Energy
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Energy Stores
There are eight energy stores — ways that energy can be held within a system.
Being able to name and identify all eight stores is the foundation — every energy question starts here.
Energy Stores — Key Knowledge
- Kinetic energy of a moving object
- Gravitational potential energy due to height in a gravitational field
- Elastic potential energy stored in a stretched/compressed object
- Thermal energy related to temperature
- Chemical energy stored in bonds — fuels, food, batteries
- Magnetic energy due to magnets attracting/repelling
- Electrostatic energy due to charged particles
- Nuclear energy stored in the nucleus of an atom
Energy Transfers
Energy moves between stores by different transfer pathways.
Students must distinguish transfers (how energy moves) from stores (where energy sits).
Energy Transfers — Key Knowledge
- Mechanically by a force acting on an object
- Electrically by charges moving through a circuit
- By heating due to a temperature difference
- By radiation by electromagnetic waves or sound
Specific Heat Capacity
How hard it is to heat a material up. Measured as the energy needed to raise the temperature of 1 kg of a substance by 1°C.
ΔE = mcΔθ
energy change = mass × specific heat capacity × temperature change
Links thermal energy store changes to measurable quantities — essential for calculation questions.
Specific Heat Capacity — Key Knowledge
- Specific heat capacity energy per kg per °C, units J/kg°C
Work Done
When a force moves an object, energy is transferred. The amount of energy transferred is the work done.
W = Fs
work done = force × distance
Connects forces to energy transfers — if a force moves something, energy has been transferred.
Work Done — Key Knowledge
- Work done energy transferred by a force, measured in joules, J
Kinetic Energy
The energy in an object's kinetic energy store depends on its mass and speed.
KE = ½mv²
kinetic energy = half × mass × speed²
Speed is squared — doubling speed quadruples kinetic energy.
Kinetic Energy — Key Knowledge
- Kinetic energy energy of a moving object, measured in J
Gravitational Potential Energy
The energy stored when an object is raised in a gravitational field.
GPE = mgh
gravitational potential energy = mass × gravitational field strength × height
Directly linked to kinetic energy in falling/projectile questions — GPE lost = KE gained (if no dissipation).
Gravitational Potential Energy — Key Knowledge
- Gravitational potential energy energy due to height, measured in J
- Gravitational field strength g = 10 N/kg on Earth
Elastic Potential Energy
The energy stored in a spring or elastic object when it is stretched or compressed.
EPE = ½ke²
elastic potential energy = half × spring constant × extension²
Extension is squared — doubling extension quadruples the stored energy.
Elastic Potential Energy — Key Knowledge
- Elastic potential energy energy in a deformed object, measured in J
- Spring constant k, how stiff the spring is, measured in N/m
Power
Power is the rate of energy transfer — how fast energy is moved, measured in watts.
P = E/t
power = energy transferred ÷ time or P = W/t
A more powerful device transfers the same energy in less time — links energy to time.
Power — Key Knowledge
- Power rate of energy transfer, measured in watts, W
- Watt 1 W = 1 joule per second
Conservation and Dissipation
Energy cannot be created or destroyed — only transferred between stores. In every transfer, some energy is dissipated to the surroundings.
Efficiency = useful output energy transfer ÷ total input energy transfer
or useful power output ÷ total power input
"Lost" energy isn't gone — it's been dissipated to thermal stores in the surroundings. Nothing is 100% efficient.
Conservation and Dissipation — Key Knowledge
- Conservation of energy total energy in a closed system stays constant
- Dissipation energy spreading out to surroundings, becoming less useful
- Efficiency the proportion of energy usefully transferred, expressed as a decimal or percentage
Reducing Unwanted Transfers
Unwanted energy transfers can be reduced by design — making devices more efficient.
Common exam question — name a method and explain how it reduces energy waste in a named device.
Reducing Unwanted Transfers — Key Knowledge
- Lubrication reduces friction between moving parts
- Thermal insulation reduces energy transfer by heating
- Thermal conductivity higher thermal conductivity = faster rate of energy transfer; thicker walls reduce cooling rate
Energy Resources
Energy resources are either renewable (won't run out) or non-renewable (finite supply). They're used for transport, electricity generation, and heating.
Renewable does not mean zero environmental impact — manufacturing, land use, and disposal all carry costs.
Energy Resources — Key Knowledge
- Renewable replenished naturally — solar, wind, wave, tidal, hydroelectric, geothermal, bio-fuel
- Non-renewable finite supply — fossil fuels: coal, oil, gas; nuclear fuel