Energy

GCSE Physics Energy covers how energy is stored, transferred and conserved in systems, including kinetic, gravitational potential, elastic potential and thermal energy stores, as well as power, efficiency, insulation and energy resources.

A system is an object or group of objects, and when a system changes, energy is transferred between different stores by heating, mechanical work, electrical work or radiation, while the total energy of a closed system remains constant.

Kinetic energy is the energy stored by a moving object and is calculated using $E_k=\frac{1}{2}mv^2$, where $m$ is mass in kilograms and $v$ is speed in metres per second, so increasing speed has a large effect because speed is squared.

Gravitational potential energy is the energy stored because of an object's position in a gravitational field and is calculated using $E_p=mgh$, where $m$ is mass, $g$ is gravitational field strength and $h$ is the vertical height gained.

Elastic potential energy is stored when an elastic object such as a spring is stretched or compressed and is calculated using $E_e=\frac{1}{2}ke^2$, where $k$ is the spring constant and $e$ is the extension, provided the elastic limit has not been exceeded.

Thermal energy and heat physics describe energy changes caused by temperature differences, and the change in thermal energy is calculated using $\Delta E=mc\Delta\theta$, where $c$ is specific heat capacity and $\Delta\theta$ is the temperature change.

Specific heat capacity is the amount of energy needed to raise the temperature of 1 kg of a substance by $1^\circ C$, so substances with a high specific heat capacity need more energy to achieve the same temperature rise.

Power describes how quickly energy is transferred or work is done and is calculated using $P=\frac{E}{t}$ or $P=\frac{W}{t}$, where power is measured in watts and 1 watt is equal to 1 joule transferred per second.

Conservation of energy means energy cannot be created or destroyed, but during energy transformation and conversion some energy may become dissipated into less useful stores, often as thermal energy in the surroundings.

Energy efficiency compares useful output with total input and can be calculated using $\text{efficiency}=\frac{\text{useful output energy}}{\text{total input energy}}$ or $\frac{\text{useful output power}}{\text{total input power}}$, with answers often expressed as percentages.

Unwanted energy transfers can be reduced using lubrication, thermal insulation and better design, because reducing friction and heat loss helps more of the input energy remain useful.

Thermal insulation and heat insulators reduce the rate of energy transfer through walls, roofs and other surfaces, and thicker materials with lower thermal conductivity generally reduce heat loss more effectively.

Non-renewable energy resources include coal, oil, natural gas and nuclear fuel, while renewable energy resources include solar, wind, hydroelectricity, geothermal, tidal, wave and biofuel, each with different advantages and limitations.

Energy resources are compared using reliability, cost, environmental impact, availability, start-up time and suitability for transport, heating and electricity generation, rather than simply deciding that one source is always best.

Global and world energy consumption is influenced by population growth, living standards, industrial development and technology use, so energy policy must balance sustainability, security of supply, environmental impact and economic cost.