Energy Stores and Transfers

Conservation of energy

Energy is never created or destroyed, only transferred between stores or dissipated to the surroundings.

Energy stores

• Kinetic – movement
• Gravitational potential – raised objects
• Elastic potential – stretched/compressed springs
• Chemical – food, fuel, batteries
• Thermal (internal) – hot objects
• Nuclear, electrostatic, magnetic

Transfer pathways

• Mechanically (a force doing work)
• Electrically (charge moving)
• By heating
• By radiation (e.g. light, sound)

Key equations

Kinetic energy:        Eₖ = ½ m v²
Gravitational PE:      Eₚ = m g h     (g ≈ 9.8 N/kg)
Elastic PE:            Eₑ = ½ k e²    (k = spring constant, e = extension)

Worked example

A 2 kg ball moving at 3 m/s:

Eₖ = ½ × 2 × 3² = ½ × 2 × 9 = 9 J

Useful vs wasted energy

Every transfer dissipates some energy (usually as thermal energy to the surroundings), which becomes less useful — so no machine is 100% efficient.

Exam tip

Say energy is transferred, not "used up". Naming the correct store and the pathway earns full marks.