HKDSE · DSE Physics

DSE Physics: Energy and Use of Energy (Elective) — Practice Questions & Answers

Electricity at home and power, heat transfer in buildings, energy efficiency and conservation, and renewable and non-renewable sources.

282 practice questions available for this topic — here are 10 with full answers and explanations.

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Practice questions with answers

1 Multiple choice · Easy

Which of the following is a renewable energy source?

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WhySolar energy is naturally replenished.
2 Multiple choice · Medium

Solar power is described as renewable because it

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WhyIt is naturally replenished and will not run out on a human timescale.
3 Multiple choice · Medium

For Hong Kong, which renewable source is generally considered most feasible, given its long coastline and windy climate?

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WhyWind (and solar) are the most practical renewables for Hong Kong.
4 Fill in the blank · Easy

An energy source that is naturally replenished and will not run out is called .

Answer: renewable

WhyRenewable sources are continually replenished.
5 Fill in the blank · Medium

Coal, oil and natural gas are examples of fuels that will eventually run out.

Answer: fossil / non-renewable / nonrenewable

WhyThese are non-renewable fossil fuels.
6 Fill in the blank · Hard

Solar energy ultimately comes from nuclear reactions in the core of the Sun.

Answer: fusion

WhyThe Sun fuses hydrogen into helium.
7 Multiple choice · Medium

Water falls through 50 m and passes the turbine of a hydro plant at 3000 kg s⁻¹. If the electrical output is 1.0 MW, the efficiency is about (g = 9.81 m s⁻²)

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WhyInput power = 3000×9.81×50 = 1.47 MW; efficiency = 1.0/1.47 ≈ 68%.
8 Multiple choice · Hard

A wind turbine has blades 5 m long and 25% efficiency in a 12 m s⁻¹ wind (air density 1.2 kg m⁻³). Its electrical output is about

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WhyP = 0.25×½×1.2×(π×5²)×12³ ≈ 2.0×10⁴ W ≈ 20 kW.
9 Multiple choice · Easy

Efficiency is defined as

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WhyEfficiency = useful output power / total input power.
10 Fill in the blank · Medium

Efficiency equals useful output power divided by total power.

Answer: input

WhyEfficiency = output/input.

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Key terms in Energy and Use of Energy (Elective)

Power rating: The rate at which an appliance transfers energy, stated by the manufacturer in watts.
Kilowatt-hour: A unit of energy equal to the energy used by a 1 kW device in one hour, used for billing electricity.
U-value: A measure of how readily heat passes through a building element, with lower values meaning better insulation.
Thermal insulation: Material used to reduce the rate of heat transfer through walls, roofs or windows.
Energy efficiency: The fraction of input energy converted to useful output energy, often expressed as a percentage.
Useful energy: The portion of energy supplied that is transferred to the desired form rather than wasted.
Wasted energy: Energy transferred to forms that are not useful, usually as heat to the surroundings.
Renewable energy: Energy from sources that are naturally replenished, such as sunlight, wind and water.

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Practise other DSE Physics topics

Topic A: Heat and Gases Topic B: Force and Motion Topic C: Wave Motion Topic D: Electricity and Magnetism Topic E: Radioactivity and Nuclear Energy Topic F: Astronomy and Space Science (Elective) Topic G: Atomic World (Elective) Topic I: Medical Physics (Elective)