A useful device transfers energy from one energy store to one or more other energy stores. A moving charge is able to transfer energy because the charge does work against the resistance in a circuit. Electrical devices transfer energy electrically to the components in the circuit when a current flows. When appliances transfer energy from one store to one or more other stores, not all of the energy is transferred to the useful energy store; some of the energy is wasted. The wasted energy store is usually the thermal energy stores of the device and the surroundings. When a larger current travels through a device, more of the energy is transferred to the thermal energy stores of the device and the surroundings – the device becomes less efficient.

We are now going to have a look at a few energy transfers in electrical appliances.

A battery-powered electric fan transfers energy electrically from the chemical energy stores of the battery in the fan to the kinetic energy stores of the motor and blades in the fan (the useful energy transfer). Some of the energy is also transferred to the thermal energy stores of the motor, fan and the surroundings; this energy is wasted.

A mains kettle just means that the kettle is plugged into the mains AC supply; it is plugged into the wall in a house. A mains kettle transfers energy electrically from the mains AC supply to the thermal energy stores of the heating element in the kettle.

A washing machine transfers energy electrically from the mains AC supply to the thermal energy stores of the water (the water becomes hot) and the kinetic energy stores of the motor (the washing machine drum spins).

We are now going to have a look at a few energy transfers in electrical appliances.

**Battery-Powered Fan**A battery-powered electric fan transfers energy electrically from the chemical energy stores of the battery in the fan to the kinetic energy stores of the motor and blades in the fan (the useful energy transfer). Some of the energy is also transferred to the thermal energy stores of the motor, fan and the surroundings; this energy is wasted.

**Mains Kettle**A mains kettle just means that the kettle is plugged into the mains AC supply; it is plugged into the wall in a house. A mains kettle transfers energy electrically from the mains AC supply to the thermal energy stores of the heating element in the kettle.

**Washing Machine**A washing machine transfers energy electrically from the mains AC supply to the thermal energy stores of the water (the water becomes hot) and the kinetic energy stores of the motor (the washing machine drum spins).

**Power**

The power of an appliance is the amount of energy that an appliance transfers per second; power is measured in watts (W) where 1 watt is 1 joule of energy per second. A more powerful appliance will transfer more energy per second than a less powerful appliance.

The total amount of energy transferred by an appliance through electrical work depends on the power of the appliance and how long the appliance is switched on for. We can work out energy transfer by using the equation below.

Energy transfer is measured in joules (J), power is measured in watts (W) and time is measured in seconds (s).

Let’s now have a few examples of using this equation.

Let’s now have a few examples of using this equation.

**Example 1**

An 800 W hairdryer is on for 3 minutes. Find the energy transferred whilst the hairdryer is on. Give your answer in kJ.

We find the energy transferred through electrical work by multiplying the power by the time.

Power needs to be in watts and time needs to be in seconds. The question tells us that the power is 800 W, which is in the correct units. We are also told that the hairdryer is on for 3 minutes, which is not in the correct units. There are 60 seconds in 1 minute, so we convert minutes to seconds by multiplying by 60. The calculation is:

The energy transferred is 144,000 joules. The question asks us to give our answer in kilojoules. There are 1,000 joules in 1 kilojoule, so we convert joules to kilojoules by dividing by 1,000.

The energy transferred is 144 kJ.

**Example 2**

A 3 kW kettle transfers 225,000 joules of energy to boil a certain quantity of water. How long is the kettle on for? Give your answer in seconds.

The question is asking us to find out time. When we cover up time in the formula triangle, we can see that we find time by dividing the energy transferred by power.

The energy transferred should be in joules and the power should be in watts. The question tells us that the energy transferred is 225,000 joules, which is in the correct units. We are also told that the power is 3 kW. Power should be in watts not kilowatts. There are 1,000 watts in 1 kilowatt, so we can convert kilowatts to watts by multiplying by 1,000; the power of the kettle is 3,000 W (3 x 1,000). We now have both of the values in the correct units; the energy transferred is 225,000 J and the power is 3,000 W.

The kettle is on for 75 seconds.

**Power Ratings**

All electrical appliances have power rating on them. The power rating tells you the maximum power that an appliance can operate at. Power is the rate of energy transfer, so the maximum power rating for an appliance is the maximum amount of energy that an appliance can transfer between stores per second. For example, a 600-watt washing machine at max power will transfer 600 joules of energy per second (1 watt is 1 joule of energy per second).

However, a more powerful device does not mean that more energy is transferred to the useful energy stores of the device. This is because some of the energy will be transferred to useless/ wasted energy stores, such as the thermal energy stores of the device. Efficiency measures the proportion of the inputted energy that is transferred to the useful output energy stores. So, it may be the case that a more powerful less efficient device will transfer less energy to the useful energy stores compared to a less powerful more efficient device.

A more powerful electrical device will transfer more energy than a less powerful device if both of the devices are on for the same amount of time. Electrical energy costs money, so if a more powerful and a less powerful device are on for the same period of time, a less powerful device will transfer less energy and be cheaper to run.

**Comparing Energy Transfer**

We are now going to work out the energy saved when a dishwasher is run on eco mode compared to when it is run on normal mode. The information for the power and time taken for the cleaning cycle are shown below.

We work out the energy saved by using eco mode by working out the energy transferred for both the normal mode and the eco mode. We then take the energy transferred when in eco mode away from the energy transferred when in normal mode. We work out the energy transferred by multiplying the power in watts by the time in seconds.

Here are the calculations for the two modes.

We now work out the energy saved by taking the energy transferred in eco mode (2,940,000 J) away from the energy transferred in normal mode (3,240,000 J).

The energy saved when the dishwasher is run on eco mode is 300,000 joules (or 300 kJ).