Practise Energy transfers in everyday appliances. 12 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How everyday appliances transfer energy, and how to calculate the energy transferred using E = P t and E = Q V. Expect questions on the energy transfers in named appliances (motors and heaters) and 2 to 4 mark calculations with unit conversions.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Describe energy transfers in simple appliancese.g. a kettle transfers energy electrically from the mains to the thermal energy store of the water.
4
State what affects the energy transferredThe power of the appliance and how long it is switched on for.
4
Calculate energy using E = P tPower in watts × time in seconds gives energy in joules.
5
Calculate energy using E = Q VCharge in coulombs × pd in volts gives energy in joules.
6
Relate power ratings to energy transferredAn appliance with a higher power rating transfers more energy each second.
7
Combine E = Q V with Q = I tFind the charge first, then the energy, in multi-step problems.
Notes
Energy transfers in appliances
Everyday electrical appliances are designed to bring about energy transfers.
Energy is transferred electrically (by electrical work) from batteries or the ac mains.
Appliances with an electric motor (e.g. a fan, a drill or a food mixer) transfer energy to the kinetic energy store of the motor.
Heating devices (e.g. a kettle, a toaster or an electric heater) transfer energy to the thermal energy store of the heating element and then of whatever is being heated.
A battery-powered device transfers energy from the chemical energy store of the battery.
Some energy is usually wasted, e.g. transferred to the thermal energy store of the surroundings.
Power ratings
The energy an appliance transfers depends on its power and on how long it is switched on for.
The power rating is the energy transferred each second in normal use: a 2000 W kettle transfers 2000 J every second.
A higher-power appliance on for the same time transfers more energy, so its energy stores change by more. For example, a 3 kW kettle can heat the same water faster than a 2 kW kettle.
Calculating energy transferred
energy transferred = power × time: E = P t (E in J, P in W, t in s).
Work is done when charge flows in a circuit. energy transferred = charge flow × potential difference: E = Q V (E in J, Q in C, V in V).
So a pd of 1 volt means 1 joule of energy is transferred for each coulomb of charge that flows.
Multi-step: find the charge with Q = I t, then use E = Q V. Or find the power with P = V I, then use E = P t. Both give the same answer. grade 7+
Cheatsheet
E = P t: energy (J) = power (W) × time (s)
E = Q V: energy (J) = charge flow (C) × pd (V)
1 V = 1 J of energy transferred per coulomb of charge
Motor: energy to the kinetic store; heater: energy to the thermal store
Energy transferred depends on the power and the time switched on
1 kW = 1000 W; 1 kJ = 1000 J; 1 min = 60 s; 1 h = 3600 s
How to answer each type of question
Describe the energy transfer in an appliance
2 marks4
Say where the energy comes from: the mains (transferred electrically) or the chemical store of a battery.
Say which store it goes to: kinetic (motor) or thermal (heater).
Mention wasted energy to the surroundings if the question asks about it.
Example. A cordless drill is powered by a rechargeable battery. Describe the energy transfer when the drill is being used.
Show the model answer
Energy is transferred electrically from the chemical energy store of the battery (1) to the kinetic energy store of the motor (and drill bit) (1).
Calculate energy using E = P t
3 marks5
Convert the power to watts and the time to seconds.
Write E = P t and substitute.
Give the answer in joules (or kJ if asked).
Example. A 1.5 kW microwave oven is used for 3.0 minutes. Calculate the energy transferred by the microwave oven.
Show the model answer
P = 1500 W and t = 180 s (1) E = 1500 × 180 (1) E = 270 000 J (1)
Calculate energy using E = Q V
2 marks5
Write E = Q V.
Substitute the charge in coulombs and the pd in volts.
Give the answer in joules.
Example. A charge of 400 C flows through a lamp. The potential difference across the lamp is 12 V. Calculate the energy transferred by the lamp.
Show the model answer
E = 400 × 12 (1) E = 4800 J (1)
Multi-step energy calculation
4 marks7
Convert the time to seconds.
Find the charge with Q = I t (or the power with P = V I).
Use E = Q V (or E = P t) to find the energy.
Example. A 6.0 V battery supplies a current of 0.50 A to a motor for 2.0 minutes. Calculate the energy transferred to the motor.
Show the model answer
t = 2.0 × 60 = 120 s (1) Q = I t = 0.50 × 120 = 60 C (1) E = Q V = 60 × 6.0 (1) E = 360 J (1)
Shortcuts and memory tricks
E = P t is the definition of power turned around: power is energy per second, so energy = power × seconds.
Remember what a volt is: joules per coulomb. So joules = coulombs × volts, E = Q V.
E = P t, E = Q V, P = V I and Q = I t all fit together: E = V I t.
Where marks are lost
Using minutes or hours in E = P t without converting to seconds.
Using kW instead of W, giving an answer 1000 times too small.
Writing that a heater transfers energy 'as heat' or 'as electricity'. Say energy is transferred electrically to the thermal energy store.
Mixing up the letters: in E = Q V, Q is the charge (not the current).
Exam technique
Name energy stores correctly: chemical (battery), kinetic (motor), thermal (heater and surroundings).
E = P t and E = Q V must be learned: they are not on the equation sheet.
If a question uses kilowatt-hours (kWh), it will tell you how to work them out. Follow its method and units exactly.
Large answers can be given in kJ or standard form, but check the unit matches the number.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Write down the equation that links charge flow (Q), energy transferred (E) and potential difference (V).
E = Q V
Write down the equation that links energy transferred (E), power (P) and time (t).
E = P t
A 100 W lamp is switched on for 10 s. Calculate the energy transferred.
1000 J
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Everyday electrical appliances are designed to bring about energy transfers.
(a) An electric kettle has a power of 2000 W. It is switched on for 180 s. Calculate the energy transferred by the kettle. Use the equation: energy transferred = power × time[2]
(b) An electric fan is switched on. Which energy store of the fan blades increases? Tick (✓) one box.[1]
chemical
elastic potential
kinetic
nuclear
(c) The energy transferred by an appliance depends on the power of the appliance and on one other factor. What is the other factor? Tick (✓) one box.[1]
how long the appliance is switched on for
the length of the cable
the mass of the appliance
the colour of the appliance
Show the answer and mark scheme
(a)Answer: 360 000 J
E = 2000 × 180
E = 360 000 (J)
(b)Answer: kinetic
(c)Answer: how long the appliance is switched on for
Question 2Medium7 marks
Energy is transferred when charge flows through electrical appliances.
(a) Write down the equation that links charge flow (Q), energy transferred (E) and potential difference (V).[1]
(b) A charge of 5.0 C flows through a motor. The potential difference across the motor is 12 V. Calculate the energy transferred.[2]
(c) An electric shower has a power of 9.0 kW. It is used for 20 minutes each day. The energy transferred in kilowatt-hours (kWh) is the power in kilowatts multiplied by the time in hours. Calculate the energy transferred by the shower each day in kilowatt-hours.[2]
(d) Electricity costs 28p per kWh. Calculate the cost of using the shower for 7 days.[2]
Show the answer and mark scheme
(a)Answer: E = Q V
E = Q × V / energy transferred = charge flow × potential difference
(b)Answer: 60 J
E = 5.0 × 12
E = 60 (J)
(c)Answer: 3.0 kWh
E = 9.0 × (20 ÷ 60) / 9.0 × 0.333
E = 3.0 (kWh)
(d)Answer: 588p (£5.88) p
3.0 × 7 × 28
588 (p) / £5.88
Question 3Hard6 marks
A car battery has a potential difference of 12 V. It is charged with a constant current of 8.0 A for 5.0 hours.
(a) Calculate the charge that flows into the battery while it is being charged.[2]
(b) Calculate the energy transferred to the battery.[2]
(c) The charger is 75% efficient. Calculate the total energy drawn from the mains to charge the battery.[2]