Energy
Energy stores, transfers, conservation, efficiency, and resources.
In this topic
Energy Stores and Transfers
Energy is stored in different ways: - Kinetic: energy of a moving object - Gravitational potential: energy of an object at height - Elastic potential: energy stored in a stretched/compressed object - Thermal: energy related to temperature - Chemical: energy stored in bonds (food, fuel, batteries) - Nuclear: energy stored in the nucleus - Electrostatic: energy of charged particles - Magnetic: energy of magnets
Energy is transferred between stores by: - Mechanically (by forces): pushing, pulling - Electrically: through circuits - By heating: conduction, convection, radiation - By radiation: light, sound, waves
Key Points
- Energy cannot be created or destroyed — only transferred (conservation of energy)
- In any transfer, some energy is dissipated (wasted), usually as heat
- KE = ½mv² (kinetic energy)
- GPE = mgh (gravitational potential energy)
- Efficiency = useful output / total input × 100%
Example Questions
2A ball of mass 0.5 kg is dropped from a height of 10 m. Calculate its gravitational potential energy at the top. (g = 10 N/kg)
GPE = mgh = 0.5 × 10 × 10 = 50 J
[2 marks]
2A car has a mass of 1200 kg and travels at 20 m/s. Calculate its kinetic energy.
KE = ½mv² = ½ × 1200 × 20² = ½ × 1200 × 400 = 240,000 J = 240 kJ
[2 marks]
2A light bulb uses 60 J of energy per second and produces 12 J of useful light energy. Calculate its efficiency.
Efficiency = (12/60) × 100 = 20%
[2 marks]
Specific Heat Capacity
Specific heat capacity is the amount of energy needed to raise the temperature of 1 kg of a substance by 1°C.
Formula: E = mcΔθ Where: - E = energy transferred (J) - m = mass (kg) - c = specific heat capacity (J/kg°C) - Δθ = change in temperature (°C)
Water has a high specific heat capacity (4200 J/kg°C), which is why it's used in heating systems and takes a long time to heat up or cool down.
Key Points
- Higher SHC = takes more energy to heat up
- Water: c = 4200 J/kg°C
- Aluminium: c = 900 J/kg°C
- This is why land heats up faster than the sea
Example Questions
3How much energy is needed to heat 2 kg of water from 20°C to 100°C? (c = 4200 J/kg°C)
E = mcΔθ = 2 × 4200 × (100-20) = 2 × 4200 × 80 = 672,000 J = 672 kJ
[3 marks]
Energy Resources
Energy resources can be classified as renewable or non-renewable.
Non-renewable (will run out): - Fossil fuels: coal, oil, natural gas — reliable but produce CO₂ - Nuclear: uranium — no CO₂ but produces radioactive waste
Renewable (won't run out): - Solar: energy from the Sun — intermittent, weather dependent - Wind: wind turbines — intermittent, visual impact - Hydroelectric: water flowing downhill — reliable, needs specific geography - Tidal: tides turn turbines — reliable, expensive to build - Geothermal: heat from Earth's core — location specific - Biomass/biofuel: burning organic material — carbon neutral in theory
Key Points
- Global energy demand is increasing
- Fossil fuels contribute to climate change
- No single energy source is perfect
- A mix of sources is needed for energy security
Example Questions
4Evaluate the use of wind power compared to natural gas for generating electricity.
Wind is renewable and produces no CO₂ during operation, but it is intermittent (depends on wind) and requires many turbines. Natural gas is reliable and can generate electricity on demand, but it is non-renewable and produces CO₂, contributing to climate change. Wind farms take up large areas and some people consider them unsightly.
[4 marks]