Work done and energy transfers (P5.1)
Energy is the cross-cutting concept in GCSE Physics. Every OCR Gateway A Physics paper has at least one calculation on kinetic or gravitational potential energy, and a question on efficiency.
Energy stores
Energy is stored in different ways and transferred between stores:
- Kinetic energy (KE) — energy due to motion.
- Gravitational potential energy (GPE) — energy due to height in a gravitational field.
- Chemical energy — energy stored in chemical bonds (e.g. food, fuel, batteries).
- Thermal energy — internal energy of particles (related to temperature).
- Elastic potential energy — energy stored in stretched/compressed materials.
- Nuclear energy — stored in atomic nuclei; released by nuclear reactions.
- Electromagnetic energy — energy carried by light and other EM waves.
Energy transfer mechanisms
Energy is transferred by:
- Mechanically — forces doing work (e.g. pushing a box).
- Electrically — charge flow (current).
- By radiation — electromagnetic waves (e.g. light, infrared, radio).
- By heating — conduction, convection, radiation.
Work done
Work is done when a force causes an object to move in the direction of the force.
Formula:
W = F × d
Where:
- W = work done (joules, J)
- F = force applied (newtons, N)
- d = distance moved in the direction of the force (metres, m)
1 joule = 1 newton × 1 metre.
Work done = energy transferred to the object. If you push a box 5 m with a 20 N force: W = 20 × 5 = 100 J of energy is transferred.
⚠ Only the component of force in the direction of motion counts. Force perpendicular to motion does no work.
Kinetic energy (KE)
KE = ½ × m × v²
Where:
- KE = kinetic energy (J)
- m = mass (kg)
- v = speed (m/s)
Speed is squared — doubling speed quadruples KE. This is why braking distances increase rapidly with speed.
Worked example: What is the KE of a 1,000 kg car travelling at 20 m/s?
- KE = 0.5 × 1,000 × 20² = 0.5 × 1,000 × 400 = 200,000 J (200 kJ)
Gravitational potential energy (GPE)
GPE = m × g × h
Where:
- GPE = gravitational potential energy (J)
- m = mass (kg)
- g = gravitational field strength (N/kg) — on Earth: 9.8 N/kg (sometimes rounded to 10)
- h = height above reference point (m)
Conservation of energy: When an object falls freely (no air resistance):
GPE lost = KE gained
m × g × h = ½ × m × v²
Worked example: A 2 kg ball falls from a height of 5 m. Find its speed just before impact.
- GPE = 2 × 9.8 × 5 = 98 J
- 98 = ½ × 2 × v² → v² = 98 → v = 9.9 m/s (√98)
Efficiency
All real energy transfers involve some energy dissipated as thermal energy (heating the surroundings) — this is "wasted" energy that cannot be used usefully.
efficiency = useful energy output ÷ total energy input
Multiply by 100 to express as a percentage. An efficiency of 1.0 (or 100%) is impossible in practice.
Worked example: A motor uses 500 J of electrical energy and does 350 J of useful mechanical work. What is its efficiency?
- efficiency = 350 / 500 = 0.70 = 70%
- Wasted energy = 500 − 350 = 150 J (dissipated as heat and sound).
Sankey diagrams
A Sankey diagram shows energy input and outputs as arrows. The width of each arrow is proportional to the amount of energy. Useful output is the main forward arrow; wasted outputs (heat, sound) branch off at right angles.
Common Gateway-paper mistakes
- Using diameter instead of radius in a centripetal question (not at GCSE, but be careful with height calculations).
- Forgetting to square v in KE = ½mv².
- Dividing useful OUTPUT by useful INPUT instead of total input in efficiency.
- Quoting an efficiency > 1 or > 100% — always check your answer.
- Using g = 10 when the question gives g = 9.8 (use whichever the question specifies).
AI-generated · claude-opus-4-7 · v3-ocr-combined-science