| Electromotive Force |
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Electromotive force ( Emf ), often denoted by or ℰ (script capital E), is a measure of the strength of a source of electrical energy and is measured in volts. TERMINOLOGY The term "electromotive force" originally referred to the 'force' with which positive and negative charges could be separated (i.e. moved, hence "electromotive"), and was also called "electromotive power" (although it is not a Power in the modern sense). (c.f. ''Oxford English Dictionary'', "electromotive force".) The term is attributed to Alessandro Volta . Maxwell's 1865 explication of what are now called Maxwell's Equations used the term "electromotive force" for what is now called the Electric Field . The unit of emf is the Volt (energy per unit electric charge) and so the term ' Force ' is misleading. Thus, the expansion of the acronym is considered obsolete. Nonetheless, it is sometimes helpful to picture emf as analogous to a force or a pressure such as when making a mechanical or liquid analogy of an electric circuit. The use of the term emf is in decline but it is still found in introductory and technical level texts on electricity. Within Electrical Engineering, the term emf is occasionally used for a voltage produced by electromagnetic induction. However, the term ''induced voltage'' is preferred even though this is in the opposite sense to the emf. DISTINCTION BETWEEN EMF AND P.D. If an external circuit is not connected to a source of emf, an electric current cannot exist. Thus, between the terminals of the source, there must exist an electric field that exactly cancels the generated emf. The source of this field is the electric charges separated by the mechanism generating the emf. For example, the chemical reaction in the battery proceeds only to the point that the electric field between the separated charges is strong enough to stop the reaction. This electric field between the terminals of the battery creates an electric Potential Difference that can be measured with a voltmeter. The polarity of this measured pd is always opposite to that of the generated emf. The value of the emf for the battery (or other source) is the value of this 'open circuit' voltage. '''emf''' itself cannot be measured directly. GENERATION OF EMFS Radiant energy generation Absorption of Radiant or Thermal Energy (e.g., a solar cell or a thermocouple). Motional emf Electromagnetic induction is a means of converting mechanical energy, i.e., energy of motion into electrical energy. The emf generated in this way is often referred to as ''motional emf''. Motional emf is ultimately due to the electrical effect of a time-varying by which the line integral of the electric field around the circuit is not zero. The emf is then given by : where ''i'' is the current and ''L'' is the Inductance of the circuit. Given this emf and the Resistance of the circuit, the instantaneous current can be computed with Ohm's Law , for example, or more generally by solving the differential equations that arise out of Kirchhoff's Laws . The current at any instant ''t'' is then given by : where ''E'' is the Emf of the source, ''i'' is the instantaneous current, and ''R'' is the Resistance of the Resistor connected in series with the Inductor , in the circuit. emfs generated by Electrochemical Reaction Commonly, emf is generated by Electrochemical Reaction (e.g., a Battery or a Fuel Cell ). Dissimilar metals in contact also produce what is know as a Contact Emf or contact potential. Other sources of emf Some other sources are: EFFECTS Regardless of how it is generated, emf causes an electric current through a closed circuit connected to the terminals of the source. For example, the chemical reaction that separates electric charge onto the two terminals of a battery proceeds as long as there is an external circuit through which electrons can flow from the '+' terminal to the '-' terminal and thereby recombine with the positive ions. SEE ALSO |
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