Any electrical component which has some resistance will convert
electrical potential energy to thermal energy, when current
flows through. 

The rate at which energy is converted is easily calculated if we
just remember the definitions of current and voltage. 

Consider the simple circuit shown below. 





The current, I, is a measure of
the number of Coulombs of charge which pass through the
resistor per second. 



The voltage, V, is a measure of the number of Joules of
energy lost by each Coulomb of charge passing through the
resistor. 



This tells us that the number of Joules of energy converted to
thermal energy by the resistor per second (the power, P dissipated
in the resistor) is simply given by 



From the definition of resistance we have 



Substituting for V or I in the
power equation gives us two very useful expressions 





To calculate the quantity of energy converted to thermal energy,
w, in t seconds we have 


