What you'll learn
This topic explores how energy is transferred, stored and transformed in physical systems. You will master calculations involving work done by forces, power in mechanical and electrical systems, and the efficiency of energy conversions. CIE IGCSE Physics exam papers regularly test these concepts through calculations, explanations of energy transfers, and analysis of real-world applications such as motors, heating systems and renewable energy devices.
Key terms and definitions
Energy — the capacity to do work, measured in joules (J). Energy exists in many forms including kinetic, gravitational potential, elastic potential, chemical, thermal, nuclear, light and sound.
Work done — the energy transferred when a force moves an object in the direction of the force, calculated as W = Fd, measured in joules (J).
Power — the rate at which energy is transferred or work is done, measured in watts (W) where 1 watt = 1 joule per second.
Efficiency — the ratio of useful energy output to total energy input, expressed as a percentage or decimal fraction.
Kinetic energy — the energy possessed by a moving object, calculated using KE = ½mv².
Gravitational potential energy — the energy stored in an object due to its position in a gravitational field, calculated using GPE = mgh.
Conservation of energy — the principle that energy cannot be created or destroyed, only transferred from one form to another.
Dissipated energy — energy that is transferred to the surroundings, usually as thermal energy, and becomes less useful.
Core concepts
Forms of energy and energy transfers
Energy exists in multiple forms, and CIE IGCSE Physics examinations require you to identify and describe transfers between these forms:
- Kinetic energy: possessed by moving objects (vehicles, projectiles, flowing water)
- Gravitational potential energy: stored when objects are lifted against gravity
- Elastic potential energy: stored in stretched or compressed springs, rubber bands
- Chemical energy: stored in fuels, food, batteries
- Thermal (heat) energy: energy of particles due to their temperature
- Electrical energy: energy transferred by moving charges in circuits
- Nuclear energy: stored in atomic nuclei, released in fission or fusion
- Light energy: electromagnetic radiation transmitted by photons
- Sound energy: energy transferred by vibrating particles
Energy transfers occur constantly in physical systems. A falling object converts gravitational potential energy to kinetic energy. A car engine converts chemical energy from petrol into kinetic energy of the vehicle, thermal energy in the exhaust, and sound energy. Exam questions frequently require you to trace these energy transfer chains using Sankey diagrams or written descriptions.
The principle of conservation of energy states that the total energy in a closed system remains constant. Energy transformations never create or destroy energy—they only convert it from one form to another. In real systems, energy is often dissipated as thermal energy to the surroundings, making it less useful but not lost.
Work done calculations
Work done occurs when a force causes displacement. The equation is:
W = Fd
where:
- W = work done (J)
- F = force applied (N)
- d = distance moved in the direction of the force (m)
Critical points for CIE IGCSE Physics calculations:
- The force and displacement must be in the same direction. If you push horizontally and an object moves horizontally, use the full force value.
- If the force acts at an angle, only the component in the direction of motion does work.
- If there is no movement (d = 0), no work is done regardless of how large the force is. Holding a heavy box stationary involves no work in the physics definition.
- 1 joule is defined as the work done when a force of 1 newton moves through 1 metre.
Common applications tested:
- Lifting objects: work done against gravity equals the gain in gravitational potential energy
- Pushing or pulling objects: work done by applied forces
- Friction: work done against friction is converted to thermal energy
Kinetic and potential energy equations
Two formulae appear frequently in CIE IGCSE Physics exam papers:
Kinetic energy: KE = ½mv²
where:
- KE = kinetic energy (J)
- m = mass (kg)
- v = speed (m/s)
Note the speed is squared, meaning doubling the speed quadruples the kinetic energy. This explains why high-speed collisions cause much greater damage.
Gravitational potential energy: GPE = mgh
where:
- GPE = gravitational potential energy (J)
- m = mass (kg)
- g = gravitational field strength (N/kg) — use 10 N/kg unless specified otherwise
- h = height above reference point (m)
The reference point (h = 0) can be chosen arbitrarily, but must be consistent within a problem. Usually ground level or the lowest point in the system is selected.
When objects fall freely, GPE converts to KE:
mgh = ½mv²
The mass cancels, giving: gh = ½v²
This relationship allows calculation of impact speeds for falling objects when air resistance is negligible.
Power calculations and applications
Power measures how quickly energy is transferred or work is done. Two key equations:
P = E/t (power = energy transferred ÷ time)
P = W/t (power = work done ÷ time)
where:
- P = power (W)
- E = energy transferred (J)
- W = work done (J)
- t = time (s)
Alternative units:
- 1 kilowatt (kW) = 1000 W
- 1 megawatt (MW) = 1,000,000 W
Rearranging P = E/t gives E = Pt, which is crucial for electrical energy calculations. Electricity companies measure energy consumption in kilowatt-hours (kWh):
Energy (kWh) = Power (kW) × Time (hours)
1 kWh = 3.6 × 10⁶ J (since 1 kW × 3600 s = 3,600,000 J)
CIE IGCSE Physics exam questions test power in contexts including:
- Electric motors: power output compared to input
- Vehicles: power required to maintain speed against resistive forces
- Lifting mechanisms: power needed to raise loads
- Household appliances: comparing energy consumption
Example: a 2 kW electric heater running for 3 hours transfers 2 × 3 = 6 kWh of electrical energy.
Efficiency calculations
Efficiency quantifies how effectively devices convert input energy to useful output energy:
Efficiency = (useful energy output / total energy input) × 100%
Alternatively:
Efficiency = (useful power output / total power input) × 100%
Efficiency values:
- Always between 0% and 100%
- No real device achieves 100% efficiency
- Can be expressed as a decimal (0 to 1) or percentage (0% to 100%)
Energy not converted usefully is wasted or dissipated, typically as:
- Thermal energy due to friction in mechanical systems
- Thermal energy due to resistance in electrical systems
- Sound energy from vibrations
- Light energy where not required
Sankey diagrams represent energy flows visually. The width of arrows is proportional to energy quantities, making wasted energy immediately visible.
Typical efficiencies tested in CIE IGCSE Physics:
- Electric motors: 70-90%
- Petrol engines: 20-30%
- Power stations: 30-40%
- Electric heaters: ~100% (but not always useful)
- LED lamps: 85-90%
- Filament lamps: ~5%
Improving efficiency:
- Lubrication reduces friction
- Thermal insulation reduces heat loss
- Streamlining reduces air resistance
- Using more efficient materials (e.g., LED instead of filament bulbs)
Worked examples
Example 1: Work done and gravitational potential energy
Question: A student of mass 50 kg climbs stairs of vertical height 12 m in 30 s. The gravitational field strength is 10 N/kg.
(a) Calculate the work done by the student against gravity. [2]
(b) Calculate the power developed by the student. [2]
Solution:
(a) Work done = force × distance
Weight = mg = 50 × 10 = 500 N [1]
Work done = Fd = 500 × 12 = 6000 J [1]
Alternatively: GPE gained = mgh = 50 × 10 × 12 = 6000 J
(b) Power = work done / time [1]
P = 6000 / 30 = 200 W [1]
Example 2: Kinetic energy calculation
Question: A car of mass 1200 kg accelerates from rest to 25 m/s.
(a) Calculate the kinetic energy gained by the car. [2]
(b) The kinetic energy comes from chemical energy in the fuel. If the efficiency of the engine is 25%, calculate the chemical energy used. [2]
Solution:
(a) KE = ½mv² [1]
KE = ½ × 1200 × 25² = ½ × 1200 × 625 = 375,000 J or 375 kJ [1]
(b) Efficiency = (useful energy output / total energy input) × 100%
25 = (375,000 / total input) × 100 [1]
Total input = 375,000 / 0.25 = 1,500,000 J or 1500 kJ [1]
Example 3: Energy transfers and efficiency
Question: An electric motor lifts a load of weight 600 N through a vertical distance of 8.0 m in 10 s. The motor has an electrical power input of 600 W.
(a) Calculate the work done lifting the load. [2]
(b) Calculate the useful power output of the motor. [2]
(c) Calculate the efficiency of the motor. [2]
Solution:
(a) Work done = Fd [1]
W = 600 × 8.0 = 4800 J [1]
(b) Power = work done / time [1]
P = 4800 / 10 = 480 W [1]
(c) Efficiency = (useful power output / total power input) × 100% [1]
Efficiency = (480 / 600) × 100% = 80% [1]
Common mistakes and how to avoid them
Mistake: Confusing energy and power. Students write "the car has more power" when they mean kinetic energy.
Correction: Energy is measured in joules (capacity to do work). Power is measured in watts (rate of energy transfer). A high-power device transfers energy quickly, but a low-power device running longer can transfer the same total energy.
Mistake: Forgetting to square the velocity in KE = ½mv². Students calculate ½ × m × v instead.
Correction: Always write v² explicitly. Doubling speed increases kinetic energy by a factor of four (2² = 4), not two.
Mistake: Using incorrect units, especially mixing metres and centimetres, or using grams instead of kilograms.
Correction: Convert all quantities to SI base units before calculating: mass in kg, distance in m, force in N, time in s. Check the units in your final answer.
Mistake: Calculating efficiency greater than 100%.
Correction: Efficiency can never exceed 100% due to conservation of energy. If your calculation gives >100%, you have divided output by input incorrectly—useful output must be smaller than total input.
Mistake: Stating that energy is "lost" or "used up" in energy transfers.
Correction: Energy is conserved (never created or destroyed). Use precise terms: energy is "dissipated" or "transferred to the surroundings" or "wasted as thermal energy", but never lost.
Mistake: Omitting units from final answers or using incorrect units (e.g., writing power in joules).
Correction: Always state units: work and energy in J, power in W, efficiency as % or as a decimal. CIE mark schemes deduct marks for missing or incorrect units.
Exam technique for Energy, Work and Power
Command word 'calculate': Show the formula, substitute values with units, then give the answer with the correct unit. Full working is essential for method marks even if the final answer is wrong. CIE typically awards 1 mark for correct formula/method and 1 mark for correct answer with unit.
Command word 'explain': Link cause and effect using physics principles. For energy transfers, identify the initial form, final form(s), and the process. Use the conservation of energy principle where relevant. Aim for 2-3 clear sentences for 2-3 mark questions.
Sankey diagrams: Width of arrows must be proportional to energy values. Label each arrow with energy type and value. The input arrow splits into useful output and wasted energy branches. Practice drawing these accurately.
Multi-step calculations: Break problems into stages. For example, finding efficiency may require first calculating work done, then power, then efficiency. Write each stage clearly. If one step is wrong, you can still gain subsequent method marks.
Quick revision summary
Energy is measured in joules and exists in multiple forms (kinetic, gravitational potential, thermal, chemical, electrical). Work done W = Fd occurs when forces cause displacement. Kinetic energy KE = ½mv²; gravitational potential energy GPE = mgh. Power P = E/t measures the rate of energy transfer in watts. Efficiency = (useful output / total input) × 100% is always less than 100% in real systems. Energy is conserved but often dissipated as thermal energy. CIE IGCSE Physics exams test calculations, energy transfer descriptions, and applications to real devices.