# Electromotive force (EMF)

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Definition of emf

Electromotive force (emf) is a source of energy that maintains a potential difference between different points in an electrical circuit or a device or a conductor and can cause an electric current to flow in it. It is a measure of the work done per unit charge by a source in moving an electric charge or creating a separation of positive from negative charges, thereby creating a voltage difference. It is the rate at which energy is drawn from this source when unit current flows through the circuit or device. The word "force" in "electromotive force" is a misnomer. Electromotive force is not really a force. It has the dimensions of energy per charge. Electromotive force is often denoted by ξ or ℰ (script capital E). The unit of emf in SI system is volt which is equivalent to joules /coulomb.

Types of voltage

There are two types of voltage as follows

1. Electrostatic potential due to (configuration / position of) stationary charges

                                    $\xi=-\int_{A}^{B} E_{cs}.dl$


Here Ecs is the electrostatic field created by the charge separation associated with the emf, dℓ is an element of the path from terminal A to terminal B. This equation is applicable only to terminals A and B and not to paths between points A and B. Electrostatic energy is conserved during a cyclic process within the associated system. The system is conservative or irrotational (that is, the work done against the field around a closed path is zero). Its sources obey the equation

                                        $\triangledown.E=\frac{\rho}{\epsilon_0}$


where

                                        $E=-\triangledown \phi$


2. Electromotive force due to moving charges: When magnetic field varies, emf is "induced " around a stationary closed path C

                                        $\xi=\oint_{C} E.dl$


the integral is around an arbitrary, stationary closed curve C surrounding a region of varying magnetic field. E is the entire electric field- electrostatic (conservative) and field due to changing magnetic flux density (nonconservative). Out of the two origins, the electrostatic field does not contribute to the net emf around a circuit. In the case of the contribution due changing magnetic flux there is transfer of energy in a cyclic process in the associated system (e. g. magnetic field in a coil). The field is described by curl sources and its divergence is zero.

Devices and Mechanisms producing emf

Devices that produce emf have different mechanisms/ principles e.g. electrical generators and transformers (electromagnetic induction), voltaic cells (chemical reactions), thermocouple devices (electricity by thermal gradient), solar cells (electricity by light energy), piezoelectric (electricity by mechanical pressure) etc. Electromotive force by Magnetic Induction

Electromotive force is induced in a coil or conductor whenever there is a change in the magnetic flux linkages. Depending on the way in which the changes are brought about, there are two types of induced emf as follows;

Types of Induced emf

1. When the change in the magnetic flux linkage is brought about by moving a conductor in a stationary magnetic field, the electromotive force generated by motion of conductor is referred to as motional emf.

2. When the change in flux linkage arises from a change in the magnetic field around the stationary conductor, the electromotive force generated by a time-varying magnetic field is referred to as transformer emf.