From Wikipedia, the free encyclopedia
Electromotive force (emf) is the amount of
energy gained per unit charge that passes through a device in
the opposite direction to the electric field existing across
that device. It is measured in
Sources and unit of measurement
Sources of electromotive force include
electric generators (both
alternating current and
continuous current types),
thermocouples (in a heat gradient).
Electromotive force is often denoted by
or ℰ (script capital E).
Electromotive force is measured in (V)
International System of Units equal in amount to a
coulomb of electric charge). Electromotive force in
electrostatic units is the
statvolt (in the
centimeter gram second system of units equal in amount to an
per electrostatic unit of
The term origin is attributed to
Alessandro Volta (1745–1827), who invented the
voltaic pile. The term "electromotive force" originally
referred to the 'force'
with which positive and negative charges could be separated
(i.e. moved, hence "electromotive"), and was also called
"electromotive power" (although it is not a
power in the modern sense). Maxwell's 1865 explication of
what are now called
Maxwell's equations used the term "electromotive force" for
what is now called the
electric field strength.
Electromotive force has been stated to be the force that has
the disposition to produce a circuit's electric current and is,
under normal conditions, called
physics, the unit of emf is the "energy per unit electric
charge", so the "force" term of "electromotive force" is
misleading to a degree. The expansion of the acronym is
considered obsolete.emf" is in decline
but it is still found in introductory and technical level texts
Nonetheless, it is sometimes
helpful to picture emf as analogous to a force or a pressure
such as when making a mechanical or liquid analogy of an
electric circuit. The use of the term "
Explanation of electromotive force
electrodynamics, a measure of electromotance
indicates the tendency for electric charge to flow around a
circuit or other
closed curve. An emf is also commonly used to express the
strength of a compact source of electrical energy. The
electromotive force of a device is defined to be the amount of
energy gained per unit charge that passes through it in the
"uphill" direction. It has units of joules per coulomb,
otherwise more commonly known as the volt.
If the vector field f is the force per unit charge on
charge carrier, the emf around a circuit C is
electric potential at a point and the voltage between two
points, the emf around a loop is measured in
Unlike the first two quantities, the emf is sensitive to
non-electrostatic forces, since the force f can include
magnetic, chemical, mechanical, and gravitational components.
Electromotive force in thermodynamics
When multiplied by an amount of charge de the emf ℰ
yields a thermodynamic work term ℰde that is used in the
formulism for the change in Gibbs free energy when charge is
passed in a battery:
- dG = -SdT + VdP + ℰde
The combination ℰ.e is an example of a
conjugate pair of variables. At constant pressure the above
relationship produces a
Maxwell relation that links the change in open cell voltage
with temperature (a measurable quantity) to the change in
entropy when charge is passed
isobarically. The latter is closely related to the reaction
entropy ΔrS of the electrochemical reaction that
lends the battery its power.
Electromotive force and potential
If no external circuit is connected to a source of emf, an
electric current cannot exist (Ohm's Law). Thus, between the
terminals of the source, there must exist an electric field that
exactly cancels the generated emf.
The source of this field is the electric charges separated by
the mechanism generating the emf
For example, the chemical reaction in the battery proceeds only
to the point that the electric field between the separated
charges is strong enough to stop the reaction.
This electric field between the terminals of the battery
creates an electric
potential difference that can be measured with a
voltmeter. The polarity of this measured potential
difference is always opposite to that of the generated
emf. The value of the emf for the battery (or other source) is
the value of this 'open circuit' voltage. The emf itself
cannot be measured directly.
Electromotive force generation
Commonly, electromotive force is generated by
electrochemical reaction (e.g., a
fuel cell). Dissimilar metals in contact also produce what
is know as a
contact electromotive force or
contact potential (eg., the
volta effect). Absorption of
thermal energy (e.g., a solar cell or a thermocouple). Some
other sources include thermocouples,
Electromagnetic induction is a means of converting
mechanical energy, i.e., energy of motion into electrical
energy. The electromotive force generated in this way is often
referred to as motional electromotive force. Motional emf
is ultimately due to the electrical effect of a time-varying
magnetic field. In the presence of such a magnetic field,
electric potential and hence the
potential difference (commonly known as voltage) is
undefined (see the former) — hence the need for distinct
concepts of emf and potential difference. Technically, the emf
is an effective potential difference included in a circuit to
Kirchhoff's voltage law valid: it is exactly the amount from
Faraday's law of induction by which the line integral of the
electric field around the circuit is not zero. The emf is then
where i is the current and L is the
inductance of the circuit.
Given this emf and the
resistance of the circuit, the instantaneous current can be
Ohm's Law, for example, or more generally by solving the
differential equations that arise out of
Kirchhoff's laws. The current at any instant t is
then given by
where E is the electromotive force of the source, i
is the instantaneous current, and R is the
resistance of the
resistor connected in series with the
inductor, in the circuit.
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