Compare devices that change energy from one form to another

Answer Key - Grade 11 Science Worksheet

Circle the best answer, or write your answer on the line.

1. 1.

   Bo's generator runs on 800 J of fuel. The engine is 20% efficient, the generator 85% and the lamp 40%. How much light energy comes out?

   54.4 J
2. 2.

   Esme's generator runs on 2000 J of fuel. The engine is 40% efficient, the generator 95% and the lamp 80%. How much light energy comes out?

   608 J

3. Read this first

   Three small wind turbines (example data)

   design	energy in (J)	electrical out (J)	mass (g)	cost ($)
   A	400	120	600	24
   B	500	160	900	30
   C	300	105	450	45

4. 3.

   Using the turbine table, which design is the most efficient, and by how much, Jun?

   • C, at 35%
   • A, at 30%, since it has the lowest cost
   • C, at 105%, its electrical energy as a percent
   • B, at 32%, since it gives the most electrical energy
5. 4.

   Using the turbine table, the turbine must weigh at most 700 g and cost at most $40. Which design qualifies, and how efficient is it, Kira?

   • A and B, since both cost under $40
   • B, since it gives the most electrical energy
   • C, since it is the most efficient
   • Only A qualifies, at 30%
6. 5.

   A 750 W motor is 85% efficient and runs 4.0 h. How much energy does it waste, Milo?

   • 112.5 Wh, one hour of the waste
   • 2,550 Wh, the useful energy
   • 3,000 Wh, the energy put in
   • 450 Wh
7. 6.

   A pump must lift 60 kg of water each second through 5.0 m. It is 60% efficient. What input power does it need, Noah?

   • 4,900 W
   • 1,764 W, from 2,940 × 0.60
   • 2,940 W, the useful power
   • 490 W
8. 7.

   An electric car stores 60 kWh. Its motor is 90% efficient and its wheels need 0.18 kWh per km. What is its range, Pia?

   • About 270 km, taking 90% of 300 km
   • About 300 km
   • About 333 km, leaving out the motor loss
   • About 370 km, from 60 ÷ (0.18 × 0.90)
9. 8.

   Kettle A is 90% efficient at 1,800 W; kettle B is 85% efficient at 2,400 W. Which heats 1.0 kg of water by 80 °C (334,880 J) in under 3 min, Ren?

   • Both, since both are over 85% efficient
   • Only B, in about 164 s
   • A, since it is more efficient
   • Neither, since both take over 3 minutes
10. 9.

    A 1,200 kg car slows from 20 m/s to rest. Regenerative braking recovers 60% of its kinetic energy. How much is recovered, Tala?

    • 240 kJ, the whole of the kinetic energy
    • 144 kJ
    • 96 kJ, 40% of the kinetic energy
    • 288 kJ, using mv² without the one half
11. 10.

    Sunlight gives 1,000 W per m². A solar panel is 20% efficient. What area gives 1,200 W, Zee?

    • 1.2 m², leaving out the efficiency
    • 0.24 m², multiplying by 0.20
    • 30 m², using 4% efficiency
    • 6.0 m²