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Edexcel International GCSE Physics · Section 2 Electricity · 2 (b) Mains electricity
Power and Energy in Circuits
In this lesson you'll learn to: use P = I × V to find the power, use E = I × V × t to find the
energy transferred, take the three readings you need with an ammeter, a voltmeter and a stop clock,
explain why adding a parallel branch takes more energy from the cell, and use a power rating with
E = P × t.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
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Start here — have a play
Don't read anything yet. On tab 1, switch the circuit on, stop the clock after a few seconds, and use the three readings to work out the energy. On tab 2, add a second lamp or a motor in parallel: which energy goes up, and which stays the same? On tab 3, read a kettle's rating plate and find the energy. Press Test yourself to hide the answers.
Cell
Ammeter · I
0 A
Voltmeter · V
0 V
Stop clock · t
0.0 s
Power · P = I × V
0 W
Energy · E = I × V × t
0 J
J
Switch on, stop the clock, then use your three readings.
The method: the ammeter goes in series (it measures the current through the lamp), the voltmeter goes in parallel across the lamp (it measures the p.d. across it), and the stop clock times how long the switch is on. Three readings, I, V and t, give you the energy: E = I × V × t.
Branch 2
The rule: in parallel, every branch gets the full p.d. of the cell. So lamp 1 keeps the same current and the same voltage: it gets the same energy as before. The new branch draws an extra current, so the current from the cell goes up and more energy is transferred from the cell every second.
Appliance
Switched on for
Power · P
0 W
Time · t
0 s
Energy · E = P × t
0 J
J
Read the rating plate, then work out the energy.
The shortcut: the rating plate gives you the power, so you don't need I and V: E = P × t. It is the same equation, because P = I × V. Two traps: kW into W (× 1000) and minutes into seconds (× 60) before you multiply.
▲ The cell is ideal: the full p.d. reaches every branch, however many you add.
Two equations, one idea
Power is the energy transferred every second. The notes show both equations, the units, how to measure each reading, and the two conversions that cost marks.
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Revision notes
The two equations
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2.4 Power: P = I × V
power = current × voltage
Power is the energy transferred every second. 1 W = 1 J/s.🔋
2.5 Energy: E = I × V × t
energy transferred = current × voltage × time
Since P = I × V, this is the same as E = P × t.| Quantity | Symbol | Unit | Watch out for |
|---|---|---|---|
| Power | P | watt (W) | kW: × 1000 to get W |
| Current | I | ampere (A) | mA: ÷ 1000 to get A |
| Voltage (p.d.) | V | volt (V) | mains is 230 V |
| Time | t | second (s) | minutes: × 60 · hours: × 3600 |
| Energy transferred | E | joule (J) | kJ: ÷ 1000 from J |
Measuring it: three readings
⭐ What “power” really means
A 3 W lamp transfers 3 joules every second. Leave it on for 10 s and it transfers 3 × 10 = 30 J. That is all E = P × t says. Power tells you how fast the energy is transferred; energy tells you how much in total.
Adding a branch in parallel
Add a second lamp (or a motor) on its own branch, in parallel with the first:
- Each branch gets the full p.d. of the cell, so lamp 1 keeps the same current and the same voltage. It transfers the same energy as before and is just as bright.
- The new branch draws an extra current. The currents add up at the junction, so the current from the cell increases.
- E = I × V × t with a bigger I: more energy is transferred from the cell every second, so the cell goes flat sooner.
Power ratings: E = P × t
An appliance's rating plate usually gives its power, for example a kettle: 230 V, 2.2 kW. Then you don't need the current at all: E = P × t. If the plate gives the current instead (a hair dryer: 230 V, 9.6 A), work out P = I × V first.
⚠ The two conversions that cost marks
kW into W: 2.2 kW = 2.2 × 1000 = 2200 W.
Minutes into seconds: 5.0 minutes = 5.0 × 60 = 300 s. Do this before you multiply.
A kettle of 2200 W on for 3 minutes: E = 2200 × 180 = 396 000 J (396 kJ). Using 3 instead of 180 gives 6600 J, which is wrong by a factor of 60.
How to write it in the exam
Every calculation, in the same four lines
Equation (E = I × V × t) → change units (min → s, kW → W)
→ substitute the numbers → answer + unit (J or W).
the equation, written first units, then numbers answer with its unit
These are the questions from page 6 of the workbook.
Make up your own example · Choose a current, a voltage and a time. Calculate the power and the energy transferred.
A 3.0 A device on a 230 V supply, used for 60 s:
P = I × V = 3.0 × 230 = 690 W
E = P × t = 690 × 60 = 41 400 J
Exam-style (i) · A hair dryer has a current of 9.6 A. The mains supply voltage is 230 V. State the formula linking power, current and voltage. [1]
power = current × voltage (P = I × V)
Exam-style (ii) · Calculate the power of the hair dryer. Give the unit. [3]
- P = 9.6 × 230 (1)
- = 2200 W (1) (2208 W or 2.2 kW also score)
- unit: watts (W) (1) (J/s is allowed)
Exam-style (iii) · The hair dryer is used for 5.0 minutes. Calculate the energy transferred. Give the unit. [3]
- t = 5.0 × 60 = 300 s; E = I × V × t = 9.6 × 230 × 300 (1) (or E = P × t = 2208 × 300)
- = 662 400 J (1) (about 6.6 × 10⁵ J or 660 kJ)
- unit: joules (J) (1)
Where marks go missing: using t = 5.0 gives 11 040 J. Always change minutes into seconds first.
✅ Check your answer
Did you write the equation first? Is the time in seconds and the power in watts? Does your answer have a unit: W for power, J for energy? The unit mark is given on its own, so never leave it out.
📘 Now do it in your workbook
Write both formulae, make up your own example with a current, a voltage and a time, then answer the 7-mark hair dryer question.
Edexcel iGCSE Physics Workbook · ELECTRICITY · page 6Free sample = the page for this lesson. The full workbook covers the whole of Section 2 Electricity, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
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Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Page 6 of the Electricity workbook is free: both formulae, your own example and the 7-mark hair dryer question.
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