Mon May 23, 2011 By: Hussain Shaikh

what is actual difference between emf & p.d?.& explain the term internal resistances

Expert Reply
Mon May 23, 2011

Potential difference is the amount of work done when a unit charge is brought from infinity to a given point in electric field. 

Electromotive force is the amount of work done to rotate a unit +ve charge in a given circuit . 

Note : EMF is always greater than Pd 

A potential difference (voltage) can exist across any circuit component. For example, the fact that current is flowing through each of several resistors in a series circuit means that there must be a potential difference across each of those resistors (which we also term 'voltage drop'). 

An electromotive force is the name we give to the open-circuit potential difference provided by the device that separates charges - i.e. a generator, battery, etc. For example, the open circuit voltage of a battery would be its electromotive force. 

So, if we use a series resistive circuit as an example, the battery would provide the electromotive force, while potential differences would appear across each of the resistances. And the electromotive force is then the sum of the voltage drops. 
A practical electrical power source which is a linear electric circuit may, according to Thevenin's theorem, be represented as a ideal voltage source in series with an impedance.This resistance is termed the internal resistance of the source.
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