# 6.05 Power in d.c. circuits.

Power is just the rate at which energy is supplied.

## P = W/t .

• P = Power in Watts (W).
• W = Energy (work done) in Joules (J).
• t = Time in seconds (s).

For an electrical circuit we have already seen that W = VIt (or W = EIt) therefore :

## P = VI.

• P = Power in Watts (W).
• V = Voltage (V).
• I = current in amps (A).

Using Ohm's law, (R = V/I) and rearranging it to give expressions for voltage and current:

• V = IR .
• I = V/R .

Substituting for V or I in the equation for power, gives two alternative equations:

.
• ## P = V2/R

.

When using the equations for power and energy, it is important (as with Ohm's Law) to put the correct values into the equation. i.e. For the power supplied by the e.m.f., you must use the supply current and supply voltage, for the power dissipated by a resistor, you must use the voltage across that resistor and the current through it, etc.
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