CURRENT ELECTRICITY EMF and internal resistance.

E.M.F. AND P.D.

E.M.F., the electromotive force is a measure of the amount of energy transfered to electrical energy at the source per coulomb of charge.
P.D., the potential difference between two points is a measure of the energy transfered from electrical energy between the two points per coulomb of charge.

Sources usually have a internal resistance within the cell, so do not give a constan voltage as the currant is increased. If a currant is being drawn then the p.d. V across the cell will be equal to the E.M.F E minus the currant I multiplied by the internal resistance r. V = E - I r

Rob Coles 27 May 1996 16:23:52