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2 definitions by Nicker doodle doo

 
1.
The counter-electromotive force (abbreviated counter emf, or CEMF ) 1 is the voltage, or electromotive force, that pushes against the current which induces it, is caused by a changing electromagnetic field. It's represented by Lenz's Law of electromagnetism. Back electromotive force is a voltage that occurs in electric motors where there is relative motion between the armature of the motor and the external magnetic field. Counter emf is a voltage developed in an inductor network by a pulsating current or an alternating current. 2 The voltage's polarity is at every moment the reverse of the input voltage 3 4

In a generator using a rotating armature and, in the presence of a magnetic flux, the conductors cut the magnetic field lines as they rotate. The changing field strength produces a voltage in the coil; the motor is acting like a generator.. (Faraday's law of induction.) This voltage opposes the original applied voltage; therefore, it is called "counter-electromotive force". (by Lenz's law.) With a lower overall voltage across the armature, the current flowing into the motor coils is reduced. 5

Pretty self explanatory dont you think, i added this as the previous entry was rubbish and he's obviously visited an engineer who doesn't know what back EMF is.
by Nicker doodle doo May 14, 2007
 
2.
Capactive reactance is the analysis of an alternating-current electrical circuit (for example a RLC series circuit), reactance is the imaginary part of impedance, and is caused by the presence of inductors or capacitors in the circuit. Reactance produces a phase shift between the electric current and voltage in the circuit.
This is determined by the equation XC=1/2piFC XC is an impedance and is measured in ohms.
*capacitive reactance
by Nicker Doodle Doo May 14, 2007