Power & Efficiency
Year 9 🌡️ Energy & Thermodynamics Calculate power P = W/t; calculate efficiency as useful/total energy.
💡 Power
Power is the rate of energy transfer or work done. A powerful machine transfers more energy per second.
Power
$$P = \frac{W}{t} = \frac{E}{t}$$
$P$ = power (W) · $W$ = work done (J) · $t$ = time (s)
Example: A motor does 1200 J in 60 s → P = 1200 ÷ 60 = 20 W
♻️ Efficiency
No machine is 100% efficient — some energy is always wasted, usually as heat or sound.
Efficiency
$$\text{Efficiency} = \frac{\text{useful energy output}}{\text{total energy input}} \times 100\%$$Example: A motor takes in 500 J and produces 350 J useful work. Efficiency = (350/500) × 100% = 70%
📊 Sankey Diagrams
Sankey diagrams show energy transfers. Arrow width represents amount of energy. Wasted energy branches off.
In a Sankey diagram: input energy (left) = useful output + wasted energy (heat/sound).
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