Year 9 Science · Unit 3 · Lesson 11
Challenge Worksheet
Learning Goals
1. An electricity bill counts energy in kilowatt-hours. Show that 1 kWh is the same amount of energy as 3.6 MJ.
1 kWh J
2. An electric heater’s job is to heat a room. Explain why an electric heater is close to 100% efficient, while an incandescent light globe of the same power is only about 5% efficient, even though both mainly give out heat.
Find the mistakes: a student’s paragraph
“When you use an inefficient electric heater, a lot of energy is destroyed as waste. This wasted energy just disappears. Also, convection can occur in solids: for example, heat moves along a metal rod by convection. Copper is a great material to store heat in a solar hot-water tank, because it has a very high specific heat capacity. Finally, thermal expansion happens because each particle grows bigger when it absorbs heat energy.”
3. The paragraph contains four scientific errors. For each one, say what is wrong and write the correct science.
| Error | What the student wrote | Correct science |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 | ||
| 4 |
Investigation: how much of a globe’s energy becomes heat?
A teacher sealed a light globe inside a clear, waterproof glass tube and lowered it into a foam cup holding 0.20 kg of water. The globe ran for 300 s, and a thermometer measured the rise in the water’s temperature. The light escaped through the clear tube, cup wall and open top. The teacher then repeated the test with a new cup of water and a different globe. Use c = 4,200 J/kg·°C for water (the unit’s value).
| Globe | Power (W) | Time (s) | Temperature rise of the water (°C) |
|---|---|---|---|
| Incandescent | 60 | 300 | 19.0 |
| LED | 10 | 300 | 2.6 |
Source: sample class results.
4. For each globe, calculate (a) the electrical energy supplied, using E = P × t, (b) the energy that heated the water, using Q = mcΔT, and (c) the energy that did not heat the water. (d) If all of (c) was light, what would each globe’s efficiency be? Round to one decimal place.
| Quantity | Incandescent | LED |
|---|---|---|
| (a) Electrical energy supplied (J) | ||
| (b) Energy that heated the water (J) | ||
| (c) Energy that did not heat the water (J) | ||
| (d) Efficiency if (c) were all light (%) |
5. The lessons give about 5% for an incandescent globe and about 20% for an LED globe. Compare your answers to (d) with these values. Explain why this method gives an efficiency that is too high, and suggest one change that would make it more accurate.
Someone claims…
“Energy is always conserved, so the world can never run out of energy. Switching off lights and buying efficient appliances to ‘save energy’ makes no sense.”
6. Evaluate this claim. Use the law of conservation of energy, the idea of dissipated energy, and at least one efficiency value from this sheet or the unit.