Efficiency, Sankey diagrams, and work done · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Efficiency, Sankey diagrams, and work done

Science · Energy · ages 12-13
Name ______________________   Date ____________

  1. A constant force of 25 N pushes a box along its 8 m motion. How much work does this force do, in joules?

    Answer: ______________

  2. A student lifts a 3 kg mass to a height of 2 m. How much work is done against gravity? Use g = 9.8 m/s^2.

    Answer: ______________

  3. A car engine transfers 1000 J from fuel. Of this, 250 J does useful work moving the car. The other outputs warm the engine and surroundings or make noise. How is the remaining 750 J transferred?

    • It disappears
    • It becomes kinetic energy of the car
    • It becomes heat and sound
    • It is stored in the fuel

  4. In a Sankey diagram for a light bulb, which arrow usually points to 'wasted energy'?

    • The arrow labeled 'light'
    • The arrow labeled 'heat'
    • Both arrows equally

  5. A worker pushes a crate with a constant 40 N force along its 3 m motion. How much work does this force do, in joules?

    Answer: ______________

  6. The bars compare two arrow widths in an energy Sankey diagram. One arrow is twice as wide as the other. What does that mean?

    Arrow A40Arrow B20
    • The wider arrow carries twice as much energy
    • The wider arrow is twice as fast
    • The wider arrow is twice as old

  7. A pulley system has an efficiency of 75 percent. If you do 400 J of work pulling the rope, how much work is done on the load?

    • 100 J
    • 300 J
    • 400 J
    • 533 J

  8. A motor takes in 150 J of energy. Of this, 30 J is wasted as heat and the rest is useful output. What is its efficiency, as a percentage?

    Answer: ______________

  9. A Sankey diagram for a phone charger shows a wide arrow splitting into a very wide 'useful charge' arrow and a very thin 'wasted heat' arrow. Priya says this charger is inefficient. Is she right?

    • Yes, any wasted energy means the charger is inefficient
    • Yes, because the wasted arrow should be wider than the useful one
    • No, a wide useful arrow and a thin wasted arrow mean high efficiency

  10. A light bulb is 10% efficient and uses 60 J of energy. How many joules become useful light?

    Answer: ______________

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Answer key

For grown-ups. Fold this page away before handing over the rest.

Efficiency, Sankey diagrams, and work done W1-mt_JyfLtl_nhw-s1

  1. 200 · Work = force × distance = 25 N × 8 m = 200 J.
  2. 58.8 · Work = force × distance. Force = weight = mg = 3 × 9.8 = 29.4 N. Work = 29.4 × 2 = 58.8 J.
  3. It becomes heat and sound · The stated remaining outputs are heat and sound. Their energy totals 1000 - 250 = 750 J.
  4. The arrow labeled 'heat' · The bulb is used for lighting, so unwanted heat counts as wasted output.
  5. 120 · Work done = F × d = 40 × 3 = 120 J.
  6. The wider arrow carries twice as much energy · Arrow width in a Sankey diagram is proportional to the amount of energy it carries.
  7. 300 J · Output work = efficiency × input work = 0.75 × 400 = 300 J.
  8. 80 · Useful energy = 150 - 30 = 120 J. Efficiency = 120 ÷ 150 × 100 = 80%.
  9. No, a wide useful arrow and a thin wasted arrow mean high efficiency · Most of the energy reached the useful job, with very little wasted. That means high efficiency, not low.
  10. 6 · 10% of 60 J = 6 J becomes light. The rest, 54 J, is wasted mostly as heat.
Worksheet · LightMySky