Energy Use and Sustainable Sources
Energy Stores and Transfers
Energy cannot be created or destroyed — it can only be transferred between different energy stores. Scientists recognise eight main stores: chemical, nuclear, kinetic, gravitational potential, elastic potential, thermal, electrostatic and electromagnetic. When energy is transferred from one store to another, it is moved by processes called working (for example, by an electric current or a force) or heating.
Electricity is a particularly convenient way to transfer energy. Chemical energy stored in a fuel or battery is converted to electrical energy, which can then be transmitted over large distances and converted into other useful forms — thermal energy in a heater, kinetic energy in a motor, or light in a lamp.
Power and Energy Transfer
Power is the rate at which energy is transferred. It is measured in watts (W), where 1 W = 1 joule per second. A 2 kW kettle transfers twice as much energy per second as a 1 kW toaster. The energy transferred by any electrical appliance depends on both its power rating and the time for which it is used:
$$E = P \times t$$
where \(E\) is energy in joules (or kilowatt-hours), \(P\) is power in watts (or kilowatts) and \(t\) is time in seconds (or hours).
Energy transferred = power × time. In SI units: joules = watts × seconds. In everyday units: kilowatt-hours (kWh) = kilowatts × hours. A device with a power rating of 2 kW running for 3 hours transfers 6 kWh of energy.
Efficiency and Dissipation
No energy transfer is perfectly efficient. Some energy is always dissipated — it spreads into the surroundings as thermal energy and becomes increasingly difficult to make use of. A light bulb converts electrical energy into useful light and wasted thermal energy; a car engine converts chemical energy into useful kinetic energy and wasted thermal energy.
Efficiency is the fraction of total energy input that is converted into a useful output:
$$\text{efficiency} = \frac{\text{useful energy transferred (J)}}{\text{total energy transferred (J)}}$$
An efficiency of 0.75 means 75% of the input energy becomes useful energy; the remaining 25% is dissipated. Ways to increase efficiency include lubrication (reducing friction in motors) and thermal insulation (slowing heat loss from buildings).
Question: An electric motor receives 500 J of electrical energy and produces 375 J of useful kinetic energy. Calculate its efficiency.
Step 1 — identify the equation: efficiency = useful energy ÷ total energy
Step 2 — substitute values: efficiency = 375 ÷ 500
Step 3 — calculate: efficiency = 0.75 (or 75%)
This means 75 J in every 100 J of electrical input becomes useful kinetic energy; 25 J is dissipated as thermal energy.
Renewable and Non-Renewable Energy Sources
The main renewable energy sources include wind, solar, hydroelectric, tidal and biofuels. They produce little or no carbon dioxide in use and will not run out. Non-renewable sources — fossil fuels (coal, oil, gas) and nuclear fuel — provide reliable on-demand power but fossil fuels release carbon dioxide when burned, contributing to global warming, and nuclear fuel produces radioactive waste.
Renewable sources (wind, solar, hydroelectric, tidal, biofuel) do not run out and produce little CO2 in use. Non-renewable sources (coal, oil, natural gas, nuclear) provide reliable generation but fossil fuels produce CO2 and nuclear fuel creates radioactive waste.
In most power stations, a generator produces electricity by spinning a magnet near a coil of wire. The fuel source (fossil fuel, nuclear, or a renewable source) is used to drive the turbine that turns the generator. The UK mains supply is alternating current (a.c.) at 50 Hz and approximately 230 volts.
Electricity is distributed across the country through the National Grid at very high voltages. High voltage reduces the current for a given power, which reduces energy losses in the cables (because power dissipated in a wire depends on the square of the current). Transformers step up the voltage for transmission and step it back down for domestic use. Most home appliances are connected by a three-core cable containing live, neutral and earth wires.
Decisions About Energy Sources
Choices about energy sources involve weighing up scientific evidence, environmental impact, reliability, cost and social factors. Different people in different contexts may come to different conclusions about which sources to prioritise — a country with strong winds may favour offshore wind differently from one with high solar irradiance. These decisions are shaped by risk-benefit analysis and are ultimately a matter of policy, informed by but not determined solely by science.
Building Insulation and Reducing Energy Loss
One of the most cost-effective ways to reduce energy consumption is to reduce the energy lost from buildings. Heat loss through walls, roofs and windows is determined by the thickness and thermal conductivity of the materials used. A thicker wall slows heat transfer; a wall made of a material with low thermal conductivity (such as foam insulation) slows it further. Double-glazing traps a layer of air or gas between two panes of glass: air is a poor thermal conductor, so heat transfer across the window is significantly reduced.
Sankey diagrams are a useful tool for visualising energy transfers. In a Sankey diagram, the width of the arrows is proportional to the amount of energy they represent. The main arrow shows the total energy input; branches show where useful energy goes and where energy is wasted (dissipated). Reading a Sankey diagram allows you to estimate efficiency quickly: a diagram where the useful output arrow is wide relative to the wasted energy arrows indicates a more efficient device.