Electromotive Force and Voltage Difference
An electrical voltage difference is sometimes called an emf. The points below illustrate the more formal usage, in terms of the distinction between emf and the voltage it generates:
- For a circuit as a whole, such as one containing a resistor in series with a voltaic cell, electrical voltage does not contribute to the overall emf, because the voltage difference on going around a circuit is zero. (The ohmic IR drop plus the applied electrical voltage is zero. See Kirchhoff's Law). The emf is due solely to the chemistry in the battery that causes charge separation, which in turn creates an electrical voltage that drives the current.
- For a circuit consisting of an electrical generator that drives current through a resistor, the emf is due solely to a time-varying magnetic field that generates an electrical voltage that in turn drives the current. (The ohmic IR drop plus the applied electrical voltage again is zero. See Kirchhoff's Law)
- A transformer coupling two circuits may be considered a source of emf for one of the circuits, just as if it were caused by an electrical generator; this example illustrates the origin of the term "transformer emf".
- A photodiode or solar cell may be considered as a source of emf, similar to a battery, resulting in an electrical voltage generated by charge separation driven by light rather than chemical reaction.
- Other devices that produce emf are fuel cells, thermocouples, and thermopiles.
In the case of an open circuit, the electric charge that has been separated by the mechanism generating the emf creates an electric field opposing the separation mechanism. For example, the chemical reaction in a voltaic cell stops when the opposing electric field at each electrode is strong enough to arrest the reactions. A larger opposing field can reverse the reactions in what are called reversible cells.
The electric charge that has been separated creates an electric potential difference that can be measured with a voltmeter between the terminals of the device. The magnitude of the emf for the battery (or other source) is the value of this 'open circuit' voltage. When the battery is charging or discharging, the emf itself cannot be measured directly using the external voltage because some voltage is lost inside the source. It can, however, be inferred from a measurement of the current I and voltage difference V, provided that the internal resistance r already has been measured: ℰ = V + Ir.
Read more about this topic: Electromotive Force
Famous quotes containing the words force and/or difference:
“If writers were too wise, perhaps no books would get written at all. It might be better to ask yourself “Why?” afterwards than before. Anyway, the force from somewhere in Space which commands you to write in the first place, gives you no choice. You take up the pen when you are told, and write what is commanded. There is no agony like bearing an untold story inside you.”
—Zora Neale Hurston (1891–1960)
“I have always suspected that too much knowledge is a dangerous thing. It is a boon to people who don’t have deep feelings; their pleasure comes from what they know about things, and their pride from showing off what they know. But this only emphasizes the difference between the artist and the scholar.”
—Margaret Anderson (1886–1973)