Edexcel IGCSE Physics · Spec 2.13-2.16
Electrical Energy & Power
Electrical energy and power in circuits.
Physics revision video
Electrical Energy & Power
Prefer to watch on YouTube? Open this video on YouTube.
Explained
Electrical power, and three ways to calculate it
Power is the rate of energy transfer: the energy moved each second. It is measured in watts, and one watt is one joule per second.
A 60 W lamp transfers 60 joules every second it is switched on. A 2000 W kettle transfers 2000 joules a second, which is why it boils water quickly and why it costs more to run.
The equations
Power equals current multiplied by potential difference, P equals I times V. Watts, amperes and volts.
Energy transferred equals power multiplied by time, E equals P times t. Joules, watts and seconds.
Combining them gives energy transferred equals current times potential difference times time, which is the one to use when a question gives a current and a voltage and asks for the energy over a period.
You can also reach power from resistance, since V equals I times R, giving power equals current squared times resistance. That form explains why the current, rather than the voltage, is what heats a cable.
Choosing a fuse
Rearrange P equals I times V to give current equals power divided by voltage, then pick the next standard fuse rating above the answer.
A 920 W appliance on 230 V draws 4 A, so a 5 A fuse. A 3 A fuse would blow in normal use and a 13 A fuse would let a dangerous current flow before acting.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme awards a mark simply for stating that power equals current times voltage, allowing standard symbols and any rearrangement such as I equals P over V. Its note then says to ignore a lower case c written for current.
Writing the equation before you touch the numbers is therefore worth a mark on its own, even on a question that only asks for a value.
On a longer efficiency question the same series shows how these build. It awards five marks in sequence: using power equals current times voltage, evaluating the input power, evaluating the output power, substituting into the efficiency formula, and evaluating the efficiency. An error carried forward is allowed from the earlier parts, and from incorrect power values if the working makes them clear.
Five marks, and only the last is the answer. A candidate who calculates both powers correctly and then mishandles the final division still has four of them, provided every step is written down.
Kilowatt hours
Electricity meters do not measure joules, because a joule is far too small for domestic use. They measure kilowatt hours.
One kilowatt hour is the energy transferred by a 1 kW appliance running for one hour. To find it, multiply the power in kilowatts by the time in hours, which means dividing watts by 1000 and leaving the time in hours rather than seconds.
A 2 kW heater for 3 hours uses 6 kWh. Multiply by the price per unit and you have the cost.
Take care with the units in this one calculation, because they are deliberately not the SI ones. Every other energy calculation in the course uses joules and seconds; this one uses kilowatts and hours, and mixing the two conventions is the standard mistake.
Why transmission uses high voltage
Power is current times voltage, so the same power can be delivered as a large current at low voltage or a small current at high voltage.
The energy wasted heating the cables depends on the current squared, so halving the current cuts the loss to a quarter. Transmitting at hundreds of thousands of volts keeps the current small and the losses low, which is the whole reason the national grid uses transformers.
Spec 2.13-2.16
What you need to know
- Describe electrical power
- Use power equals current times voltage
- Use the energy equations
Active recall
Quick check
Answer each question before opening the answer.
Give two equations for electrical power.
P = I × V and P = I² × R.
What is the equation for electrical energy transferred?
E = I × V × t (or E = P × t).
Physics revision app
Take this topic further in the app
275 guided topics with questions marked as you answer them, section checkpoints and timed practice papers. Fifteen topics are free to try, with no card needed.