Energy — stores, transfers and efficiency

Energy stores

Energy is stored in different ways:

Energy transfers

Energy is transferred between stores by:

• Mechanical work (force acting through a distance).
• Heating (thermal conduction, convection, radiation).
• Electrical work (current through a component).
• Radiation (light, sound, electromagnetic waves).

Key energy equations

Kinetic energy: KE = ½ × m × v² (joules; m in kg, v in m/s)

Gravitational potential energy: GPE = m × g × h (joules; m in kg, g = 10 N/kg, h in m)

Work done: W = F × d (joules; F in N, d in m)

Conservation of energy: Energy is never created or destroyed. It is transferred from one store to another. The total energy in a closed system is constant.

Example — ball dropped from height h:

• At top: GPE = mgh, KE = 0.
• Falling: GPE decreases, KE increases.
• Just before hitting ground: GPE = 0, KE = mgh (if no air resistance).

Efficiency

Efficiency measures how much of the energy input is usefully transferred (and how much is wasted).

Efficiency = useful energy output / total energy input × 100%

OR

Efficiency = useful power output / total power input × 100%

A value of 100% would mean no energy is wasted (impossible in practice). Most devices waste energy as heat.

Example: An electric motor has 800 J input; 640 J useful output. Efficiency = 640/800 × 100% = 80%.

The wasted energy (160 J) is usually dissipated as heat.

Reducing wasted energy

• Lubrication reduces friction between moving parts → less wasted thermal energy.
• Insulation reduces heat loss from buildings/boilers.
• Regenerative braking in electric vehicles converts KE back to electrical energy.

Power

Power (P) = energy transferred (E) / time (t) P = E/t (watts, W; where 1 W = 1 J/s)

Also: P = W/t = F × v (force × velocity, for constant speed).