NCEA Level 1 Energy Practice Test

An LED consumes 5 J of electrical energy. If its efficiency is 20% for light, how much light energy is produced?

0.5 J 4 J 2 J 1 J

Correct Answer:

1 J

Explanation:

Key idea: efficiency is the fraction of input energy that becomes light. If 20% of the electrical energy is converted to light, you multiply the input energy by 0.20. So with 5 joules of electrical energy, light energy produced = 5 × 0.20 = 1 joule. The remaining energy isn’t light (it's wasted as heat or other losses). The other numbers would come from using different fractions (for example, 0.10 would give 0.5 J, 0.80 would give 4 J, and 0.40 would give 2 J).

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