Energy
Energy stores and transfers, kinetic and potential energy, power and efficiency, and national and renewable energy resources.
Learn
Energy, explained point by point
Everything the GCSE specification expects you to be able to do, and how to actually do it - the same lesson a signed-in student studies from.
Describe energy stores and transfers
Energy is stored in different ways - kinetic (moving), gravitational (raised up), elastic (stretched), thermal, chemical and more - and is transferred between stores by forces, heating, electricity or waves. Energy is never created or destroyed, only shifted around, so a swinging pendulum trades gravitational store for kinetic and back.
Calculate kinetic and gravitational potential energy
Kinetic energy depends on mass and speed: Ek = ½mv², so doubling the speed quadruples the energy. Gravitational potential energy depends on height: Ep = mgh, where g is about 9.8 N/kg. Lifting a 2 kg book 1.5 m stores 2 × 9.8 × 1.5 ≈ 29 J, ready to be released if it falls.
Calculate work done and power
Work done is energy transferred by a force: work = force × distance, measured in joules. Power is how fast that happens: power = energy ÷ time, in watts. A motor transferring 600 J in 5 seconds has a power of 120 W. A more powerful device does the same job in less time.
Calculate efficiency
No device transfers all its energy usefully - some is wasted, usually as heat. Efficiency = useful energy out ÷ total energy in, often given as a percentage. A bulb that gives 20 J of light for every 100 J of electricity is 20% efficient. Reducing wasted energy, such as friction, raises efficiency.
Describe conduction, convection and radiation, and how to reduce losses
Heat moves three ways: conduction through solids as vibrating particles pass energy along, convection in fluids as warm regions rise, and infrared radiation across empty space. Homes cut losses with insulation, cavity walls and double glazing that trap air, a poor conductor. Matching the method to the situation is the key idea.
Compare renewable and non-renewable energy resources
Non-renewable resources - coal, oil, gas and nuclear fuel - will run out and, when burned, release pollution or waste. Renewables such as wind, solar, hydro, tidal and geothermal will not run out and pollute less, though they can be less reliable or more costly. Choosing a mix balances reliability, cost and the environment.
Energy key terms
The words the specification and the mark schemes use, each defined the way an examiner wants it.
- Energy store
- Where energy is held in a system: kinetic, gravitational potential, elastic potential, thermal, chemical, nuclear, magnetic and electrostatic stores.
- Conservation of energy
- Energy can be transferred usefully, stored or dissipated, but it can never be created or destroyed.
- Specific heat capacity
- The energy needed to raise the temperature of 1 kg of a substance by 1°C.
- Power
- The rate at which energy is transferred or work is done. Measured in watts: 1 watt is 1 joule per second.
- Dissipation
- Energy spreading out into less useful stores, often thermal energy warming the surroundings. Dissipated energy is often described as wasted.
- Efficiency
- The fraction of input energy that is transferred usefully: useful output ÷ total input.
- Non-renewable energy resource
- An energy resource that will run out because it cannot be replenished as it is used: coal, oil, gas and nuclear fuel.
Practice
Try a Energy question
A GCSE-style original question from this topic. Have a go before you open the working - deciding on an answer first is what makes the working stick.
A cyclist and bike have a combined mass of 50 kg and travel at 12 m/s. Use Eₖ = ½ × m × v² to work out their kinetic energy, in joules (J).
- 7200
- 300
- 600
- 3600
Show the answer and the working
Answer: 3600
Eₖ = ½ × 50 × 12² = ½ × 50 × 144 = 3600 J.
- The kinetic energy equation is Eₖ = ½ × m × v².
- Square the speed: 12² = 144.
- Multiply: 50 × 144 = 7200, then halve: 7200 ÷ 2 = 3600.
- The kinetic energy is 3600 J.