Energy Resources and Transfer
Summary: Covers the forms of energy, conservation of energy, work done, kinetic and gravitational potential energy, efficiency, power, renewable and non-renewable energy resources, and thermal/heat transfer (conduction, convection, radiation). This topic bridges mechanics and real-world energy issues. Tags: igcse physics energy resources heat-transfer Created: 2026-07-16 Last Updated: 2026-07-16
Forms of Energy
Energy is the capacity to do work. It is measured in joules (J). Energy can exist in different forms:
| Form of Energy | Description | Example |
|---|---|---|
| Kinetic (KE) | Energy of motion | A moving car, a thrown ball |
| Gravitational Potential (GPE) | Energy due to position in a gravitational field | Water behind a dam, a book on a shelf |
| Chemical | Energy stored in chemical bonds | Food, batteries, fossil fuels |
| Nuclear | Energy stored in the nucleus of atoms | Uranium in nuclear reactors, the Sun |
| Elastic / Strain | Energy stored in stretched or compressed materials | Stretched rubber band, compressed spring |
| Thermal / Internal | Energy due to the random motion of particles (total KE + PE of particles) | Hot water, heated air |
| Electrical | Energy transferred by electric current | Mains electricity, lightning |
| Light (radiant) | Energy transferred as electromagnetic waves | Sunlight, light bulbs |
| Sound | Energy transferred as longitudinal pressure waves | Speakers, musical instruments |
Energy Transfers and Conservation of Energy
The principle of conservation of energy states that energy cannot be created or destroyed — it can only be transferred (moved between objects), transformed (changed from one form to another), or stored.
Common energy transfer chains:
- Falling object: GPE → KE (all GPE converted to KE if no air resistance)
- Pendulum: GPE ⇄ KE (continuous exchange, maximum KE at lowest point, maximum GPE at highest points)
- Battery-powered torch: Chemical (battery) → Electrical (wires) → Light + Thermal (bulb)
- Hydroelectric dam: GPE (water behind dam) → KE (falling water) → KE (turbine) → Electrical (generator)
- Burning fuel: Chemical (fuel) → Thermal (heat) → KE (moving pistons / expanding gases) → KE (vehicle)
Work done is a measure of energy transferred when a force moves an object:
Work done = force × distance moved in the direction of the force
W = Fd
- W = work done (J), F = force (N), d = distance (m)
- 1 joule = 1 newton-metre
Example 1: A force of 25 N pushes a box 3 m across a floor. Calculate the work done.
W = Fd = 25 × 3 = 75 J
Kinetic Energy and Gravitational Potential Energy
Kinetic Energy (KE):
KE = ½mv²
- m = mass (kg), v = speed (m/s)
- KE is proportional to the square of speed — doubling speed quadruples KE
Gravitational Potential Energy (GPE):
GPE = mgh
- m = mass (kg), g = gravitational field strength (N/kg or m/s²), h = height (m)
Example 2: A ball of mass 0.5 kg is dropped from a height of 20 m (g = 10 m/s²). Find: (a) its GPE at the top, (b) its speed just before hitting the ground (assuming no air resistance).
- (a) GPE = mgh = 0.5 × 10 × 20 = 100 J
- (b) All GPE converts to KE: ½mv² = 100 → ½ × 0.5 × v² = 100 → 0.25v² = 100 → v² = 400 → v = 20 m/s
Example 3: A cyclist of mass 80 kg (including bike) accelerates from rest to 10 m/s. Calculate the kinetic energy gained.
KE = ½mv² = ½ × 80 × 10² = ½ × 80 × 100 = 4000 J
Efficiency
No device is 100% efficient — some energy is always transferred to less useful forms (typically thermal energy/heating).
Efficiency = (useful energy output / total energy input) × 100%
Efficiency = (useful power output / total power input) × 100%
- Efficiency can be expressed as a percentage or as a decimal (0 to 1)
- A 100% efficient machine is impossible in practice (perpetual motion machines are impossible)
Example 4: A motor lifts a weight, doing 40 J of useful work. The motor receives 100 J of electrical energy. Calculate the efficiency.
Efficiency = (40 / 100) × 100% = 40%
Example 5: An electric motor has an efficiency of 75%. If it outputs 150 W of useful power, what is the input power?
0.75 = 150 / input power → input power = 150 / 0.75 = 200 W
Power
Power is the rate of doing work or the rate of energy transfer:
Power = work done / time = energy transferred / time
P = W / t = E / t
- P = power in watts (W), 1 W = 1 J/s
Example 6: A crane lifts a 500 kg mass through a vertical height of 12 m in 20 seconds (g = 10 m/s²). Calculate the power output.
Work done = mgh = 500 × 10 × 12 = 60,000 J
Power = 60,000 / 20 = 3000 W = 3 kW
Energy Resources
Non-Renewable Energy Resources
These are finite — once used, they cannot be replaced within a human timescale.
| Resource | How it Works | Advantages | Disadvantages |
|---|---|---|---|
| Coal | Burned → heat water → steam → turbine → generator | Reliable, large reserves, established infrastructure | Produces CO₂ (greenhouse gas) and SO₂ (acid rain), non-renewable, mining damages landscape |
| Oil | Burned → heat water → steam → turbine, or refined into petrol/diesel for vehicles | High energy density, easy to transport as liquid, established infrastructure | Produces CO₂ and NOₓ, oil spills damage ecosystems, non-renewable, price volatility |
| Natural Gas | Burned → hot gases → turbine directly, or heat water → steam → turbine | Cleanest fossil fuel (less CO₂ per J than coal/oil, no SO₂), reliable | Still produces CO₂, non-renewable, methane leaks are a potent greenhouse gas |
| Nuclear (fission) | U-235/Pu-239 fission → heat → steam → turbine → generator | No CO₂ or SO₂ produced, very high energy density (small amount of fuel produces huge energy), reliable baseload | Radioactive waste (needs safe storage for thousands of years), risk of catastrophic accidents (Chernobyl, Fukushima), high decommissioning costs, non-renewable fuel (uranium is finite) |
Renewable Energy Resources
These can be replenished as they are used and will not run out.
| Resource | How it Works | Advantages | Disadvantages |
|---|---|---|---|
| Solar (photovoltaic) | Solar cells convert sunlight directly to electricity | No pollution during operation, free ‘fuel’, good for remote locations | Intermittent (night, cloudy days), low efficiency, requires large area, manufacturing solar cells uses toxic chemicals |
| Solar (heating) | Solar panels absorb sunlight to heat water directly | Simple technology, low running costs | Intermittent, supplementary heating often needed in winter |
| Wind | Wind turns turbine blades → generator → electricity | No pollution during operation, land beneath turbines can still be farmed | Intermittent (needs wind), visual impact, noise, can affect birds, requires backup or storage |
| Hydroelectric | Water stored behind dam flows down → turns turbine → generator | Reliable (can be turned on/off quickly), no pollution, also provides water storage/flood control | Dams flood large areas (habitat destruction, displacement of people), high initial cost, depends on rainfall |
| Tidal | Tidal water flows through barriers → turns turbines | Very predictable (tides are regular), no pollution, barrages can also serve as flood barriers | High initial cost, alters marine ecosystems, only works in locations with large tidal range |
| Wave | Wave motion drives generators (several designs exist) | Free energy once built, no pollution | Intermittent, can be damaged by storms, difficult to maintain, currently small-scale |
| Geothermal | Cold water pumped down to hot rocks → steam returns → turbine | Reliable (always available), no pollution during operation, low running costs | Only viable in volcanic/geologically active areas, high drilling costs, may release underground gases |
| Biomass | Organic matter (wood, crops, waste) burned → heat → steam → turbine | Renewable (plants can be regrown), uses waste materials, ‘carbon neutral’ in theory (CO₂ released = CO₂ absorbed during growth) | Still produces CO₂ (and particulates if not properly controlled), competes with land for food crops, deforestation risk |
Thermal (Heat) Energy Transfer
Heat can be transferred from a hotter region to a cooler region by three mechanisms:
Conduction
- Occurs mainly in solids
- Particles vibrate and pass energy to neighbouring particles through collisions
- In metals, conduction is much faster because of free/delocalised electrons that can move through the lattice, transferring energy rapidly
- Insulators (non-metals, gases, liquids) have few or no free electrons and conduct heat poorly
- Good conductors: copper, aluminium, silver, iron
- Good insulators: wood, plastic, glass, air, wool, polystyrene
Convection
- Occurs in fluids (liquids and gases)
- When a fluid is heated, it expands, becomes less dense, and rises
- Cooler, denser fluid sinks to replace it, creating a convection current
- Convection cannot occur in solids because particles cannot move freely
- Examples: hot air rising above a radiator, sea breezes (land heats up faster than sea during day), magma convection in Earth’s mantle, hot water rising in a kettle
Radiation (Infrared)
- Transfer of energy by electromagnetic waves (infrared radiation)
- Does not require a medium — can travel through a vacuum (how the Sun’s energy reaches Earth)
- All objects emit and absorb infrared radiation
- Black/dark, matt (dull) surfaces: best absorbers AND best emitters of radiation
- White/silver, shiny/polished surfaces: best reflectors of radiation (poor absorbers and poor emitters)
- The hotter an object, the more infrared radiation it emits
Practical applications summary table:
| Application | Principle Used |
|---|---|
| Vacuum flask | Silvered walls reflect radiation, vacuum stops conduction/convection between walls, stopper prevents convection at top |
| Domestic hot water system | Convection — hot water rises from boiler to storage tank; cold water sinks to be reheated |
| Car radiators | Matt black — good emitters of heat |
| Cooking pans | Metal base (good conductor), plastic/wooden handle (good insulator) |
| Clothing | White clothes in summer reflect radiation, black clothes in winter absorb radiation to stay warm |
| Double glazing | Trapped air gap between glass panes is a poor conductor |
| Greenhouse effect | Glass is transparent to visible light (Sun’s radiation gets in) but opaque to infrared (traps re-radiated heat) |
Sankey Diagrams
A Sankey diagram is a visual representation of energy transfers. The width of each arrow is proportional to the amount of energy. Input energy is shown on the left; useful output energy and wasted energy branch off to the right.
Features of a Sankey diagram:
- Arrow widths are drawn to scale (or labelled with energy values)
- Total input energy = useful output energy + wasted energy
- Wasted energy is typically shown branching downwards
Sources
- BBC Bitesize GCSE Physics — Energy topic, BBC (free educational resource)
- OpenStax College Physics — Work and Energy chapter, Rice University (free, CC BY 4.0)
- Cambridge IGCSE Physics 0625 — Energy resources and energy transfer section, Cambridge Assessment International Education
- CK-12 Physics for High School — Energy chapter, CK-12 Foundation (free, CC BY-NC 3.0)
Related Notes
- Forces and Motion — Work done links to force and distance, KE links to speed
- Thermal Physics — Thermal expansion and convection, specific heat capacity
- Electricity and Magnetism — Electrical power, P = IV, electrical energy = IVt
- Nuclear Physics — Nuclear power from fission
- IGCSE-Phys-Index — Full IGCSE Physics index
Keywords
kinetic energy, gravitational potential energy, work done, conservation of energy, efficiency, power, watt, joule, conduction, convection, radiation, infrared, renewable, non-renewable, fossil fuels, nuclear, solar, wind, hydroelectric, geothermal, Sankey diagram, free electrons, convection current
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Energy is ‘used up‘“ | Energy is never used up — it is transferred or transformed into other forms (often thermal energy that dissipates) |
| “A cold object contains no thermal energy” | All objects above absolute zero contain thermal energy — it’s just less than hotter objects |
| ”Heavier objects fall faster because they have more KE” | In a vacuum, all objects fall at the same rate. Heavier objects do have more KE at the same speed, but they also have more GPE to convert |
| ”Heat rises” | Hot air/fluid rises because it expands and becomes less dense. Heat itself transfers in all directions from hot to cold |
| ”Metals are good at conducting heat because they are dense” | No — metals conduct well because they have free/delocalised electrons that can transfer energy rapidly through the lattice |
| ”Black objects absorb radiation better because they are thicker” | Absorption depends on surface colour and texture, not thickness. Black matt surfaces are the best absorbers |
| ”Renewable energy resources have no environmental impact” | All energy resources have some environmental impact (e.g., dams flood valleys, wind turbines affect birds, solar farms use land) |
| “100% efficiency is possible with good design” | 100% efficiency is impossible — some energy is always transferred to less useful stores (usually thermal) due to friction, resistance, etc. |