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Voltage (Electric Potential Difference)

Voltage, or electric potential difference, is the measure of energy change per unit charge that drives electric current. This article explains definitions, units, types, history, measurement, uses and safety.

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Overview

Voltage is the quantity that describes how much electric potential energy is available to move electric charge between two points. In everyday language it is often called "voltage" or "potential difference," while the formal scientific term is electric potential difference. Voltage does not itself flow; rather, it provides the condition that causes electric charges to move when a conductive path exists, producing an electric current. For a concise introduction to the underlying quantity of charge see electric charge, and for conductors see electrical conductor.

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20030230-01 Connecting High Voltage Cable - Bonny Nigeria
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Definition and units

Technically, voltage between two points is the difference in electric potential energy per unit charge between those points. The SI unit of voltage is the volt (symbol V), named after Alessandro Volta. One volt equals one joule of energy change per coulomb of charge (1 V = 1 J/C). The volt is a unit; the quantity it measures is a potential difference or voltage. For related basic quantities consult joule and coulomb. The International System of Units guidance on unit symbols and usage can be found at SI conventions.

Key characteristics

Voltage is inherently a difference: it must be specified between two points, for example between the positive and negative terminals of a battery or between a wire and earth ground. Common symbols used in formulas include uppercase V for the measured value in volts and lowercase v or e for an instantaneous or theoretical potential. Voltage is not a force in the mechanical sense, though it is sometimes described informally as a "push" because it is the potential that drives charge motion; for a development of how voltage relates to electric forces, see force and electric potential.

Types and behavior

There are two broad types of voltage used in electricity and electronics: direct current (DC) voltage and alternating current (AC) voltage. DC voltage maintains a steady polarity and magnitude (as from a battery), while AC voltage varies in magnitude and reverses polarity periodically (as supplied by many power grids). Typical domestic mains voltages vary by country; for examples and frequency information see AC and DC. Voltage can be constant, time-varying, or have complex waveforms used in communication and signal processing.

History and terminology

The name volt honors Alessandro Volta, whose experiments in the late 18th and early 19th centuries led to the development of the first chemical batteries. The term electromotive force (EMF) historically describes the source tendency to produce a voltage (for example a battery's open-circuit voltage), though EMF is not a mechanical force. For historical context and biography see Volta and for the EMF concept see electromotive force. The development of standardized units and measurement practice tied these ideas together under the modern concept of electric potential difference.

Measurement, use and safety

Voltage is measured with instruments such as voltmeters and oscilloscopes. A voltmeter reads the difference of potential between its two terminals when connected across the points of interest; an oscilloscope displays how voltage changes over time. Practical applications range from tiny millivolt signals in sensors to high-voltage transmission lines. High voltage alone does not determine hazard; electric shock risk depends on available current, path through the body, and exposure time. Examples illustrating this distinction include live power lines that may be deadly under some contact conditions but harmless to birds perched on a single conductor because no current flows through them. For measurement practice and units see volt (unit) and unit.

Applications, formulas and notable distinctions

Voltage is central to nearly all electrical engineering and electronics. Ohm's law relates voltage (V), current (I) and resistance (R): V = I·R. Electrical power delivered to a load is the product of voltage and current: P = V·I. In circuits, the difference between voltage and volts is important: volts quantify potential differences while voltage refers to the physical quantity being measured. Practical lists of common voltages and their typical uses include household mains, battery voltages, and signal levels; for examples of how these are specified and standardized consult unit symbols, charge, and introductory circuit texts at basic conductors. Below are quick reference points:

  • Voltage is a scalar quantity defined between two points, measured in volts.
  • One volt = one joule per coulomb (1 V = 1 J/C).
  • DC supplies steady polarity; AC alternates magnitude and sign.
  • Hazard depends on current and path as well as voltage.

For further elementary reading and authoritative references, see introductory physics and electrical engineering resources: force vs potential, electric potential, and basic electrical safety guides at coulomb context and SI guidance.

Alessandro Volta, physicist and eponym of the unit of tension
Alessandro Volta, physicist and eponym of the unit of tension

Electrical potential

→ Main article: Electrical potential

Formula symbols for the electric potential are Vand φ \varphi . This potential at a point P in space is {\vec {A}}defined by means of the electric field strength {\vec {E}}and the magnetic vector potential by

{\displaystyle -{\vec {\nabla }}V={\vec {E}}+{\frac {\partial {\vec {A}}}{\partial t}}}

with the remark: The electric potential is not unique, because any constant scalar quantity can be added to a given potential without changing its gradient.

For uniqueness, a reference point P0 is established which receives zero potential {\displaystyle V_{\mathrm {P_{0}} }=0}. There are conventions for choosing the reference point in many fields, so it is often not mentioned linguistically. In electrical engineering, the reference point is placed on that part of the conductor that is labeled "ground"; in electric field theory, the reference point is often placed "at infinity".

The integral value with respect to the reference point P0 is called the electric potential.

{\displaystyle V=-\int _{\mathrm {P_{0}} }^{\mathrm {P} }\left({\vec {E}}+{\frac {\partial {\vec {A}}}{\partial t}}\right)\cdot \mathrm {d} {\vec {s}}}.

With the potentials {\displaystyle V_{\mathrm {A} }}and {\displaystyle V_{\mathrm {B} }}at points A and B, the definition of the voltage gives

{\displaystyle U_{\mathrm {AB} }=\int _{\mathrm {A} }^{\mathrm {B} }{\vec {E}}\cdot \mathrm {d} {\vec {s}}=-(V_{\mathrm {B} }-V_{\mathrm {A} })-\int _{\mathrm {A} }^{\mathrm {B} }{\frac {\partial {\vec {A}}}{\partial t}}\cdot \mathrm {d} {\vec {s}}}.

In a potential field, ∂ {\displaystyle {\frac {\partial {\vec {A}}}{\partial t}}=0} . In the following, we will assume only the electric voltage in a potential field. There apply

{\displaystyle V_{A}=-\int _{\mathrm {P_{0}} }^{\mathrm {A} }{\vec {E}}\cdot \mathrm {d} {\vec {s}}=\int _{\mathrm {A} }^{\mathrm {P_{0}} }{\vec {E}}\cdot \mathrm {d} {\vec {s}}\quad ;\qquad V_{B}=\int _{\mathrm {B} }^{\mathrm {P_{0}} }{\vec {E}}\cdot \mathrm {d} {\vec {s}}}

{\displaystyle U_{\mathrm {AB} }=V_{\mathrm {A} }-V_{\mathrm {B} }}.

The electric voltage between these locations is therefore equal to the difference of the electric potentials at these locations.

The specification of a voltage at one point is only possible in exceptional cases if the second point for the voltage is known from the circumstances; otherwise the voltage can always only be specified between two points. In contrast, the potential depends only on the selected point in space and can therefore be specified as a location-dependent function. It thus represents a scalar field which (except for one constant) can be determined from the electric field and, conversely, uniquely determines the electric field.

Positive charge carriers move - if no further forces act on them - in the direction of the field strength. Because they lose potential energy in the process, the electric potential decreases in this direction. Negatively charged objects, on the other hand, move against the field strength in the absence of other forces, in the direction of increasing potential.

For a shift along an equipotential line, the integral is zero because on this path {\vec {E}}\perp \mathrm {d} {\vec {s}}, so the scalar product is zero.

If a charge is transported from A to B and via any other path back to A, the ring integral over the closed circuit disappears in the potential field:

\oint {\vec {E}}\cdot \mathrm {d} {\vec {s}}=0

Electrical voltage measurement

→ Main article: Voltmeter

The voltmeter used to measure a voltage is connected in parallel with the object whose voltage is to be measured.

When using a moving-coil movement, which by its physics is a current measuring device, a current divider circuit is created for voltage measurement. The current is measured by the internal resistance R_{{\text{i}}} of the measuring device as a measure of the voltage. Since each measuring device has a limited measuring range, the current must be reduced via a series resistor if the maximum measurable value is exceeded, {\displaystyle R_{\text{v}}}thus extending the measuring range.

The measurement error caused by the current branching (the feedback error due to the measuring device) is kept small if {\displaystyle R_{\text{i}}+R_{\text{v}}} is R_{1}large compared to the DUT This is because it is the only way that the total resistance of the measuring circuit remains approximately unchanged, and the measuring circuit has negligible influence on the rest of the circuit. For the comparison, the factor is xintroduced here

{\displaystyle R_{\text{i}}+R_{\text{v}}=x\cdot R_{1}\ .}

In a parallel circuit, the currents in the parallel branches add up to the total current and the conductances of the branches add up to the total conductance.

{\displaystyle {\frac {1}{R_{\mathrm {ges} }}}={\frac {1}{R_{1}}}+{\frac {1}{R_{\text{i}}+R_{\text{v}}}}}

and using this results inx

{\frac {1}{R_{\mathrm {ges} }}}={\frac {1}{R_{1}}}+{\frac {1}{x\cdot R_{1}}}\quad \Rightarrow R_{\mathrm {ges} }={\frac {x}{x+1}}\cdot R_{1}\ .

If the term relative measurement deviation is used

{\displaystyle f={\frac {x_{\text{a}}-x_{\text{r}}}{x_{\text{r}}}}}

with the correct value {\displaystyle x_{\text{r}}} and the value deviating from it to {\displaystyle x_{\text{a}}}this circuit, the following results

f={\frac {R_{\mathrm {ges} }-R_{1}}{R_{1}}}={\frac {x}{x+1}}-1={\frac {-1}{x+1}}\quad \Rightarrow \quad x={\frac {1}{-f}}-1\ .

For example, if this always negative variance is required to be |f|<1\;\%=0{,}01, then {\displaystyle x>99}. If {\displaystyle R_{\text{i}}+R_{\text{v}}}100 times as large as R_{1}, then is R_{\mathrm {ges} }1% less than . R_{1}

If the current from A to B comes from a constant current source, in this case the voltage is measured with a relative deviation f= -1 %. If there is a constant voltage source between A and B, f= 0. For any other supply, the measurement deviation is in between.

If the relative deviation fthat someone is willing to accept is given, the requirement for the resistance in the measurement branch is:

{\displaystyle R_{\text{i}}+R_{\text{v}}\geq \left({\frac {1}{|f|}}-1\right)\cdot R_{1}\ .}

With electronic voltage measuring instruments (digital measuring instrument, oscilloscope or compensation measuring recorder), the measuring range extension with a series resistor is not common; the internal resistance with these measuring instruments is typically 1 to 20 MΩ in all ranges. The series resistor would be of a magnitude that could not be reliably implemented. Instead, if the maximum measurable voltage value is exceeded, U_{\mathrm {max} }a voltage divider must be used. At a tap further to the right in the adjacent circuit, a smaller voltage can be fed to the indicating part, thus increasing the full scale value U_{\mathrm {MBE} }be increased.

The problem that the measuring device is connected in parallel toR_{1} as above, and thus the total resistance of the measuring circuit is changed, is the same as above for the measurement with moving-coil movement. Only the series resistor of different size depending on the measuring range is {\displaystyle R_{\text{v}}}omitted.

Voltage measurement at R_{1} with moving-coil instrument

Voltage measurement with digital-electronic measuring device
Voltage measurement with digital-electronic measuring device

Questions and answers

Q: What is voltage?

A: Voltage is an electrical potential difference, the difference in electric potential between two places. It can be thought of as the force that pushes charges to move in a wire or other electrical conductor.

Q: What unit is used to measure voltage?

A: The unit for measuring voltage is the volt. The symbol for this unit is written with an uppercase V (9V).

Q: How does voltage cause current?

A: Voltage can cause charges to move, and since moving charges create a current, voltage can cause a current.

Q: Who was Alessandro Volta and why was the volt named after him?

A: Alessandro Volta was an Italian physicist who invented the first battery in 1800. The volt was named after him as a way of honoring his contribution to science.

Q: Are volts and voltage two different things?

A: Yes, volts are units by which we measure something while voltage refers to what we measure using those units.

Q: What are the two types of voltage?

A: There are two types of voltage - DC (direct current) and AC (alternating current). DC always has the same polarity while AC alternates between positive and negative polarities.

Q: Is it possible for birds to land on high-voltage lines without being harmed?

A:Yes, birds can land on high-voltage lines such as 12kV and 16kV without dying because there must be both voltage and current present in order for power (energy) to transfer through them - if only one element is present then nothing will happen.

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