# Electrical power

Electrical energy is easy to move from place to place, using wire or cables. However, electrical energy must be changed into other forms of energy before we can use it.

The rate at which energy is transferred (or changed) is called – how much energy is used per second.

$Power = \frac{{Energy}}{{time}}$

$P = \frac{E}{t}$

The symbol for power is $$P$$, it is measured in Watts ($$W$$).

The symbol for energy is $$E$$, it is measured in Joules ($$J$$).

The symbol for time is $$t$$, it is measured in seconds ($$s$$).

The electrical energy transferred each second is found by multiplying voltage by .

$Power = Voltage \times current$

$P = V \times I$

The symbol for power is $$P$$, it is measured in Watts ($$W$$).

The symbol for voltage is $$V$$, it is measured in Volts ($$V$$).

The symbol for current is $$I$$, it is measured in Amperes ($$A$$).

Sometimes we don't know the current or voltage, so $$P = IV$$ cannot be used to calculate the power. In these situations, we use Law to find an alternative power equation where more of the information is available.

## When current is unknown:

$P = \frac{{{V^2}}}{R}$

The symbol for power is $$P$$, it is measured in Watts ($$W$$).

The symbol for voltage is $$V$$, it is measured in Volts ($$V$$).

The symbol for resistance is $$R$$, it is measured in Ohms $$(Ω)$$.

## When voltage is unknown:

$Power = curren{t^2} \times resistance$

$P = {I^2}R$

The symbol for power is $$P$$, it is measured in Watts ($$W$$).

The symbol for current is $$I$$, it is measured in Amperes ($$A$$).

The symbol for resistance is $$R$$, it is measured in Ohms ($$Ω$$).