Electromotive force


The electromotive force (EMF) is the energy originated from any medium, channel or device that provides an electric current , being all energy capable of providing or serving to supply a viable electric current, which can be used by a device without require moving parts, such as portable cells or batteries.


What is the electromotive force?

The electromotive force, represented as EMF , would be any relationship between the electrical energy that is consumed and the electrical charge that passes through it , causing the current to flow in the direction of the generated electrical circuit. This can take place both in an open circuit and in a closed circuit. The utility of electromotive force is to provide energy to a device without the use of moving parts , for example, cells and batteries that require EMF for proper operation.

  • Electromotive force characteristics
  • How to measure electromotive force
  • Formula
  • Sources
  • How the electromotive force is generated
  • Examples

Electromotive force characteristics

For the electromotive force relationship to take place, the existence of a potential difference between two poles is required , where one is negative and one positive, in order to be able to promote electric charges through an open circuit or closed.


Electromotive force circuits can be both open and closed, an open circuit is characterized by generating an electric current, which, being interrupted or not communicated by a medium by an electrical conductive medium, does not circulate . This means that it cannot produce the proper continuity of the energy flow, causing that it is not used by the electric current and the connection is not effective when it is open. On the other hand, the closed or parallel electrical circuit works correctly because the electric current advances from the power generating source , for example a battery or cell; the common thread to the receiversFor example, the light bulb passes through the switch and turns the generator where it is connected, thus providing the effectiveness of the electrical circuit.

How to measure electromotive force

Electromotive force is measured or calculated from a voltage , which is called potential or voltage . The force with the electric charges make their way from the unit of negative charge to the positive pole. This is divided by its value in coulombs of said charge, (this being represented in joules / coulomb). It is summarized when talking about volts (V), which is equivalent to joules between coulomb (J / C).


The electromotive force in a closed circuit is equivalent to the variation of the induction flux of the magnetic field . This is expressed with the following formula that alludes to Faraday’s law :

Electromotive force formula

This formula can also have a minus sign “-“.


Electromotive force can be produced by different sources:

  • Direct electromotive force source: Obtains the flow of electrical charge from direct devices, such as the use of batteries, rechargeable batteries, non-rechargeable and solar batteries, accumulators. This causes the generated current to be of a constant value and a relatively large duration interval proportional to the size and strength of the generating device.
  • Alternating electromotive force source : In this case, the electrical current obtained is variable over time, as is the case with automobile generators, which are responsible for providing electricity for proper operation.
  • Non-alternating variable electromotive force source : There is an energy source that is variable, as in the case of piezoelectric lighters in a kitchen, since they produce an electric discharge in the air, which is variable to the intensity and subject to the environment. ; thus acquiring the factor of little durability.

How the electromotive force is generated

It can be generated through static , where the attractive and repulsive forces on other electrical charges are generated by rubbing, for example, when a plastic comb is rubbed and a static electrical charge is generated. Likewise, the electromotive force can be produced through induction , where electric charges acquire movement when they are in a viable magnetic field.


  • Batteries or batteries: They are classified as sources of electromotive energy since they are capable of generating energy by chemical means. Some of them, as in the case of batteries made of carbon-zinc and alkaline ones, when consumed to their maximum capacity, cannot be rechargeable. On the other hand, batteries composed of nickel-cadmium (NiCd), nickel and metal hydride (Ni-MH) and lithium ion, are capable of being rechargeable.
  • Electromagnetic machines: They are electric power generating machines based on magnetic and mechanical means, as is the case of small dynamos and generators, such as those frequently used in motor vehicles, portable power plants, among others. They are capable of supplying electrical energy to large structures, such as industries and cities.

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