Energy transfer

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    linh.tranLittle Lab Notes

    • 🎯 Try it

    Single choice

    Input 400 J, useful output 100 J. Efficiency?

    📝 Start by naming the system

    Energy is transferred between stores and surroundings; it is not used up. The useful output depends on the task.

    Kettle: electrical work transfers energy, increasing the thermal store of water and kettle. Some energy warms the surrounding air. If 100 kJ enters and 80 kJ heats the intended water, efficiency is 80 / 100 × 100 = 80%.

    Falling ball: the gravitational store decreases while the kinetic store increases. Air resistance transfers some energy to the surroundings.

    Bicycle braking: the moving bicycle slows as energy transfers to thermal stores of brakes, tyres, road, and surroundings. The energy has not disappeared.

    📊 Calculate and explain

  • 1.
    A lamp receives 200 J and transfers 30 J as useful visible light. Find its efficiency and the other transfers.
  • 2.
    A motor receives 500 J and transfers 350 J to a lifted load. Find its efficiency.
  • 3.
    Two devices both have 80% efficiency. Must their useful outputs be equal?
  • 4.
    A learner divides 40 watts by 200 joules and calls the result efficiency. What is missing?

    • State the system boundary and units before calculating.

    🔑 Worked corrections

  • 1.
    30 / 200 × 100 = 15%; the other transfers total 170 J.
  • 2.
    350 / 500 × 100 = 70%.
  • 3.
    No. With inputs of 100 J and 200 J, useful outputs would be 80 J and 160 J.
  • 4.
    Power and energy are different quantities. Compare useful output energy with input energy for the same process, or corresponding powers over the same conditions.

    • ✅ Check your understanding

    Single choice

    What happens to energy transferred by friction?

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    linh.tranLittle Lab Notes