Power Formulas Used in the Study of Electricity

When current flows through a resistive circuit, energy is dissipated in the form of heat. Recall that voltage can be expressed in the terms of energy and charge as given in the expression:

Current I, can also be expressed in terms of charge and time as given by the expression:

When voltage W/Q and current Q/t are multiplied, the charge Q is divided out leaving the basic expression from physics:

For a simple DC electrical system, power dissipation can then be given by the equation:

If a circuit has a known voltage of 24 volts and a current of 2 amps, then the power in the circuit will be:

Now recall Ohm’s laws which states that E = I(R). If we now substitute IR for E in the general formula, we get a formula that uses only current I and resistance R to determine the power in a circuit.

Second Form of Power Equation

If a circuit has a known current of 2 amps and a resistance of 100 Ω, then the power in the circuit will be:

Using Ohm’s law again, which can be stated as I = E/R, we can again make a substitution such that power can be determined by knowing only the voltage (E) and resistance (R) of the circuit.

Third Form of Power Equation

If a circuit has a known voltage of 24 volts and a resistance of 20 Ω, then the power in the circuit will be: