Energy Stores and Transfers

Energy explains how and why things change. Think of stores as places where energy is kept, and transfers as the ways energy moves between stores. Like money moving between bank accounts, the total amount is conserved.

Main energy stores

• Kinetic – in moving objects
• Gravitational potential – in raised objects
• Elastic (strain) – in stretched or compressed objects
• Chemical – in fuels, food, and batteries
• Internal (thermal) – in hot objects (the energy of particles moving/vibrating)
• Electrostatic – in charged objects due to their positions
• Nuclear – in atomic nuclei

How energy is transferred (pathways)

• By forces (mechanical work) – a force moves something, transferring energy
• By electrical currents (electrical work) – charges move through components
• By heating – from hot to cold objects (by conduction, convection or radiation)
• By waves – light, infrared, and sound carry energy from place to place

Conservation of energy

Energy cannot be created or destroyed; it is only transferred between stores. Some becomes useful (what you want) and some becomes wasted (often heating the surroundings). Sankey diagrams show inputs and outputs; arrow thickness shows the amount.

Key equations for stores and work

Kinetic energy:

$$E_k = \tfrac{1}{2} m v^2$$

Change in gravitational potential energy:

$$\Delta E_p = m g \Delta h$$

Mechanical work done (equals energy transferred):

$$W = F d = \Delta E$$

Common transfer chains

• Phone charging: chemical (battery) increases via electrical work from mains
• Bicycle braking: kinetic → internal (thermal) in brake pads and air (heating)
• Stretching a spring: mechanical work → elastic store
• Sun warming skin: light/infrared waves → internal (thermal)

Misconceptions to avoid

• Energy is not “used up”; it spreads out and becomes less useful
• A force is not a store; it is a way to transfer energy
• Temperature is not a store; the store is internal (thermal) energy
• Electrical current is a pathway, not a store