OCR GCSE Physics A (J249) — Global challenges
Every time you switch on a light, the electricity powering it likely began its journey millions of years ago, or millions of miles away. Almost all the energy resources we use on Earth originally received their energy from the Sun. The Sun is powered by nuclear fusion, meaning its primary store is a nuclear energy store.
Energy is transferred from the Sun to Earth via the radiation pathway. Once on Earth, this energy is kept in an energy store until it is needed. For example, fossil fuels (coal, oil, and natural gas) and biofuels maintain a chemical energy store originating from ancient or recent photosynthesis.
Wind possesses a kinetic energy store due to uneven solar heating of the atmosphere, while hydroelectricity relies on the gravitational potential energy (GPE) store of water held in a high-altitude reservoir. Not all resources originate from the Sun, however. Nuclear fuels like uranium rely on their own internal nuclear energy store.
Geothermal energy uses the thermal energy store of extremely hot rocks in the Earth's crust. Finally, tides are unique because they possess both kinetic and GPE stores driven primarily by the gravitational pull of the Moon, rather than the Sun.
Most power stations work by forcing a fluid (like steam, water, or air) to spin a machine. The standard electricity generation pathway follows a specific, required sequence:
Different resources use different methods to provide that initial fluid motion:
Nuclear reactors achieve their heating through nuclear fission, where large, unstable nuclei split to release thermal energy. To safely control this extraction, control rods made of boron or cadmium absorb excess neutrons, while a moderator like water or graphite slows the neutrons down. The reactor explicitly uses a separate water loop to prevent the steam driving the turbine from becoming contaminated with radioactive materials.
Some methods bypass the turbine entirely. Solar photovoltaic (PV) cells harness the radiation pathway directly from the Sun and convert it into a direct current (DC) electrical transfer. It is important to distinguish these from solar thermal panels, which do not generate electricity but simply extract radiation to heat water for homes.
Globally, about 84% of energy is currently generated by non-renewable sources, though the UK has significantly reduced its reliance on coal in favour of natural gas (which releases less ) and renewables. Energy resources are broadly classified based on their replenishment rate. A renewable energy resource is replenished faster than it is used, meaning it will never run out.
Conversely, a non-renewable energy resource takes millions of years to form and is used much faster than it can be replaced. The table below compares these two broad categories of energy.
| Feature | Renewable Resources (e.g., Wind, Solar, Hydro) | Non-Renewable Resources (e.g., Fossil Fuels, Nuclear) |
|---|---|---|
| Similarities | Many use spinning turbines and generators to produce an electrical transfer. | Many use spinning turbines and generators to produce an electrical transfer. |
| Replenishment | Replenished faster than they are used (will not run out). | Used much faster than they are replenished (finite supply). |
| Carbon Emissions | Mostly zero emissions during operation (biofuels are carbon neutral). | Fossil fuels release huge amounts of (nuclear releases none). |
| Reliability | Often unreliable as they depend heavily on the weather or time of day. | Highly reliable and can provide continuous energy on demand. |
Power networks require a constant base-load of electricity to meet minimum national demand. This is typically provided by highly reliable sources like nuclear, coal, or geothermal energy, which can run 24/7. Hydroelectricity is also highly reliable; facilities often use pumped storage to act like a giant battery, releasing water immediately to meet sudden surges in demand.
Other resources are less consistent. Tides are entirely predictable but only generate power at specific, restricted times of the day. Wind and solar power are fundamentally unreliable because their output depends heavily on unpredictable weather conditions and daylight hours.
Every energy resource has an environmental cost. Burning fossil fuels releases carbon dioxide (), which contributes to global warming, and sulfur dioxide (), which causes acid rain. Nuclear power produces no greenhouse gases but leaves behind dangerous radioactive waste that requires expensive, long-term storage.
While renewable sources like wind and hydro do not emit , wind farms can cause visual and noise pollution. Hydroelectric dams often lead to severe habitat loss due to massive flooding. Biofuels offer a middle ground; they are often carbon neutral because the they absorb during growth matches the they release during combustion.
When calculating the performance of an energy resource, you may need to use the efficiency formula:
To calculate the energy stored in a hydroelectric reservoir, use the formula for gravitational potential energy:
Where:
A wind turbine captures kinetic energy from moving air. In one second, of kinetic energy is transferred to the turbine, and it transfers electrically to the National Grid. Calculate the efficiency of the wind turbine.
Step 1: Identify the required values.
Step 2: Substitute into the efficiency equation.
Step 3: Calculate the final answer.
Students often mistakenly refer to nuclear power as a fossil fuel. Remember that OCR mark schemes strictly distinguish nuclear fuel as a completely separate type of non-renewable resource.
When defining renewable resources, do not use the words 'recycled' or 'reusable'. You must use the specific term 'replenished' to earn the mark.
In 6-mark 'Compare' questions, examiners expect you to discuss both advantages (e.g., zero emissions) and disadvantages (e.g., high initial costs or unreliability) to access the top band of marks.
Always use the specific phrase 'transferred electrically' when describing the final energy pathway from a generator to the National Grid, rather than saying it 'produced electricity'.
Energy store
A system where energy is kept until it is transferred to another system or across an energy pathway (e.g., chemical, kinetic, thermal, nuclear).
Renewable energy resource
An energy source that is replenished at a faster rate than it is being used, meaning it will not run out.
Non-renewable energy resource
An energy source that is used up faster than it is replenished, meaning it has a finite supply and will eventually run out.
Nuclear fission
The process where a large, unstable nucleus splits into two smaller nuclei, releasing a significant amount of thermal energy.
Base-load
The continuous, minimum level of electricity demand on a national grid, typically met by reliable, constant resources like nuclear or fossil fuels.
Carbon neutral
A fuel or process where the amount of carbon dioxide released during combustion exactly equals the amount absorbed during its growth (e.g., biofuels).
Generator
A device in a power station that transfers energy from a kinetic store into an electrical transfer through the interaction of magnetic fields and conductors.
Turbine
A machine with a rotor made to revolve by a fast-moving flow of fluid, such as water, steam, or air.
Photovoltaic (PV) cells
Devices that directly convert electromagnetic radiation from the Sun into a direct current (DC) electrical transfer without using a turbine.
Control rods
Safety components in a nuclear reactor, typically made of boron or cadmium, that absorb excess neutrons to regulate the rate of the fission chain reaction.
Moderator
A material, such as water or graphite, used in a nuclear reactor to slow down fast-moving neutrons so that nuclear fission can be sustained.
Fossil fuels
Non-renewable energy resources (coal, oil, and natural gas) formed from the remains of ancient organisms that contain a chemical energy store.
Biofuels
Renewable energy resources derived from recently living organic matter, such as wood or crops, containing a chemical energy store.
Wind
A renewable energy resource that harnesses the kinetic energy store of moving air using turbines.
Hydroelectricity
A renewable energy resource that uses the gravitational potential energy store of water held in high-altitude reservoirs to generate electricity.
Nuclear fuels
Non-renewable radioactive materials, such as uranium-235, that release energy from their nuclear store during fission.
Geothermal
A renewable energy resource that utilizes the thermal energy store of hot rocks within the Earth's crust to produce steam.
Tides
A renewable energy resource that uses the kinetic and gravitational potential energy stores of moving water, driven primarily by the Moon's gravity.
Nuclear energy store
Energy stored in the nucleus of an atom; it is the source of energy for nuclear reactors and the Sun.
Put your knowledge into practice — try past paper questions for Physics A
Energy store
A system where energy is kept until it is transferred to another system or across an energy pathway (e.g., chemical, kinetic, thermal, nuclear).
Renewable energy resource
An energy source that is replenished at a faster rate than it is being used, meaning it will not run out.
Non-renewable energy resource
An energy source that is used up faster than it is replenished, meaning it has a finite supply and will eventually run out.
Nuclear fission
The process where a large, unstable nucleus splits into two smaller nuclei, releasing a significant amount of thermal energy.
Base-load
The continuous, minimum level of electricity demand on a national grid, typically met by reliable, constant resources like nuclear or fossil fuels.
Carbon neutral
A fuel or process where the amount of carbon dioxide released during combustion exactly equals the amount absorbed during its growth (e.g., biofuels).
Generator
A device in a power station that transfers energy from a kinetic store into an electrical transfer through the interaction of magnetic fields and conductors.
Turbine
A machine with a rotor made to revolve by a fast-moving flow of fluid, such as water, steam, or air.
Photovoltaic (PV) cells
Devices that directly convert electromagnetic radiation from the Sun into a direct current (DC) electrical transfer without using a turbine.
Control rods
Safety components in a nuclear reactor, typically made of boron or cadmium, that absorb excess neutrons to regulate the rate of the fission chain reaction.
Moderator
A material, such as water or graphite, used in a nuclear reactor to slow down fast-moving neutrons so that nuclear fission can be sustained.
Fossil fuels
Non-renewable energy resources (coal, oil, and natural gas) formed from the remains of ancient organisms that contain a chemical energy store.
Biofuels
Renewable energy resources derived from recently living organic matter, such as wood or crops, containing a chemical energy store.
Wind
A renewable energy resource that harnesses the kinetic energy store of moving air using turbines.
Hydroelectricity
A renewable energy resource that uses the gravitational potential energy store of water held in high-altitude reservoirs to generate electricity.
Nuclear fuels
Non-renewable radioactive materials, such as uranium-235, that release energy from their nuclear store during fission.
Geothermal
A renewable energy resource that utilizes the thermal energy store of hot rocks within the Earth's crust to produce steam.
Tides
A renewable energy resource that uses the kinetic and gravitational potential energy stores of moving water, driven primarily by the Moon's gravity.
Nuclear energy store
Energy stored in the nucleus of an atom; it is the source of energy for nuclear reactors and the Sun.
