Electromotive Force Electromotive Force 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.
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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.
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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. Counter Electromotive Force :- Counter Electromotive Force is also called as the Back Electromotive force. To explain this we will first explain the principle of Self Induction. Here we will differentiate the Electromotive forces and currents which are generated by the batteries etc. and electromotive forces which are due to change in the magnetic fields. Source emf and source currents be the emf and current which are generated by battery etc. and induces emf and induced current induced by the change in Magnetic fields. Consider again the circuit shown in which Resistor, switch and source of emf are connected. When switch is closed, the current starts from zero but it does not reach to its maximum value immediately. According to Faradays law of the electromagnetic induction, as the source current increases, the magnetic flux associated with the current in the circuit loop also increases with time. As the magnetic flux increases with time, it results in induced electromotive force. flux creates an induced emf in the circuit. Induced emf in the circuit opposes the change in magnetic flux due to the change in source magnetic field. That is, the direction of the induced current is opposite to the source emf. That is why source current does not reach to its maximum value from zero immediately but there is a gradual increase in the source current with time. This principle is called as the self-induction principle. This is due to the change in the source magnetic flux in the circuit because of which emf gets induced in the circuit. The emf induced is called a self-induced emf. It is also called as Counter Electromotive Force or Back emf. It is always proportional equal to the time rate change of the source current.
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