Voltage

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.^[1] It is the measurement of the potential for an electric field to cause an electric current in an electrical conductor. Voltage is a property of an electric field, not individual electrons. Voltage, as a definition, can more easily be described as a representation of the "carrier" of electrons. 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.^[2]

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.

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 can be described as a high voltage, although some writers do refer to negative voltages as "lower".

Voltage between two stated points

Another usage of the term "voltage" is in specifying how many volts are dropped across an electrical device (such as a resistor). In this case, the "voltage," or, more accurately, the "voltage drop across the device," is really the first voltage taken, relative to ground, on one terminal of the device minus a second voltage taken, relative to ground, on the other terminal of the device. In practice, the voltage drop across a device can be measured directly and safely using a voltmeter that is isolated from ground, provided that the maximum voltage capability of the voltmeter is not exceeded.

Two points in an electric circuit that are connected by an "ideal conductor," that is, a conductor without resistance and not within a changing magnetic field, have a potential difference of zero. However, 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.

Addition of voltages

Voltage is additive in the following sense: the voltage between A and C is the sum of the voltage between A and B and the voltage between B and C. The various voltages in a circuit can be computed using Kirchhoff's circuit laws.

When talking about alternating current (AC) there is a difference between instantaneous voltage and average voltage. Instantaneous voltages can be added as for direct current (DC), but average voltages can be meaningfully added only when they apply to signals that all have the same frequency and phase.

Hydraulic analogy

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

This hydraulic analogy is a useful method of teaching a range of electrical concepts. In a hydraulic system, the work done to move water is equal to the pressure multiplied by the volume of water moved. Similarly, in an electrical circuit, the work done to move electrons or other charge-carriers is equal to 'electrical pressure' (an old term for voltage) multiplied by the quantity of electrical charge moved. Voltage is a convenient way of quantifying the ability to do work. In relation to electric current, the larger the gradient (voltage or hydraulic) the greater the current (assuming resistance is constant).

Mathematical definition

The electrical potential difference is defined as the amount of work needed to move a unit electric charge from the second point to the first, or equivalently, the amount of work that a unit charge flowing from the first point to the second can perform. The potential difference between two points a and b is the line integral of the electric field E:

V_{a}-V_{b}=\int _{a}^{b}\mathbf {E} \cdot d\mathbf {l} .

Useful formulae

DC circuits

V={\sqrt {PR}}

V=IR

where V = potential difference (volts), I = current intensity (amps), R = resistance (ohms), P = power (watts).

AC circuits

V={\frac {P}{I\;\cos \phi }}

V={\frac {\sqrt {P\;Z}}{\sqrt {\cos \phi }}}\!\

V={\frac {I\;R}{\cos \phi }}

Where V=voltage, I=current, R=resistance, P=true power, Z=impedance, φ=phase difference between I and V

AC conversions

V_{avg}=.637\,V_{pk}={\frac {2}{\pi }}V_{pk}={\frac {\omega }{\pi }}\int _{0}^{\pi /\omega }V_{pk}\sin(\omega t-kx){\rm {d}}x\!\

V_{rms}=.707\,V_{pk}={\frac {1}{\sqrt {2}}}V_{pk}=V_{pk}{\sqrt {\langle \sin ^{2}(\omega t-kx)\rangle }}\!\

V_{pk}=0.5\,V_{ppk}\!\

V_{avg}=.319\,V_{ppk}\!\

V_{rms}=.354\,V_{ppk}={\frac {1}{2{\sqrt {2}}}}V_{ppk}\!\

V_{avg}=0.900\,V_{rms}={\frac {2{\sqrt {2}}}{\pi }}V_{rms}\!\

Where V_pk=peak voltage, V_ppk=peak-to-peak voltage, V_avg=average voltage over a half-cycle, V_rms=effective (root mean square) voltage, and we assumed a sinusoidal wave of the form $V_{pk}\sin(\omega t-kx)$ , with a period $T=2\pi /\omega$ , and where the angle brackets (in the root-mean-square equation) denote a time average over an entire period.

Total voltage

Voltage sources and drops in series:

V_{T}=V_{1}+V_{2}+V_{3}+...+V_{n}\!\

Voltage sources and drops in parallel:

V_{T}=V_{1}=V_{2}=V_{3}=...=V_{n}\!\

Where $n\!\$ is the nth voltage source or drop

Voltage drops

Across a resistor (Resistor R):

V_{R}=IR_{R}\!\

Across a capacitor (Capacitor C):

V_{C}=IX_{C}\!\

Across an inductor (Inductor L):

V_{L}=IX_{L}\!\

Where V=voltage, I=current, R=resistance, X=reactance.

Measuring instruments

A multimeter set to measure voltage.

Instruments for measuring potential differences include the voltmeter, the potentiometer (measurement device), and the oscilloscope. The voltmeter works by measuring the current through a fixed resistor, which, according to Ohm's Law, is proportional to the potential difference across the resistor. The potentiometer works by balancing the unknown voltage against a known voltage in a bridge circuit. The cathode-ray oscilloscope works by amplifying the potential difference and using it to deflect an electron beam from a straight path, so that the deflection of the beam is proportional to the potential difference.

Safety

Electrical safety is discussed in the articles on High voltage (note that even low voltage, e. g. of 50 Volts, can lead to a lethal electric shock) and Electric shock.

References

^ "voltage", A Dictionary of Physics. Ed. John Daintith. Oxford University Press, 2000. Oxford Reference Online. Oxford University Press.
^ "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.

External links

[1] "voltage", A Dictionary of Physics. Ed. John Daintith. Oxford University Press, 2000. Oxford Reference Online. Oxford University Press.

[2] "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.

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