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Energy and Power

<^< Worksheet: Ohm's Law | Course Index | Examples: Electricity >^>

At first you may be a little confused about the difference between energy and power. Energy is the ability to do work whilst power is the rate at which work is done. In electrical circuits, energy is supplied by batteries or generators. It may also be stored in components such as capacitors and inductors. Electrical energy is converted into various other forms of energy by components such as resistors (producing heat), loudspeakers (producing sound energy), and light emitting diodes (producing light).

The unit of energy is the joule (J). Power is the rate of use of energy and it is measured in watts (W). A power of 1W results from energy being used at the rate of 1J per second. Thus:

P = E / t

where P is the power in watts (W), E is the energy in joules (J), and t is the time in seconds (t).

The power in a circuit is equivalent to the product of voltage and current. Hence:

P = I x V

where P is the power in watts (W), I is the current in amps (A), and V is the voltage in volts (V).

The formula may be arranged to make P, I or V the subject, as follows:

P = I x V or I = P/V and V = P/I

The relationship, P = I x V, may be combined with that which results from Ohm's law (V = I x R), to produce two further relationships. Firstly, substituting for V gives:

P = I x (I x R) = I²R

Secondly, substituting for I gives:

P = (V / R) x V = V²/R

<^< Worksheet: Ohm's Law | Course index | Examples: Electricity >^>