The problems are on the energy transfers worksheet. Try them on your own first, then compare with the answers below.
Quick answer: An energy transfer is the movement of energy from one store to another or its change from one form to another. These worked answers correct the 20 problems of the grade 7 science worksheet on energy transfers, from name the transfer to is a heater wasteful?: each solution shows the formula, the substitution and the final answer with its unit.
Each answer shows the formula, the substitution and the final result in bold with its unit. Check that useful plus wasted energy always equals the input, and remember that efficiency has no unit apart from the percentage sign.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-PS3-5
Problem 1 Name the transfer
1. Toaster: electrical in, thermal (and a little light) out.
2. Loudspeaker: electrical in, sound out.
3. Solar cell: light in, electrical out.
4. Candle: chemical in, light and thermal out.
Problem 2 The conservation law
Energy cannot be created or destroyed. It can only be transferred from one store to another.
Problem 3 Phone in a pocket
1. Thermal energy.
2. It is wasted, because the aim is to show video and play sound, not to heat the phone.
Problem 4 Efficiency of a lamp
1. Efficiency = (useful energy output ÷ total energy input) × 100 %.
2. 40 ÷ 100 × 100 = 40 %.
Problem 5 Complete the energy chain
1. The first arrow carries light energy.
2. The second arrow carries electrical energy.
3. The fan produces kinetic energy in the moving air.
Problem 6 Kilojoules and joules
1. 18 × 1000 = 18 000 J.
2. 240 000 ÷ 1000 = 240 kJ.
3. 3.5 × 1000 = 3500 J.
Problem 7 Wasted energy of a bulb
1. Wasted = 50 − 8 = 42 J.
2. It is mostly thermal energy.
Problem 8 Reading a Sankey diagram
1. 56 J of kinetic energy.
2. 24 J (thermal and sound).
3. Efficiency = 56 ÷ 80 × 100 = 70 %.
Problem 9 Two kettles
1. X wastes 150 − 132 = 18 kJ. Y wastes 150 − 120 = 30 kJ.
2. X: 132 ÷ 150 × 100 = 88 %. Y: 120 ÷ 150 × 100 = 80 %.
3. Kettle X is better because it wastes less energy for the same input.
Problem 10 Complete the table
Fan: wasted 200 − 120 = 80 J; efficiency 120 ÷ 200 × 100 = 60 %.
Drill: useful 500 − 150 = 350 J; efficiency 350 ÷ 500 × 100 = 70 %.
Heater: wasted 1000 − 950 = 50 J; efficiency 950 ÷ 1000 × 100 = 95 %.
Problem 11 A warm charger
1. Electrical energy from the socket is transferred to chemical energy stored in the battery.
2. Some of the electrical energy is transferred to thermal energy in the charger, because no device is perfectly efficient.
3. It spreads into the surrounding air, where it cannot be used again.
Problem 12 Solar panel on a roof
1. Useful = 20 % of 5000 = 0.20 × 5000 = 1000 J.
2. Wasted = 5000 − 1000 = 4000 J.
Problem 13 Bicycle lamp chain
Chemical energy (in her muscles) → kinetic energy (pedals and wheel) → electrical energy (dynamo) → light energy (lamp). Thermal energy is wasted in the muscles, the bearings, the dynamo and the lamp.
Problem 14 Which lamp is better?
1. Lamp P: 6 ÷ 40 × 100 = 15 %. Lamp Q: 6 ÷ 12 × 100 = 50 %.
2. Lamp Q: it needs only 12 J for the same light, so it takes less electrical energy from the supply.
Problem 15 Find the errors
1. Energy cannot be created, so a device can never give out more energy than it takes in. Efficiency can never exceed 100 %.
2. Possibly the input was measured wrongly, or the water was already warm, or the output included heat from the surroundings.
Problem 16 Wind turbine in two stages
1. 0.40 × 50 = 20 kJ.
2. 0.90 × 20 = 18 kJ.
3. Overall efficiency = 18 ÷ 50 × 100 = 36 % (also 0.40 × 0.90 = 0.36).
Problem 17 Replacing school lamps
1. 3 ÷ 60 × 100 = 5 %.
2. Input = 3 ÷ 0.30 = 10 J every second.
3. Saving per lamp = 60 − 10 = 50 J every second. For 20 lamps: 20 × 50 = 1000 J every second. In 8 hours: 8 × 3600 = 28 800 s, so 1000 × 28 800 = 28 800 000 J = 28.8 MJ.
Problem 18 Diesel water pump
1. 2.4 ÷ 12 × 100 = 20 %.
2. Input = 2.4 ÷ 0.30 = 8 MJ.
3. Saved = 12 − 8 = 4 MJ.
Problem 19 Battery toy car
1. Thermal = 900 − 540 − 90 = 270 J.
2. Efficiency = 540 ÷ 900 × 100 = 60 %.
3. 270 ÷ 900 × 100 = 30 %.
Problem 20 Is a heater wasteful?
1. 1000 ÷ 1000 × 100 = 100 % as a heater, because all the thermal energy is useful.
2. Whether energy is wasted depends on the purpose: thermal energy is the aim for a radiator but an unwanted by-product for a lamp.
3. Once the room warms, the thermal energy leaks to the outside air and spreads out, so it cannot be gathered and used again.
