Energy Stores and Transfers
Energy stores and transfers
Energy is never made or destroyed — it is stored and transferred from one store to another. This is the conservation of energy: the total energy in a closed system stays the same.
Energy stores
| Store | Example |
|---|---|
| Kinetic | Anything moving |
| Gravitational potential | An object raised up high |
| Elastic potential | A stretched spring or elastic band |
| Chemical | Food, fuel, batteries |
| Thermal (internal) | A hot object |
| Nuclear | Energy in the nucleus of atoms |
| Magnetic / Electrostatic | Magnets; charged objects |
Energy transfers (pathways)
Energy moves between stores by four pathways:
- Mechanically (a force doing work)
- Electrically (a current)
- By heating (conduction, convection, radiation)
- By radiation (e.g. light, sound waves)
Example: a falling ball transfers gravitational potential → kinetic (mechanically). A kettle transfers electrical → thermal (by heating).
Key equations
Kinetic energy:
KE = ½ × m × v² (joules) — mass in kg, speed in m/sGravitational potential energy:
GPE = m × g × h (g ≈ 9.8 N/kg on Earth)Elastic potential energy:
Ee = ½ × k × e² (k = spring constant, e = extension)Dissipation (wasted energy)
In real transfers, some energy is always dissipated — spread out to the surroundings, usually as wasted heat (e.g. friction warming up surfaces). This energy isn't destroyed, but it becomes less useful because it's spread out.
- Reducing waste: lubrication (less friction), and insulation (thicker/lower thermal conductivity materials reduce heat loss from buildings).
Worked example
A 2 kg ball moves at 3 m/s. What is its kinetic energy?
KE = ½ × m × v² = ½ × 2 × 3² = ½ × 2 × 9 = 9 J✓
Common mistakes
- Forgetting to square the speed (v²) in the KE equation.
- Saying energy is "used up" — it's transferred and some is dissipated, never destroyed.
- Mixing up energy stores (kinetic, GPE...) with pathways (mechanical, electrical...).
Exam tips
- Learn the three energy equations and their units (joules, kg, m/s, m).
- Describe transfers as "from store X to store Y by pathway Z".
- Link friction/insulation to reducing dissipated (wasted) energy.
Key facts to remember
- Conservation of energy: energy is transferred between stores, never created or destroyed.
- Stores: kinetic, GPE, elastic, chemical, thermal, nuclear, magnetic/electrostatic. Pathways: mechanical, electrical, heating, radiation.
- KE = ½mv², GPE = mgh; wasted energy is dissipated (spread out, usually as heat), reduced by lubrication/insulation.