Year 9 Science · Unit 3 · Lesson 9
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Learning Goals
1. Water takes far more energy than iron to warm by 1 °C, kilogram for kilogram. Which property explains this, and what is its unit?
Real-world context: a rooftop solar hot water system
A solar hot water system in Adelaide heats 200 L (200 kg) of water from 18 °C to 60 °C during the day. The water flows through copper pipes with a total mass of 12 kg, which also warm from 18 °C to 60 °C. Use c = 4,200 J/kg·°C for water (the lesson’s value). For copper, c = 390 J/kg·°C.
2. Calculate the energy needed to heat (a) the 200 kg of water and (b) the 12 kg of copper pipes. Give (a) in megajoules too (1 MJ = 1,000,000 J).
(a) Water J
(a) Water MJ
(b) Copper pipes J
3. About how many times more energy does the water need than the pipes? Give two reasons why the water needs so much more.
The water needs about times as much
4. Copper heats up faster than water. Why is water, and not copper or sand, used to store and carry the energy in this system?
Investigation: water or cooking oil?
A class heated 0.5 kg of water and 0.5 kg of cooking oil in identical beakers on identical hotplates, so both received energy at the same rate. They recorded the temperature every minute.
| Time (min) | Water (°C) | Cooking oil (°C) |
|---|---|---|
| 0 | 20.0 | 20.0 |
| 1 | 22.0 | 24.2 |
| 2 | 24.0 | 28.4 |
| 3 | 26.0 | 32.6 |
| 4 | 28.0 | 36.8 |
| 5 | 30.0 | 41.0 |
Source: sample class results.
5. (a) Calculate the energy the water took in during the 5 minutes. (b) The oil took in the same energy. Calculate the specific heat capacity of the cooking oil.
(a) Energy into the water J
(b) c of cooking oil J/kg·°C
6. The oil warmed about twice as fast as the water, although both received energy at the same rate. Explain what this tells you about the specific heat capacity of each liquid, and which one would be better for storing heat.
7. A family is choosing between an 80 L tank and a 300 L tank of hot water. Each tank starts the night at 65 °C. Assume each tank loses 3.0 MJ of energy to the colder air overnight. Calculate the temperature of each tank in the morning, and recommend a tank. Use c = 4,200 J/kg·°C for water (the lesson’s value).
80 L tank: morning temperature °C
300 L tank: morning temperature °C