Electrical Power

Factors of electrical power

The electrical power used by an electrical component depends on two main factors. These factors are:

• The current $I$ passing through the component.
• The potential difference/voltage $V$ across the two ends of the component

increasing either one of these variables will increase the power proportionally. This can be formulated as an equation for power in terms of these two variables which we demonstrate in the next section of this explanation.

Electrical power formula

The electric power transferred to an electrical component in a circuit can be calculated using the electric power formula:

Or in words:

$\mathrm{Power}=\mathrm{potential}\mathrm{difference}\times \mathrm{current}$

where$P$is the electrical power,$V$is the potential difference across the component and$I$is the current passing through the component.

The electric power can also be calculated by knowing the current and resistance using the following equation

$P=I^{2}R$

where$R$is the resistance of the electrical component.

Therefore,$1\mathrm{W}$of electric power can be defined as the energy transferred when a current of$1\mathrm{A}$flows through a potential difference of$1\mathrm{V}$.

Unit of electrical power

The units of electrical power, like all other forms of power, is Watts \(W\) or often kilowatts \(kW\). Power is a measure of the time rate at which energy is transferred to or from an object or more generally some physical system. Therefore electrical power can also have units of Joules per second $(\mathrm{J}/\mathrm{s})$ which is the same the Watt $\left(\mathrm{W}\right)$.

$1\mathrm{J}/\mathrm{s}=1\mathrm{W}$.

As an example, imagine we have a lamp that requires a $12\mathrm{W}$incandescent lightbulb. The power rating of $12\mathrm{W}$ indicates that the total energy used by the lightbulb in one second, in the form of both light energy and wasted thermal energy, is equal to $12\mathrm{J}$.

Electrical power triangle

The electric power triangle is an easy way to memorize the above equation. This formula can be rearranged with the help of an Electrical power triangle shown below.

Electrical Power Electric Power triangle, StudySmarter Originals

We can derive the second electrical power formula using Ohm's law. The equation for Ohm's law is given by

$V=IR$

We can substitute the value for the potential difference into the equation for power.

$P\mathit{=}I\mathit{\times }R\mathit{\times }I$

$P\mathit{=}{I}^{2}R$

or in words

$\mathrm{Power}={\left(\mathrm{Current}\right)}^2\times \mathrm{Resistance}$

Let us look at a few examples where we calculate power:

Power transmission

For a given value of current, the power consumed increases with an increase in potential difference. The magnitude of power consumed depends on both the current and the potential difference. Therefore electric power can be delivered in the same quantity using different combinations of potential difference and current.

• Low current with a high voltage
• High current with a low voltage

Warning signs outside power stations indicate that the voltages present are dangerous to humans and could cause serious harm in the form of an electric shock, StudySmarter

The disadvantage of using a high current with a low potential difference is the heating effect. When large values of current pass through a wire they heat up to high temperatures which reduces the lifetime of the wires. The heating effect is bad as it reduces the efficiency of the electric device. This is because a part of the energy that is being transferred is being converted into heat. For this reason, High powers across the mains are transmitted in high voltages with low currents.