A battery-powered toy car transfers energy as it moves along a flat surface. Explain why the toy car slows down and eventually stops, referring to useful energy transfer, waste energy transfer, and the conservation of energy.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A battery-powered toy car is switched on and moves along a flat surface. After a short time, the car slows down and stops.
Model answer (5 marks)
The battery supplies electrical energy to the motor, which converts it into useful kinetic energy that moves the car.
While moving, friction between the wheels and the surface and air resistance act on the car. These forces cause waste energy transfers, converting some of the useful kinetic energy into thermal energy (heat) in the wheels, axle and air.
The heat produced does not help the car move; it is a loss of useful energy.
As the battery discharges, the amount of electrical energy available to the motor decreases. With less energy to convert into kinetic energy, the car’s speed reduces.
Throughout the process, energy is conserved: the electrical energy from the battery is transferred into kinetic and thermal energy, but the total energy of the system remains constant.
While moving, friction between the wheels and the surface and air resistance act on the car. These forces cause waste energy transfers, converting some of the useful kinetic energy into thermal energy (heat) in the wheels, axle and air.
The heat produced does not help the car move; it is a loss of useful energy.
As the battery discharges, the amount of electrical energy available to the motor decreases. With less energy to convert into kinetic energy, the car’s speed reduces.
Throughout the process, energy is conserved: the electrical energy from the battery is transferred into kinetic and thermal energy, but the total energy of the system remains constant.
Examiner tips
- Use the word "convert" to link battery to motor and motor to kinetic energy. Mention both friction and air resistance as waste energy sources. Show the link between battery depletion and reduced kinetic energy. End with a brief statement on conservation of energy.
- common_mistakes
- :
- Confusing kinetic energy with potential energy; forgetting to mention heat as waste energy. Saying the battery “runs out” without linking it to reduced useful energy. Omitting the conservation of energy statement or using vague wording like "energy is lost.
Mark scheme (5 marks)
- The battery supplies energy to the motor, which provides a useful energy transfer to kinetic energy (movement) of the car.
- Friction (between wheels/surfaces) and/or air resistance cause waste energy transfers.
- The waste energy transfer is to thermal energy (heat), which does not serve the purpose of the system.
- As the battery runs down, the total energy available to the car decreases, so less energy is available for useful kinetic energy transfer, causing the car to slow down.
- Energy is conserved throughout — it is not created or destroyed, only transferred; the total energy of the closed system remains constant.
Key terms in this question
useful energy transfer · waste energy transfer · conservation of energy
Related
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