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Connect energy transfers to work, power and efficiency with three worked examples and 12 original questions.
Original AI-assisted Readnary material. Not independently academically reviewed; answers may contain errors. Check important results against your course materials.
Selected content from Physics 0625 section 1.7 for 2026-2028 exams. Calculations use straight-line force and displacement, energy over time and simple useful-to-input ratios; energy resources and full practical coverage are outside this guide.
| Idea | Equation | Unit |
|---|---|---|
| Work done | force x distance moved in force direction | joule (J) |
| Power | energy transferred / time | watt (W) |
| Efficiency | useful energy output / total energy input x 100% | percent (%) |
Energy can be transferred between stores and surroundings; it is conserved overall. A lifting motor transfers energy from an electrical supply to a raised object's gravitational store. Some energy is also transferred to the surroundings by heating and sound. Calling this energy 'lost' can be useful shorthand for unavailable useful energy, but it has not vanished. Always name which output the question counts as useful before calculating an efficiency.
Mechanical work is energy transferred when a force moves an object through a distance in the direction of that force. For a constant force along the motion, work done = force x distance, with newton-metres equal to joules. A 10 N push through 3 m does 30 J of work. Do not multiply by a distance perpendicular to the force. A person holding a stationary box may become tired, but the box has zero displacement and this simple mechanical-work calculation gives zero work on the box.
Power describes how quickly energy is transferred or work is done. Divide joules by seconds to get watts: 1 W = 1 J/s. Two motors can do the same total work while one has greater power if it finishes sooner. Convert minutes to seconds before using the equation. Kilowatts are thousands of watts; keep the unit consistent with the energy and time values in a question.
Efficiency is useful output divided by total input. It may be written as a decimal or multiplied by 100 for a percentage. For a device receiving 500 J and delivering 350 J of specified useful output, efficiency is 70%; 150 J goes to other stores or surroundings. In a single-process energy account, output totals should equal input, and useful output cannot exceed input. Power ratios also work when useful output and input powers refer to the same operating conditions.
A steady 45 N horizontal force moves a trolley 8 m horizontally. Find work done by that force.
A machine transfers 2400 J in 20 s. Calculate average power.
A lamp receives 200 J of electrical energy and delivers 30 J as useful light. Find efficiency and other energy transfers.
Try each question before opening its answer. Numerical answers should include your working.
W = 25 x 4 = 100 J.
W = 60 x 0.5 = 30 J.
Zero joules in this force-times-displacement model because the box's displacement is zero.
P = 900 / 15 = 60 W.
E = P x t = 50 x 8 = 400 J.
2 minutes = 120 s, so P = 3600 / 120 = 30 W.
A: 600 / 5 = 120 W; B: 600 / 10 = 60 W. A has twice the average power.
52 / 80 x 100% = 65%.
500 - 425 = 75 J. Conservation accounts for both useful and other transfers.
0.25 x 400 = 100 J useful output.
150 / 200 x 100% = 75%, assuming powers refer to the same operating interval.
Useful output would exceed total input, contrary to energy conservation; check whether an input or unit was omitted.
Work and power are different: joules measure energy transferred; watts measure joules per second. Do not report a raw ratio such as 0.6 as 0.6%: it is 60%. Energy transferred to heating still exists.
Physics · IGCSE · STUDY GUIDE
Connect energy transfers to work, power and efficiency with three worked examples and 12 original questions.
Read the original Readnary guide in PDF view or use the web lesson above. Try the questions before revealing their explained answers.