Energy transfers in everyday appliances

What you'll learn

• How appliances change energy stores using an electrical transfer from a battery or wall socket.
• What a power rating means: the energy an appliance transfers each second.
• How to calculate energy transferred using time, charge flow, and potential difference.
• How power links to current and potential difference in a circuit.

The big picture: appliances transfer energy

An electrical appliance is a device designed to transfer energy in a useful way. Examples include kettles, drills, washing machines, lamps, heaters and phone chargers.

At GCSE, it is best to think in terms of energy stores and energy transfer pathways. Energy is not “made” by the appliance. Instead, energy is transferred from one store to another.

For example:

• A battery has a chemical store that decreases when it is used.
• The ac mains is the electrical supply from wall sockets; in the UK it provides energy to appliances through the National Grid.
• An appliance transfers energy electrically to useful stores, such as a thermal store or kinetic store.

Many appliances also transfer some energy in less useful ways, often to the thermal store of the surroundings and by sound. This less useful transfer is often called dissipation.

Common appliance transfers

A heating device transfers energy electrically to a thermal store. For example, a kettle transfers energy to the thermal store of the water.

An electric motor transfers energy electrically to a kinetic store. For example, a fan transfers energy to the kinetic store of the rotating blades and moving air.

A lamp transfers energy electrically to light and to the thermal store of the lamp and surroundings.

Power: energy transferred each second

Power tells you how quickly energy is transferred.

A higher-power appliance transfers more energy each second than a lower-power appliance. That is why a 2 kW kettle heats water much faster than a small 20 W phone charger warms up.

The key equation is:

$$E=Pt$$

where:

• $E$ is energy transferred, measured in joules, J
• $P$ is power, measured in watts, W
• $t$ is time, measured in seconds, s

So the total energy transferred depends on both the power and how long the appliance is switched on for.

Power ratings on domestic appliances

The power rating of an appliance tells you how much energy it transfers per second when used normally.

For example:

• A 2000 W kettle transfers 2000 J every second.
• A 60 W lamp transfers 60 J every second.
• A 500 W drill transfers 500 J every second.

This rating is linked to the changes in stored energy while the appliance is being used. A high-power kettle causes the thermal store of the water to increase quickly. A lower-power lamp changes energy stores more slowly.

Work is done when charge flows

In a circuit, charge is carried by moving charged particles. The amount of charge that flows is called charge flow.

When charge flows through a device, work is done. At GCSE, saying “work is done” means energy is transferred.

The energy transferred by electrical work can be calculated using:

$$E=QV$$

where:

• $E$ is energy transferred, measured in joules, J
• $Q$ is charge flow, measured in coulombs, C
• $V$ is potential difference, measured in volts, V

A bigger potential difference means each coulomb of charge transfers more energy.

Linking power, current and potential difference

Current is the rate of flow of charge.

If a device has a high current, lots of charge flows through it each second. If it also has a high potential difference, each coulomb transfers a lot of energy. Together, these determine the power.

The useful relationship is:

$$P=IV$$

where:

• $P$ is power in watts, W
• $I$ is current in amperes, A
• $V$ is potential difference in volts, V

You can see where this comes from by combining the ideas:

$$\begin{array}{rl}P& =\frac{E}{t}\\ E& =QV\\ P& =\frac{QV}{t}\\ P& =V\frac{Q}{t}\end{array}$$

Since current is charge flow per second, $\frac{Q}{t}=I$, so $P=IV$.