Energy and particles · GCSE Physics
Power and work
Teacher-written GCSE Physics revision on power and work: W = Fs, P = E/t and P = W/t, with watts as joules per second, and the substitutions examiners expect on Combined and Triple papers.
Work is the energy transferred mechanically: W = Fs. Power is how fast that transfer happens: P = E/t, in watts. One watt is one joule per second.
The important bits
What you need to know
- 1
Work done W = Fs when the force is along the displacement. W is in joules (J), F in newtons (N), s in metres (m). One joule is one newton-metre.
- 2
If the force is at an angle, only the component along the motion does work. At GCSE, most questions keep force and distance in the same direction.
- 3
Power P = E / t = W / t. Units: watts (W). 1 W = 1 J/s. A kilowatt is 1000 W. Time must be in seconds unless you are using kilowatt-hours.
- 4
A more powerful device transfers the same energy in less time, or more energy in the same time. Power is a rate, not a store.
- 5
Electrical work also uses P = VI, and energy E = Pt = VIt. A 2.0 kW kettle transfers 2000 J every second.
- 6
Kilowatt-hours are for bills: energy (kWh) = power (kW) × time (h). That unit is not for mechanics questions that ask for joules.
- 7
Lifting at steady speed, work done against gravity is W = mgh, which is the same as Fs with F = mg and s = h.
- 8
Rearrange before you substitute: t = E/P, E = Pt, F = W/s. Write the quantity, the unit, then the number.
Quotations worth analysing
Short evidence. Real method.
“Power is the rate at which energy is transferred.”
Rate means divide by time. Leaving time in minutes is the classic unit error: 2 minutes is 120 s, not 2 s.
“Work done = force × distance”
Distance must be along the force. Pushing a wall that does not move does no work on the wall, however tired you feel.
Go deeper
Work is a transfer, not a store you can “have”
Saying a person “has 400 J of work” is the wrong model. Work is the mechanical pathway: a force moves a charge of energy from one store to another. Push a 50 N crate 3.0 m and W = 50 × 3.0 = 150 J leaves your chemical store and arrives in the kinetic or thermal store of the crate and floor. If the crate does not move, s = 0 and W = 0, even if you are straining. Power then tells you how quickly that 150 J was transferred. Do it in 2.0 s and P = 150 / 2.0 = 75 W. Do it in 0.50 s and P = 300 W. Same work, different power. That distinction is what the exam is for.
Go deeper
Seconds, watts and kilowatt-hours must not be mixed
P = E/t needs seconds if E is in joules and P is in watts. A 1500 W heater left on for 10 minutes transfers E = 1500 × 600 = 900 000 J. For a bill, convert first: 1.5 kW × (10/60) h = 0.25 kWh. Students multiply 1500 by 10 and call it joules, or multiply kilowatts by seconds and call it kilowatt-hours. Write the unit next to every number. Electrical questions may also give V and I: P = VI, then E = Pt. A 12 V, 2.0 A motor has P = 24 W, so in 30 s it transfers 720 J. Check that the size of the number matches the story: hundreds of watts for a person sprinting, kilowatts for a kettle, megawatts for a power station.
See the idea in action
A student pulls a 30 N sledge 12 m in 8.0 s along the snow. Work done W = Fs = 30 × 12 = 360 J. Power P = W/t = 360 / 8.0 = 45 W. If the same work is done in 4.0 s, power doubles to 90 W because the rate has doubled. Checking units: N × m = J, and J / s = W. If the time had been left as 8 minutes, the power would have been 48 times too small.
Exam technique
Turn knowledge into marks
Write W = Fs or P = E/t, convert minutes to seconds, substitute with units, then calculate. For lifts at steady speed, W = mgh is the same mechanical work against gravity.
Common mistakes
Do not give these marks away
- 01
Leaving time in minutes in P = E/t, or mixing kilowatt-hours with joules.
- 02
Using mass in grams when calculating mgh, or quoting power in joules.
- 03
Saying work is done when a force is applied but nothing moves.
A 1500 W kettle is switched on for 2.0 minutes. How much energy is transferred from the mains?
A3000 J
B750 J
C180 000 J
D1.5 J
Show the answer
180 000 J. Time must be in seconds: 2.0 × 60 = 120 s. E = Pt = 1500 × 120 = 180 000 J. Using 2.0 as the time is the standard trap.
Quick questions
If this is the bit you searched
What is the difference between energy and power?
Energy is the quantity transferred, measured in joules. Power is how fast that transfer happens, in watts: P = E/t.
When is W = Fs the right equation?
When a force moves an object along the direction of the force. That mechanical work equals the energy transferred.
How do I convert minutes for a power calculation?
Multiply by 60 to get seconds if energy is in joules and power is in watts. Ten minutes is 600 s, not 10 s.
What is a kilowatt-hour?
A unit of energy used on electricity bills: 1 kWh = 1000 W for 3600 s = 3 600 000 J. Use kWh = kW × hours, not for mechanics in joules.