Electromotive force, also called EMF, (denoted and measured in volts), refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.
Electromotive "force" is not considered a force, as force is measured in newtons, but a potential, or energy per unit of charge, measured in volts. Formally, EMF is classified as the external work expended per unit of charge to produce an electric potential difference across two open-circuited terminals. By separating positive and negative charges, electric potential difference is produced, generating an electric field. The created electrical potential difference drives current flow if a circuit is attached to the source of emf. When current flows, however, the voltage across the terminals of the source of emf is no longer the open-circuit value, due to voltage drops inside the device due to its internal resistance.
Devices that can provide emf include electrochemical cells, thermoelectric devices, solar cells, electrical generators, transformers, and even Van de Graaff generators. In nature, emf is generated whenever magnetic field fluctuations occur through a surface. An example for this is the varying Earth magnetic field during a geomagnetic storm, acting on anything on the surface of the planet, like an extended electrical grid.
In the case of a battery, charge separation that gives rise to a voltage difference is accomplished by chemical reactions at the electrodes; a voltaic cell can be thought of as having a "charge pump" of atomic dimensions at each electrode, that is:
A source of emf can be thought of as a kind of charge pump that acts to move positive charge from a point of low potential through its interior to a point of high potential. … By chemical, mechanical or other means, the source of emf performs work dW on that charge to move it to the high potential terminal. The emf ℰ of the source is defined as the work dW done per charge dq: ℰ = dW/dq.
Around 1830 Faraday established that the reactions at each of the two electrode–electrolyte interfaces provide the "seat of emf" for the voltaic cell, that is, these reactions drive the current. In the open-circuit case, charge separation continues until the electrical field from the separated charges is sufficient to arrest the reactions. Years earlier, Volta, who had measured a contact potential difference at the metal-metal (electrode-electrode) interface of his cells, held the incorrect opinion that this contact potential was the origin of the seat of emf.
In the case of an electrical generator, a time-varying magnetic field inside the generator creates an electric field via electromagnetic induction, which in turn creates an energy difference between generator terminals. Charge separation takes place within the generator, with electrons flowing away from one terminal and toward the other, until, in the open-circuit case, sufficient electric field builds up to make further movement unfavorable. Again the emf is countered by the electrical voltage due to charge separation. If a load is attached, this voltage can drive a current. The general principle governing the emf in such electrical machines is Faraday's law of induction.
A solar cell or photodiode is another source of emf, with light energy as the external power source.
Read more about Electromotive Force: Notation and Units of Measurement, Terminology, Formal Definitions of Electromotive Force, Electromotive Force in Thermodynamics, Electromotive Force and Voltage Difference
Other articles related to "electromotive force, force":
... The strength of the pile is expressed in terms of its electromotive force, or emf, given in volts ...
... The law of total currents (B) The equation of magnetic force (C) Ampère's circuital law (D) Electromotive force created by convection, induction, and by static electricity ... This is in effect the Lorentz force) (E) The electric elasticity equation (F) Ohm's law (G) Gauss's law (H) Equation of continuity or Notation H is the magnetizing field, which ... E is called the electromotive force by Maxwell ...
... The light-induced charge separation creates a reverse current through the cell's junction (that is, not in the direction that a diode normally conducts current), and the charge separation causes a photo voltage that drives current through any attached load ... However, a side effect of this voltage is that it tends to forward bias the junction ...
... Voltage, electromotive force Name of unit Symbol Definition Relation to SI units abvolt (cgs unit) abV ≡ 1×10−8 V = 1×10−8 V statvolt (cgs unit) statV ≡ c· (1 μJ/A·m) = 299.792 458 V volt (SI unit) V The ...
... which had all appeared in "On physical lines of force" ... made at equation (112) in "On Physical Lines of Force", is the one that modifies Ampère's Circuital Law to include Maxwell's displacement current ... (C) Ampère's circuital law (D) The Lorentz force This force represents the effect of electric fields created by convection, induction, and by charges ...
Famous quotes containing the word force:
“In the new science of the twenty-first century, not physical force but spiritual force will lead the way. Mental and spiritual gifts will be more in demand than gifts of a physical nature. Extrasensory perception will take precedence over sensory perception. And in this sphere woman will again predominate.”
—Elizabeth Gould Davis (b. 1910)