Conservation of Energy & Energy Resources
Conservation of Energy
The conservation of energy is one of the most important ideas in physics. It states that energy cannot be created or destroyed — only transferred from one store to another. The total energy in a closed system always stays the same.
Energy cannot be created or destroyed. In a closed system, the total energy before a change is always equal to the total energy after the change.
Calculating Gravitational Potential Energy
When an object is raised above the ground, energy is transferred to its gravitational potential energy store. The amount of energy stored depends on the mass, the gravitational field strength, and the height gained:
\(\Delta GPE = m \times g \times \Delta h\)
Where \(\Delta GPE\) is the change in gravitational potential energy in joules (J), \(m\) is mass in kilograms (kg), \(g\) is gravitational field strength in newtons per kilogram (N/kg), and \(\Delta h\) is the change in vertical height in metres (m). On Earth, \(g = 10\) N/kg in Edexcel questions.
A 2 kg book is lifted 1.5 m onto a shelf. Calculate the change in gravitational potential energy. Use \(g = 10\) N/kg.
\(\Delta GPE = m \times g \times \Delta h = 2 \times 10 \times 1.5 = 30\) J
Calculating Kinetic Energy
The energy in the kinetic energy store of a moving object depends on its mass and the square of its speed:
\(KE = \frac{1}{2} \times m \times v^2\)
Where \(KE\) is kinetic energy in joules (J), \(m\) is mass in kilograms (kg), and \(v\) is speed in metres per second (m/s). Because speed is squared, doubling the speed quadruples the kinetic energy.
A 0.5 kg ball travels at 4 m/s. Calculate its kinetic energy.
\(KE = \frac{1}{2} \times 0.5 \times 4^2 = \frac{1}{2} \times 0.5 \times 16 = 4\) J
Wasted Energy and Efficiency
No energy transfer is perfectly efficient. Some energy is always dissipated — spread out to the thermal store of the surroundings. This wasted energy cannot easily be recovered. Mechanical processes become wasteful when friction causes a rise in temperature, heating the surroundings.
We can reduce unwanted energy transfers by lubrication (reducing friction between moving parts) and thermal insulation (reducing energy transfer by heating). For example, loft insulation and cavity wall insulation reduce the rate at which a building cools by trapping air, which is a poor thermal conductor.
Efficiency measures how much of the input energy is usefully transferred: \(\text{efficiency} = \frac{\text{useful energy output}}{\text{total energy input}}\). Efficiency has no units and is always between 0 and 1 (or 0% and 100%).
Energy Resources
The main energy resources available on Earth are: non-renewable sources (fossil fuels — coal, oil, natural gas — and nuclear fuel) and renewable sources (wind, solar, hydroelectric, tidal, geothermal, wave, and bio-fuel).
Non-renewable resources release large amounts of energy per kilogram and provide a reliable supply. However, burning fossil fuels produces carbon dioxide (contributing to climate change) and they will eventually run out. Nuclear fuel produces radioactive waste that must be stored safely.
Renewable resources will not run out and most produce no greenhouse gases during operation. However, many are intermittent (wind and solar depend on weather conditions), and some require large land areas or have visual/noise impacts. Bio-fuels are carbon neutral in theory because the plants absorb CO\(_2\) as they grow.
The global trend is a shift towards renewable energy sources. This is driven by concerns about climate change, energy security (reducing dependence on imported fossil fuels), and the falling cost of renewable technologies such as solar panels and wind turbines.
Non-renewable resources (fossil fuels and nuclear) provide reliable, high-energy output but produce waste and will run out. Renewable resources (wind, solar, tidal, hydroelectric, geothermal, bio-fuel, wave) are replenished naturally but many are intermittent.
Exam Tip: Calculation questions on energy are very common. Always write the equation, substitute the values with correct units, then calculate. Show every step — you can pick up method marks even if you make an arithmetic error. Remember that \(v\) in the kinetic energy equation is squared, so always square the speed before multiplying.
The conservation of energy principle underpins all energy calculations. You need to recall and use the equations for gravitational potential energy and kinetic energy, understand efficiency, and be able to compare renewable and non-renewable energy resources.