A delivery drone uses an electric motor to hover in place, holding a parcel at a fixed height. During hovering, the drone's motor transfers 2400 J of electrical energy every 10 seconds. Of this, 600 J per 10 seconds is dissipated as heat due to resistive forces in the motor. Explain why the drone cannot be 100% efficient, and describe what happens to the energy that is not usefully transferred.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A delivery drone hovers at a fixed height while carrying a parcel. Its electric motor transfers 2400 J of electrical energy every 10 seconds. Of this, 600 J per 10 seconds is dissipated as heat.
Model answer (5 marks)
The motor cannot be 100 % efficient because some of the electrical energy is always lost to the surroundings as heat.
1. The motor’s internal resistive forces (e.g. winding resistance) convert part of the electrical energy into thermal energy.
2. This heat is transferred to the air and the drone’s components, increasing their temperature.
3. The energy that is not used to lift the drone is therefore dissipated as thermal energy in the surroundings.
4. Because energy is conserved, the useful output energy (lifting the parcel) must be less than the total input energy when dissipation occurs, so efficiency cannot exceed 1.
5. The useful energy is 2400 J – 600 J = 1800 J, giving an efficiency of 1800 J/2400 J = 0.75 or 75 %.
1. The motor’s internal resistive forces (e.g. winding resistance) convert part of the electrical energy into thermal energy.
2. This heat is transferred to the air and the drone’s components, increasing their temperature.
3. The energy that is not used to lift the drone is therefore dissipated as thermal energy in the surroundings.
4. Because energy is conserved, the useful output energy (lifting the parcel) must be less than the total input energy when dissipation occurs, so efficiency cannot exceed 1.
5. The useful energy is 2400 J – 600 J = 1800 J, giving an efficiency of 1800 J/2400 J = 0.75 or 75 %.
Examiner tips
- Show the energy balance first (input – loss = useful).
- Use the term ‘heat’ or ‘thermal energy’ for the lost part.
- State the efficiency formula and calculate 75 %.
- Explain that conservation of energy forbids 100 % efficiency.
Mark scheme (5 marks)
- Some energy is always dissipated / lost to the surroundings
- Resistive forces in the motor cause energy to be transferred as heat / thermal energy
- The wasted energy is spread out into the surroundings (dissipation of energy)
- Efficiency cannot exceed 1 (or 100%) because of conservation of energy — useful output energy must always be less than total input energy when dissipation occurs
- Correct identification of useful output energy and correct efficiency value — 1800 J out of 2400 J is usefully transferred, giving an efficiency of 0.75 (or 75%)
Key terms in this question
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
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