A cinema uses two types of lighting in its foyer: a set of filament lamps and a set of LED spotlights. Both sets of lights produce the same brightness. The filament lamps each operate at a potential difference of 230 V with a current of 0.5 A. The LED spotlights each operate at the same potential difference but draw only 0.04 A. Explain why the cinema should replace all its filament lamps with LED spotlights, using the concepts of power and energy transfer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A cinema manager is reviewing the electricity costs of the foyer lighting. Both the filament lamps and the LED spotlights produce the same useful light output, but the manager notices the electricity bills differ significantly depending on which type of lighting is used.
Model answer (5 marks)
Power is the rate of energy transfer, P = IV.
The filament lamp operates at 230 V and 0.5 A, so P = 230 V × 0.5 A = 115 W.
The LED spotlight operates at the same voltage but only 0.04 A, giving P = 230 V × 0.04 A = 9.2 W.
Both sets of lights give the same useful light output, so the filament lamp must waste the extra power as heat.
Energy transferred in a given time is E = Pt (or E = IVt). Over the same operating period the filament lamp transfers far more energy from the supply.
Thus the LED spotlights use much less energy for the same brightness, lowering electricity bills and making them the more economical choice.
The filament lamp operates at 230 V and 0.5 A, so P = 230 V × 0.5 A = 115 W.
The LED spotlight operates at the same voltage but only 0.04 A, giving P = 230 V × 0.04 A = 9.2 W.
Both sets of lights give the same useful light output, so the filament lamp must waste the extra power as heat.
Energy transferred in a given time is E = Pt (or E = IVt). Over the same operating period the filament lamp transfers far more energy from the supply.
Thus the LED spotlights use much less energy for the same brightness, lowering electricity bills and making them the more economical choice.
Examiner tips
- Show the power calculation for each lamp type; include units. Explain that the same light output means the extra power of the filament lamp is wasted as heat. State that energy transferred = power × time, so less power = less energy used. Finish with a clear conclusion that LEDs are cheaper and more efficient.
Common mistakes
- Using the wrong formula for power (e.g. P = V²/R instead of IV). Forgetting to convert current units or mis‑multiplying voltage and current. Not linking the higher power of the filament lamp to wasted heat and higher energy use.
Mark scheme (5 marks)
- Power is the rate at which an appliance transfers energy (or: power = current × potential difference / P = IV)
- The filament lamp has a greater power than the LED spotlight (e.g. filament lamp power is 115 W; LED power is 9.2 W)
- Because both types produce the same brightness (same useful light output), the filament lamp wastes more energy / transfers more energy to the surroundings as heat
- Energy transferred equals power multiplied by time (E = Pt or E = IVt), so over the same time period the filament lamp transfers more energy from the supply
- Therefore the LED spotlights transfer less energy from the supply for the same light output, reducing electricity costs / making them the more economical/efficient choice
Key terms in this question
power · potential difference · current · energy · rate
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
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