Energy, work and power

In physics, 'work' has a very specific meaning. Work is done whenever a force causes an object to move. If you push against a wall and it doesn't move, you have done no work, even though you might feel tired! The amount of work done is directly linked to the amount of energy transferred. For example, when you lift a book, you do work against gravity, and the chemical energy from your muscles is transferred to the book as gravitational potential energy. Work done and energy transferred are both measured in the same unit: the joule (J). One joule of work is done when a force of one newton moves an object by one metre.

Energy isn't 'used up'; it's stored and moved around. We can think of energy being held in different 'stores'. When something happens, energy is transferred from one store to another. There are 8 main energy stores you need to know: Kinetic (movement), Gravitational Potential (height), Chemical (in food, fuels, batteries), Elastic Potential (in stretched or squashed objects), Thermal (heat), Nuclear (in atomic nuclei), Electrostatic (in electric charges), and Magnetic (in magnets and electromagnets). Energy can be transferred between these stores in four main ways: by mechanical work (a force moving something), by electrical work (currents flowing), by heating (due to a temperature difference), or by radiation (like light or sound waves).

Two of the most important energy stores in mechanics are Gravitational Potential Energy (GPE) and Kinetic Energy (KE). GPE is the energy an object has because of its height in a gravitational field. The higher it is, the more GPE it has. The formula is GPE = mass × gravitational field strength × height. Kinetic energy is the energy an object has because it is moving. The faster it moves and the more mass it has, the more kinetic energy it has. The formula is KE = ½ × mass × velocity². Notice that the velocity is squared, which means doubling an object's speed quadruples its kinetic energy!

This is one of the most fundamental laws in all of science. The Principle of Conservation of Energy states that energy cannot be created or destroyed, only transferred from one store to another. In a 'closed system' (where no energy can get in or out), the total amount of energy is always constant. In the real world, energy often seems to be 'lost', for example, when a bouncing ball eventually stops. The energy hasn't been destroyed; it has been transferred to less useful stores. In the case of the ball, each bounce transfers some energy to a thermal store (heating the ball and the floor) and a sound store. This dissipated energy spreads out into the environment and is difficult to use again.

Power is the rate at which work is done or energy is transferred. A more powerful machine can do the same amount of work in less time. The unit of power is the watt (W), where 1 watt is equal to 1 joule of energy transferred per second. Efficiency is a measure of how good a device is at transferring energy into the form we want. No device is 100% efficient; some energy is always wasted, usually as heat. Efficiency is calculated by dividing the useful energy (or power) output by the total energy (or power) input. It has no units and is given as a decimal (e.g., 0.8) or a percentage (e.g., 80%). A Sankey diagram is a special type of flow chart where the width of the arrows is proportional to the amount of energy. It's a great way to visualise efficiency, with the straight arrow showing useful energy and the curving arrow showing wasted energy.

Energy resources are the sources we use to generate electricity and power our world. They are broadly classified into two groups: non-renewable and renewable. Non-renewable resources are finite; they are used up much faster than they are formed and will eventually run out. The main examples are fossil fuels (coal, oil, and natural gas) and nuclear fuels (like uranium). Renewable resources are those that are replenished naturally on a human timescale or will not run out as a result of being used. Examples include solar, wind, hydroelectric, geothermal, and biomass. Most of the energy we use on Earth, including both renewable and fossil fuels, originates from the Sun. The Sun's energy drives the weather for wind and rain (hydroelectric), and it was the source of energy for the ancient plants and animals that became fossil fuels.

Non-renewable resources, primarily fossil fuels and nuclear fuel, are the world's main source of energy because they are reliable and have a high energy density. In a typical thermal power station, a fuel is used to generate heat. For fossil fuels (coal, oil, gas), the fuel is burned (combustion). For nuclear fuel (uranium), atoms are split (fission). This heat is used to boil water, creating high-pressure steam. The steam is then directed at the blades of a turbine, causing it to spin. The spinning turbine is connected to a generator, which converts the mechanical energy of rotation into electrical energy.

Fossil Fuels:

• Advantages: Reliable (can generate power 24/7), high energy density, relatively cheap to build power stations.
• Disadvantages: Release carbon dioxide (a greenhouse gas) causing climate change, release sulfur dioxide causing acid rain, they are finite and will run out.

Nuclear Fuel:

• Advantages: No greenhouse gas emissions during operation, extremely high energy density (a small amount of fuel produces a lot of energy), reliable.
• Disadvantages: Produces hazardous radioactive waste that is difficult and expensive to store for thousands of years, high cost of building and decommissioning plants, risk of catastrophic accidents.

Renewable energy resources offer an alternative to fossil fuels, helping to reduce pollution and combat climate change. Here's how the main types work:

• Solar: Photovoltaic cells (solar cells) convert sunlight directly into electricity. Solar panels use sunlight to heat water. Adv: No pollution, free energy source after setup. Disadv: Only works when sunny (intermittent), requires a large area for significant power output.
• Wind: Wind turns large blades connected to a turbine and generator. Adv: No pollution, low running costs. Disadv: Intermittent (only works when windy), can be noisy and considered a visual blight, can be a danger to birds.
• Hydroelectric Power (HEP): A dam is built to trap water, creating a reservoir. The water is allowed to flow down through turbines to generate electricity. Adv: Very reliable, can be switched on quickly, low running costs. Disadv: High initial cost, floods large areas of land destroying habitats, can be affected by drought.
• Geothermal: In volcanic regions, cold water is pumped down to hot rocks deep underground. It turns to steam, which rises to turn turbines. Adv: Reliable and consistent, no fuel costs, small footprint on land. Disadv: Only possible in specific geological locations.
• Wave & Tidal: The movement of sea water is used to turn turbines. Tidal barrages work like HEP dams across estuaries, while wave machines float on the surface. Adv: Predictable (tides), no pollution. Disadv: High construction and maintenance costs due to corrosive seawater, can disrupt marine ecosystems.