⏰ Minutes to seconds
There are 60 seconds in a minute, so multiply by 60.
15 minutes = 15 × 60 = 900 s
Mains Electricity » Energy Transferred by Appliances
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 2.5
- How to use energy transferred = current × voltage × time
- Why the time must always be in seconds, and how to change minutes and hours
- How to rearrange the equation to find a time, a current or a voltage
- How to compare the energy transferred by different appliances
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Unlock This CourseHow much energy does an appliance transfer?
Leave a phone charger plugged in for a minute and very little happens. Run an electric shower for a minute and a lot of water gets hot. Different appliances transfer very different amounts of energy, and you can calculate exactly how much.
The energy an appliance transfers depends on three things:
- the current in the appliance
- the voltage across the appliance
- the time it is switched on for
Make any one of these bigger and more energy is transferred. Double the time and the energy transferred doubles. That is why an appliance left on all day can transfer a lot of energy, even if its current is small.
Key terms:
- Energy transferred (E): the amount of energy an appliance takes from the electrical supply and passes on, for example by heating or by making something move. It is measured in joules (J).
- Kilojoule (kJ): 1000 joules. Large energies are often written in kilojoules, so 6900 J = 6.9 kJ.
The energy equation
energy transferred = current × voltage × time
E = I × V × t
E is the energy transferred in joules (J), I is the current in amperes (A), V is the voltage in volts (V) and t is the time in seconds (s).
This equation is given on the formulae page of the exam paper. You do not have to memorise it, but you must be able to use it quickly and without mistakes.
Worked example 1: time in seconds
A pair of hair straighteners is connected to the 230 V mains supply. The current in them is 0.5 A. Calculate the energy transferred in 60 s.
E = I × V × t
E = 0.5 × 230 × 60
E = 6900 J
Where the equation comes from
You already know that power = current × voltage, and that power is the energy transferred each second. So I × V tells you the number of joules transferred every second. Multiply by the number of seconds and you get the total energy:
energy transferred = power × time
E = P × t
The two equations are really the same thing, because P = I × V. In worked example 1 the power of the straighteners is 0.5 × 230 = 115 W. That is 115 J each second, so in 60 s they transfer 115 × 60 = 6900 J. Same answer.
This also shows why the time must be in seconds. A watt is a joule each second, so the equation only gives joules when you count the time in seconds.
Changing minutes and hours to seconds
Exam questions often give the time in minutes or hours. Change it before you put any numbers into the equation.
🕒 Hours to seconds
There are 60 × 60 = 3600 seconds in an hour, so multiply by 3600.
2 hours = 2 × 3600 = 7200 s
Worked example 2: time in minutes
An air fryer is used on the 230 V mains. The current in it is 6 A. Calculate the energy transferred when it is switched on for 15 minutes.
t = 15 × 60 = 900 s
E = I × V × t
E = 6 × 230 × 900
E = 1 242 000 J (which is 1242 kJ)
Worked example 3: time in hours
A laptop charger supplies a voltage of 20 V. The current in the laptop is 3 A. Calculate the energy transferred in 2 hours.
t = 2 × 3600 = 7200 s
E = I × V × t
E = 3 × 20 × 7200
E = 432 000 J
Rearranging the equation
Sometimes the question gives you the energy and asks for one of the other three quantities. The rule is simple: divide the energy by the two things you know.
⏰ To find the time
t = E ÷ (I × V)
The answer is in seconds.
⚡ To find the current
I = E ÷ (V × t)
The answer is in amperes.
🔋 To find the voltage
V = E ÷ (I × t)
The answer is in volts.
Work out the bracket first, then divide. On a calculator, type the brackets in or you will get the wrong answer.
Worked example 4: finding the time
An electric fan is used on the 230 V mains. The current in the fan is 0.2 A. Calculate how long the fan takes to transfer 27 600 J of energy. Give your answer in minutes.
E = I × V × t, so t = E ÷ (I × V)
t = 27 600 ÷ (0.2 × 230)
t = 27 600 ÷ 46
t = 600 s
600 ÷ 60 = 10 minutes
Worked example 5: finding the current
A cool box is plugged into a 12 V car battery. It transfers 86 400 J of energy in 30 minutes. Calculate the current in the cool box.
t = 30 × 60 = 1800 s
E = I × V × t, so I = E ÷ (V × t)
I = 86 400 ÷ (12 × 1800)
I = 86 400 ÷ 21 600
I = 4 A
Comparing appliances
Which appliance transfers the most energy? You cannot tell from the current alone. You need the time as well. The table shows four appliances on the 230 V mains, each used in a typical way.
| Appliance | Current | Time switched on | Energy transferred (E = I × V × t) |
|---|---|---|---|
| Electric shower | 40 A | 5 minutes = 300 s | 40 × 230 × 300 = 2 760 000 J |
| Slow cooker | 1 A | 6 hours = 21 600 s | 1 × 230 × 21 600 = 4 968 000 J |
| Games console | 0.5 A | 2 hours = 7200 s | 0.5 × 230 × 7200 = 828 000 J |
| LED lamp | 0.04 A | 5 hours = 18 000 s | 0.04 × 230 × 18 000 = 165 600 J |
The shower has by far the largest current, so it transfers energy fastest. But the slow cooker is on for so long that it transfers more energy in total. The LED lamp has such a small current that it transfers the least energy, even after five hours.
If two appliances are on for the same time at the same voltage, the comparison is easy: the one with the larger current transfers more energy.
Common mistakes
- Leaving the time in minutes. Your answer will be 60 times too small. Change the time to seconds first.
- Multiplying hours by 60. That gives minutes, not seconds. Multiply hours by 3600.
- Forgetting the last step. If the question asks for the time in minutes, divide your answer in seconds by 60.
- Dividing by only one quantity when rearranging. To find the time, divide the energy by current × voltage, not by the current alone.
- Leaving out the unit. Energy is in joules (J).
Exam-style question
A rice cooker is connected to the 230 V mains supply. The current in the rice cooker is 3 A.
(a) Calculate the energy transferred by the rice cooker in 20 minutes. [3 marks]
(b) A phone charger supplies a voltage of 5 V. The current in the phone is 2 A. Calculate the time taken for the charger to transfer 18 000 J of energy to the phone. Give your answer in minutes. [4 marks]
(c) Explain why the rice cooker transfers more energy than the phone charger when both are switched on for the same time. [2 marks]
Model answer
(a) t = 20 × 60 = 1200 s (1)
E = I × V × t = 3 × 230 × 1200 (1)
E = 828 000 J (1)
(b) t = E ÷ (I × V) (1)
t = 18 000 ÷ (2 × 5) (1)
t = 1800 s (1)
1800 ÷ 60 = 30 minutes (1)
(c) The rice cooker has a larger current and a larger voltage than the phone charger (1). Energy transferred = current × voltage × time, so with the same time the rice cooker transfers more energy (1).
Exam tip
In a question like part (a), write the time conversion as its own line of working before you use the equation. Changing 20 minutes to 1200 s is worth a mark by itself, and it stops you putting 20 into the equation by mistake.