Energy — AQA Combined Science: Trilogy
Energy is one of the most important ideas in physics. This unit covers energy stores and transfers, calculations, conservation, efficiency and energy resources.
Energy stores and systems
A system is an object or group of objects. Energy is stored in different ways:
- Kinetic (movement)
- Gravitational potential (raised objects)
- Elastic potential (stretched/compressed springs)
- Thermal (hot objects)
- Chemical, nuclear, magnetic and electrostatic stores.
When a system changes, energy is transferred between stores. Transfers happen by heating, by waves (light/sound), by an electric current, or mechanically (by forces doing work).
Energy calculations
Kinetic energy: $$E_k = \tfrac{1}{2}mv^2$$ (mass in kg, speed in m/s, energy in joules)
Gravitational potential energy: $$E_p = mgh$$ (g = gravitational field strength, about 9.8 N/kg)
Elastic potential energy: $$E_e = \tfrac{1}{2}ke^2$$ (k = spring constant, e = extension)
Work done = energy transferred: $$W = F \times d$$ (force × distance moved in the direction of the force)
When an object falls, gravitational potential energy is transferred to kinetic energy.
Specific heat capacity
The specific heat capacity of a substance is the energy needed to raise the temperature of 1 kg by 1 °C.
$$\Delta E = m \times c \times \Delta\theta$$ (mass × specific heat capacity × temperature change)
Required practical: measuring the specific heat capacity of a material (e.g. a metal block) using an electric heater, measuring energy input and temperature rise.
Power
Power is the rate of energy transfer (or work done):
$$P = \frac{E}{t} \quad\text{and}\quad P = \frac{W}{t}$$
Power is measured in watts (W); 1 watt = 1 joule per second. A more powerful device transfers the same amount of energy in less time.
Conservation and dissipation of energy
The law of conservation of energy states that energy cannot be created or destroyed, only transferred, stored or dissipated.
In any transfer, some energy is dissipated ("wasted") to the surroundings, usually as thermal energy — for example through friction. This wasted energy spreads out and becomes less useful.
Reducing unwanted transfers
- Lubrication reduces friction.
- Thermal insulation reduces heat loss. The rate of heat transfer through a building's walls depends on the thickness and thermal conductivity of the materials — thicker walls and lower conductivity reduce heat loss.
Efficiency
Efficiency is the proportion of input energy that is transferred usefully:
$$\text{efficiency} = \frac{\text{useful output energy transfer}}{\text{total input energy transfer}}$$
It can also be calculated using power. Efficiency is always less than 1 (or 100%) because some energy is always dissipated. It can be expressed as a decimal or a percentage (× 100).
National and global energy resources
Energy resources are either renewable or non-renewable.
- Non-renewable: fossil fuels (coal, oil, gas) and nuclear fuel. Reliable and high output, but finite and (for fossil fuels) release CO₂ and other pollutants.
- Renewable: solar, wind, hydroelectric, tidal, wave, geothermal and bio-fuel. They will not run out and produce little or no pollution, but many are unreliable (depend on weather/time), and some have high set-up costs or environmental impacts.
You should be able to evaluate the use of different resources, considering reliability, environmental impact, cost and political factors. There is increasing pressure to use more renewables to reduce reliance on fossil fuels and limit climate change.
Exam tips
- Learn the energy equations and what each symbol and unit means — practise rearranging them.
- Always quote energy in joules and convert units (kJ → J, etc.).
- State the conservation of energy law precisely and identify where energy is dissipated (usually as heat by friction).
- For efficiency, identify the useful output versus the total input.
- For energy resources, give balanced pros and cons (reliability, cost, pollution).