A student notices that their home has two types of lighting: old incandescent bulbs and newer LED bulbs. Both types produce the same amount of light, but the LED bulbs have a much lower power-rating. Explain what is meant by the term power-rating and explain why using LED bulbs instead of incandescent bulbs reduces the cost of the electricity bill.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An incandescent bulb has a power-rating of 60 W. An LED bulb produces the same amount of light but has a power-rating of only 8 W.
Model answer (4 marks)
Power‑rating is the maximum rate at which an appliance can transfer energy, expressed in watts.
An LED bulb transfers energy at a lower rate than an incandescent bulb for the same light output.
The incandescent bulb produces a large amount of waste energy as heat, whereas the LED converts most of its energy into light.
Because the LED uses less energy per unit time, fewer kilowatt‑hours are consumed, so the electricity bill is lower.
An LED bulb transfers energy at a lower rate than an incandescent bulb for the same light output.
The incandescent bulb produces a large amount of waste energy as heat, whereas the LED converts most of its energy into light.
Because the LED uses less energy per unit time, fewer kilowatt‑hours are consumed, so the electricity bill is lower.
Examiner tips
- Define power‑rating in watts; link to energy transfer rate.
- Show that LEDs give the same light with lower wattage.
- Explain waste heat in incandescents.
- Connect lower energy use to lower kWh and bill.
Common mistakes
- Confusing power‑rating with light output; writing ‘power‑rating is the light produced’.
- Ignoring the waste heat aspect of incandescents.
- Failing to mention kilowatt‑hours or the bill reduction.
Mark scheme (4 marks)
- Power-rating is the maximum rate of energy transfer of an appliance
- The LED bulb transfers energy at a lower rate than the incandescent bulb (for the same light output)
- The incandescent bulb produces more waste energy transfer (e.g. heat/thermal energy) than the LED
- Because the LED uses less energy (per unit time / in total), fewer kilowatt-hours are used, so the electricity bill is lower
Key terms in this question
Related
- All WJEC GCSE Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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