When an electric current flows through a conductor, electrical energy is expended n over coming the frictional resistance between eh electrons and the molecules of the wire.
If potential difference of V volts is applied across a conductor and current of I amperes flows through it for time of t seconds, then energy expended will be equal to VIt watt-secs or joules.

If R is the resistance of the conductor through which a current of I amperes flows and V is the potential difference applied across its end then by Ohm’s law

                                   V- IR
and energy expended,   W = VIt = IR X I X t = I²Rt joules

or     W also = V X V  t = V²  t joules
                             R      R

According to the law of conservation of energy this electrical energy expended must be converted in some other form of energy an I that other form is hear i.e. electrical energy expended is converted into heat energy and conversion of electrical energy into heat energy is called the thermal effect of electric current.

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