltage sources such as batteries, generators and power supplies establish potential differences through chemical, mechanical or electronic processes. A battery does not simply "contain voltage" as a material substance; rather, its internal chemical processes maintain a potential difference between its terminals. The open-circuit voltage of a source can differ from its terminal voltage when current flows because internal resistance and other effects produce voltage drops.
Voltage is closely related to Electric Field, but the two quantities are not interchangeable. The electric field describes force per unit positive charge at a point, whereas electric potential describes potential energy per unit charge. In electrostatic situations, the electric field is related to the spatial gradient of electric potential, with the field pointing in the direction of greatest decrease in potential.
A uniform electric field across a distance can produce a voltage difference proportional to the field magnitude and separation, subject to the geometry and reference direction. In complex electromagnetic situations, especially when magnetic fields vary with time, the relationship between electric field and scalar potential becomes more subtle because induced electric fields can have non-conservative circulation. Circuit voltage remains a useful macroscopic quantity, but it should not be assumed that every voltage can be represented solely by a static electrostatic potential field.
Voltage is central to Ohm's law and circuit analysis. For an ideal resistor, the relationship V = IR connects voltage, current and resistance. This equation is a constitutive relationship for an ohmic resistor rather than a universal definition of voltage or a universal law applying identically to every component. Capacitors obey a charge-voltage relationship Q = CV, while inductors obey a voltage-current relationship based on the time derivative of current.
Diodes, transistors, batteries, lamps and many other devices have nonlinear or dynamic voltage-current characteristics. Voltage measurements are normally made by connecting a voltmeter or voltage-sensing circuit in parallel with the two points of interest. An ideal voltmeter has infinite input resistance so that it draws no current, while practical instruments have finite input impedance and can alter sensitive circuits.
Voltage may be steady, alternating, transient or rapidly changing. A DC voltage can remain approximately constant in polarity and magnitude, whereas an AC voltage generally varies with time and may reverse polarity periodically. A transient voltage can arise when switches operate, capacitors charge or discharge, inductive currents change, or faults occur. Voltage spikes can exceed the normal operating voltage of components and cause insulation breakdown or semiconductor damage.
In AC power systems, instantaneous voltage varies continuously, and root-mean-square voltage is commonly used for power calculations involving resistive loads. Peak, peak-to-peak and RMS values are distinct measurements and should not be treated as interchangeable. The waveform, frequency, phase and source impedance are relevant when voltage is used to transfer power or carry information.
Voltage also occurs naturally in electrochemical, biological and atmospheric systems. Electrochemical cells produce voltage through chemical potential differences and charge separation. Nerve cells maintain membrane potentials resulting from ion concentration gradients and selective membrane permeability. Atmospheric electrical processes can create very large potential differences that contribute to lightning. Static charge accumulation can create high voltages even when the available current and stored energy are relatively small.
This distinction is important because voltage alone does not determine how dangerous or powerful an electrical source is. A source with very high voltage but extremely limited charge-transfer capability may behave differently from a lower-voltage source capable of delivering substantial current. Electrical safety therefore requires consideration of source impedance, available current, stored energy, duration, contact conditions and the path through which current could flow.
Voltage polarity and reference conventions are essential in engineering. A voltage written as V_AB generally means the potential at point A relative to point B, so V_AB = V_A − V_B. Reversing the order changes the sign. Circuit diagrams use polarity markings, plus and minus signs, reference nodes and voltage arrows to establish consistent equations.
Kirchhoff's voltage law states that the algebraic sum of voltage changes around a closed circuit is zero under the lumped-circuit approximation, reflecting conservation of energy in the circuit model. Voltage drops across series elements depend on their electrical characteristics and the current through them, while voltage division allows a source voltage to be distributed among series impedances. In practical circuits, parasitic resistance, inductance, capacitance and source impedance can make measured voltages differ from simple ideal calculations.
A frequent misconception is that voltage is something that "flows". Voltage does not flow; charge flows as current, while voltage represents a potential difference that can drive or accompany charge movement. Another misconception is that voltage is always consumed by a component. In circuit analysis, a component may experience a voltage drop as electrical energy is transferred or may experience a voltage rise when energy is supplied to the circuit.
Voltage can exist across an open switch even though the current through the switch is essentially zero. Likewise, an ideal short circuit has zero voltage across it while potentially carrying substantial current. Understanding voltage as a difference in potential rather than as a substance or flow quantity provides the basis for correctly interpreting batteries, generators, resistors, capacitors, inductors, power supplies and electromagnetic systems.
