Conservation of Energy, Efficiency and Energy Resources
Conservation of Energy
The law of conservation of energy states that energy can never be created or destroyed. In any energy transfer, the total energy before and after is always the same.
In a closed system, where nothing can enter or leave, the total energy stays constant. However, in most real-world situations energy is dissipated — it spreads out to the surroundings as thermal energy and becomes increasingly difficult to use.
Energy is never “lost” — when we say energy is wasted, we mean it has been transferred to less useful stores (usually thermal energy in the surroundings). The total energy in the system has not changed.
Efficiency
Efficiency tells you how good a device is at transferring energy to useful stores. It is calculated using:
$$\text{efficiency} = \frac{\text{useful output energy}}{\text{total input energy}}$$
Efficiency can also be calculated using power:
$$\text{efficiency} = \frac{\text{useful output power}}{\text{total input power}}$$
Efficiency is a ratio and has no units. It is always between 0 and 1. To express it as a percentage, multiply by 100. No device can ever be more than 100% efficient.
A motor uses 500 J of electrical energy and transfers 350 J to the kinetic store. Calculate the efficiency.
\(\text{efficiency} = \frac{350}{500} = 0.70\) or 70%
This means 70% of the input energy is usefully transferred. The remaining 30% (150 J) is dissipated as thermal energy due to friction and sound.
Ways to increase efficiency include reducing friction (lubrication, streamlining), using thermal insulation to reduce unwanted heating, and improving design to minimise energy dissipation.
Reducing Unwanted Energy Transfers
Thermal insulation reduces the rate of energy transfer by heating. In a house, loft insulation, cavity wall insulation, and double glazing all reduce the amount of thermal energy lost to the surroundings.
Lubrication reduces friction between moving parts. This means less energy is wasted as heat in the thermal store, so more energy goes to the useful output.
The thicker the walls of a house, the slower the rate of thermal energy transfer. Cavity wall insulation traps air between the inner and outer walls. Since air is a poor conductor, this reduces energy transfer by conduction. Double glazing works on the same principle — a layer of gas between two panes of glass reduces conduction and convection.
Energy Resources
Energy resources can be classified as non-renewable or renewable.
Non-renewable resources include fossil fuels (coal, oil, natural gas) and nuclear fuel. These are finite — once used up, they cannot be replaced within a human timescale.
Renewable resources include solar, wind, hydroelectric, tidal, geothermal, wave power, and biofuels. These are replenished naturally and will not run out.
Fossil fuels are reliable and produce large amounts of energy, but release carbon dioxide (contributing to climate change) and are running out. Nuclear power is reliable and produces no carbon dioxide during operation, but produces radioactive waste that must be stored safely for thousands of years.
Renewable resources produce no greenhouse gases during operation and will not run out. However, many are weather-dependent (solar, wind) or location-dependent (tidal, geothermal), and some have high initial costs or low power output compared to fossil fuels.
The UK is working towards generating more electricity from renewable sources to reduce carbon emissions and meet climate targets. However, a mix of energy resources is needed to ensure a reliable supply.