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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

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