A delivery drone carries a parcel from a warehouse to a rooftop. As the drone rises, some of the electrical energy from its battery is transferred to gravitational potential energy in the parcel, but some energy is dissipated. Explain why the process of lifting the parcel is not 100% efficient, and describe what happens to the energy that is not transferred to the parcel's gravitational potential energy store.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A delivery drone uses electrical energy from a rechargeable battery to lift parcels vertically to rooftop delivery points. The motors in the drone produce a resistive force on the air as the drone rises.
Model answer (5 marks)
1. The battery supplies electrical energy that is converted by the motors into mechanical work.
2. The motors and other moving parts experience friction and other resistive forces.
3. Work is therefore done against these resistive forces, which does not raise the parcel.
4. The energy used to overcome friction is converted into heat, raising the temperature of the motors and the surrounding air.
5. Because the useful output (gravitational potential energy of the parcel) is less than the total electrical input, the efficiency is less than 100 %.
2. The motors and other moving parts experience friction and other resistive forces.
3. Work is therefore done against these resistive forces, which does not raise the parcel.
4. The energy used to overcome friction is converted into heat, raising the temperature of the motors and the surrounding air.
5. Because the useful output (gravitational potential energy of the parcel) is less than the total electrical input, the efficiency is less than 100 %.
Examiner tips
- Use the word "friction" or "resistive forces" to show understanding of energy loss.
- Explain that the lost energy becomes heat in the motors and air.
- Show the efficiency formula: η = useful output / total input < 1.
- Keep the answer concise and directly linked to the marks scheme points.
Common mistakes
- Saying the energy is lost to "air resistance" only, ignoring internal friction.
- Claiming the energy is stored elsewhere instead of being converted to heat.
- Using vague terms like "waste" without explaining the mechanism.
Mark scheme (5 marks)
- Not all the input energy is converted into useful (gravitational potential) energy — some energy is dissipated/wasted
- The drone's motors/moving parts experience friction / resistive forces, causing the mechanical components to do work against those resistive forces
- This causes the drone's components to heat up, so energy is transferred to thermal energy in the surroundings
- The wasted energy is spread out into the surroundings (dissipated) and cannot be recovered to do useful work
- The efficiency is less than 1 (or 100%) because the useful output energy (gravitational potential energy gained) is less than the total electrical energy input from the battery
Key terms in this question
gravitational potential energy
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →