Energy transfers in everyday appliances — AQA GCSE Physics
Test yourself on Energy transfers in everyday appliances with AQA GCSE practice questions.
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Energy transfers in everyday appliances explained
An appliance is a designed pathway: it takes energy supplied electrically and shifts it into whichever store the user wants.
Read the full explanation
A kettle moves energy to the thermal store of the water, an electric drill to the kinetic store of the rotating bit, and a lamp transfers it by light and by heating. The source is the chemical store of a battery or the mains supply, and the transfer happens because charge does work as it passes through the device. Almost no appliance transfers everything usefully - the exception is a resistive heater, whose useful store is the thermal store of the room, so it is treated as (near) 100% efficient. In every other appliance some energy is dissipated to the thermal store of the surroundings by resistance in the wires and friction in moving parts. Designers cut those losses, by insulating a kettle or oiling a motor, raising the efficiency but not to 100%.
The amount of energy an appliance transfers depends on how long the appliance is switched on for and the power of the appliance.
The energy transferred is the power multiplied by the time, E = P t, so the two factors matter equally: a high power appliance used briefly can transfer more energy than a low power one left on for hours. A 2000 W kettle boiling for 180 s transfers 2000 × 180 = 360 000 J. A 10 W phone charger left on for five hours, 18 000 s, transfers only 10 × 18 000 = 180 000 J, half as much. To use the equation the power must be in watts and the time in seconds, so convert kilowatts and minutes first. Energy bills use the kilowatt-hour, which is power in kW multiplied by time in hours, but every exam calculation works in joules unless you are told otherwise.
Students should be able to describe how different domestic appliances transfer energy from batteries or ac mains to the kinetic energy of electric motors or the energy of heating devices.
Domestic appliances fall into two families. In a motor appliance, such as a fan, a drill or a washing machine drum, a current in a coil sitting in a magnetic field makes the coil turn, so energy is transferred electrically to the kinetic store of the moving part. In a heating appliance, such as a kettle, a toaster or an iron, the current passes through a high resistance element and collisions between the moving charges and the metal ions transfer energy to the thermal store of the element, and then to the food, water or air. Some appliances do both: a hairdryer holds a motor driving a fan and a heating element in one casing. The supply is either the chemical store of a battery, giving a few volts of dc, or the 230 V ac mains.
Work is done when charge flows in a circuit.
Work done means energy transferred by a force, and in a circuit the source does work pushing charge round the loop while the charge does work as it passes through each component. The bookkeeping is E = Q V, where the potential difference is the energy transferred per coulomb: a 1 V potential difference means 1 joule is transferred for every coulomb that passes. Moving 2 C through a 6 V lamp therefore transfers 2 × 6 = 12 J. Because charge flow is Q = I t, the equations combine: E = Q V = I t V = P t, which is why the circuit energy and power equations always agree. Work is only done while charge is actually moving, which is why an appliance transfers nothing while its switch is open.
The amount of energy transferred by electrical work can be calculated using the equation: energy trans f erred = power × time E = P t energy trans f erred = charge f low × potential di f f erence E = Q V energy transferred, E, in joules, J power, P, in watts, W time, t, in seconds, s charge flow, Q, in coulombs, C potential difference, V, in volts, V Students should be able to explain how the power of a circuit device is related to: the potential difference across it and the current through it the energy transferred over a given time.
Two equations do this job and they have to agree. E = P t uses the power rating and the running time, while E = Q V uses the charge that has passed and the potential difference it passed through. They connect through P = V I and Q = I t, because P t = V I t = V Q. Take a 230 V appliance drawing 2.0 A for 60 s. Its power is 230 × 2.0 = 460 W, so E = 460 × 60 = 27 600 J. Working the other way, the charge is Q = 2.0 × 60 = 120 C, and E = 120 × 230 = 27 600 J, the same answer. Pick the route that matches the data, keep power in watts, time in seconds and charge in coulombs, and the two methods always agree. Power is simply that energy divided by the time.
Students should be able to describe, with examples, the relationship between the power ratings for domestic electrical appliances and the changes in stored energy when they are in use.
The power rating printed on an appliance is the energy it transfers each second, in joules per second. An LED lamp near 10 W, a television near 100 W, a kettle near 2 000 W and an electric shower near 9 000 W are the numbers worth carrying into the exam. A larger rating means the stored energy changes faster: a 2 000 W kettle raises the thermal store of the water two hundred times faster than a 10 W lamp lights a room. To compare appliances properly you must combine the rating with the running time using E = P t, because a low power device left on all day can change stores by more than a high power device used for two minutes. Heating appliances carry the highest ratings, because raising temperature needs a great deal of energy.
Your focus
- Name the starting store, the pathway and the useful end store for a named appliance such as a kettle or a drill.
- Identify the wasted energy transfer in a given appliance and say where that energy ends up.
- Explain one design change that increases the useful share of the energy an appliance transfers.
Show all 18 objectives
- Calculate the energy transferred by an appliance from its power rating and its running time in seconds.
- Compare the total energy transferred by two appliances of different power used for different times.
- Convert a power in kilowatts and a time in minutes into the units required by E = P t.
- Describe the energy transfer in a named motor appliance from the supply to the kinetic store.
- Explain how a heating element raises the temperature of its surroundings, in terms of resistance and the thermal store.
- Identify both transfers taking place in an appliance that contains a motor and a heater.
- Calculate the energy transferred when a stated charge passes through a stated potential difference.
- Combine Q = I t with E = Q V to find the energy transferred from a current and a time.
- Explain what a potential difference of 1 V means in terms of joules and coulombs.
- Calculate the energy transferred by an appliance using E = P t with the time converted to seconds.
- Calculate the same energy by the charge route, finding Q = I t and then using E = Q V.
- Explain how the power of a circuit device depends on the potential difference across it and the current through it.
- State what the power rating on an appliance label says about the energy it transfers each second.
- Compare two appliances by calculating the energy each transfers over its own realistic running time.
- Name the store that changes when a named appliance is used and say what causes that change.
Energy transfers in everyday appliances exam tips
Marking Points
- one mark for naming the store the energy is transferred from, such as the chemical store of the battery
- one mark for naming the useful store it is transferred to, such as a thermal or kinetic store
- one mark for naming the pathway, electrical work, heating or light
- one mark for identifying energy dissipated to the thermal store of the surroundings as the wasted share
- one mark for selecting E = P t
- one mark for converting the time to seconds and the power to watts before substituting
- one mark for correct substitution of both values
- one mark for the energy transferred with the unit joules, J
- one mark for naming the supply as the chemical store of a battery or as the ac mains
- one mark for electrical work as the pathway from the supply to the appliance
- one mark for the kinetic store of the motor as the useful end store in a motor appliance
- one mark for the thermal store of the heating element as the useful end store in a heater
- one mark for selecting E = Q V, energy transferred equals charge flow multiplied by potential difference
- one mark for using Q = I t to find the charge when the current and time are given
- one mark for correct substitution into the chosen equation
- one mark for the energy transferred with the unit joules, J
- one mark for selecting the equation that matches the data, E = P t or E = Q V
- one mark for a correct intermediate step such as P = V I or Q = I t
- one mark for correct substitution with the time in seconds
- one mark for the energy transferred with the unit joules, J
- one mark for the power rating stated as the energy transferred each second
- one mark for a sensible power rating quoted for a named appliance
- one mark for naming the store that changes, such as the thermal store of the water in a kettle
- one mark for combining the rating with a running time using E = P t to compare two appliances
Examiner Tips
- 💡Name the store at each end and the pathway between them; that structure earns the marks.
- 💡Use 'dissipated to the surroundings' as the standard phrase for wasted energy.
- 💡Match the useful store to the job of the appliance, since the useful output of a kettle is not light.
- 💡Underline the time in the question and convert it to seconds before writing anything else.
- 💡When comparing two appliances, calculate both energies in full rather than arguing from the power ratings.
- 💡Write large answers in standard form, or with a space every three digits, so the magnitude is readable.
- 💡Decide first whether the appliance is a motor device, a heating device or both, then name the store to match.
- 💡Keep 'chemical store' for the battery and 'electrical work' for the transfer; the two are not interchangeable.
- 💡A three-mark description usually wants supply, pathway and useful store, one mark each.
- 💡If the question gives current and time but the equation needs charge, do Q = I t on its own line first.
- 💡Define a volt as a joule per coulomb when asked to explain what potential difference means.
- 💡Check that both routes agree, since E = Q V and E = P t must give the same number for the same situation.
- 💡List the quantities you have been given before choosing an equation, because the data shows which route is intended.
- 💡Both equations are on the equation sheet, so check the printed form rather than recalling it under pressure.
- 💡Keep full accuracy through the intermediate steps and round only the final answer.
- 💡Learn one plausible rating for a lamp, a television, a kettle and a shower so you can always quote an example.
- 💡'With examples' in a question means at least one named appliance with a number attached.
- 💡Say which store changes and in which direction, because 'energy increases' alone is not enough.
Common Mistakes
- writing 'electrical energy' as a store instead of naming electrical work as the pathway
- saying energy is lost rather than dissipated to the surroundings
- describing an appliance as converting energy into a new type rather than transferring it between stores
- saying a kettle is 100% efficient because it is a heater, ignoring the energy transferred to the kettle body and the surroundings
- leaving the time in minutes or hours, so the answer is 60 or 3600 times too small
- leaving the power in kilowatts while reporting an answer in joules
- assuming the appliance with the larger power rating always transfers more energy whatever the running time
- adding power and time instead of multiplying them
- saying a heating element works because electricity turns into heat, without naming resistance or the thermal store
- treating a hairdryer as only a heater and ignoring its motor
- describing the mains as a store of energy in the same way as a battery
- naming the useful store of a washing machine as thermal when the question is about the drum turning
- treating potential difference as an energy rather than as energy per coulomb
- putting the time into the wrong equation, for example writing E = Q V t
- leaving the current in the final answer because Q = I t was never applied
- saying no work is done in a resistor because nothing visibly moves
- mixing the two routes, for example multiplying power by charge
- using minutes for the time in E = P t
- quoting the power of a device as its energy transfer without multiplying by time
- rounding an intermediate power to one significant figure and carrying that error into the final answer
- describing a power rating as the total energy an appliance uses
- quoting a kettle as 2 W or an LED lamp as 2 000 W because the magnitudes have not been learned
- comparing appliances by power rating alone when their running times differ
- saying the energy of the appliance increases rather than naming the store that changes