Year 9 Science · Unit 3 · Lesson 7
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Learning Goals
1. When two objects at different temperatures touch, which way does heat flow between them, and when does the flow stop?
Compare two
Complete the table to compare temperature with thermal energy.
| Feature | Temperature | Thermal energy |
|---|---|---|
| What it tells you about the particles | ||
| Unit | ||
| Does it depend on how much of the substance there is? |
Real-world context: a river in flood
In a big flood, the Fitzroy River in Western Australia carries an enormous volume of water at about 25 °C past the town of Fitzroy Crossing. In a house in the town, a small pot of water boils on a stove at 100 °C.
2. The flooded river at 25 °C contains far more thermal energy than the pot of boiling water at 100 °C. Use the particle model to explain why. Refer to the number of particles and their average kinetic energy.
3. The hot pot is dipped into the floodwater. Heat flows from the pot into the river, even though the river has far more thermal energy. Explain why heat flows in this direction.
Investigation: a small and a large beaker cooling
A class poured 100 mL of water at 80 °C into one beaker and 400 mL of water at 80 °C into another. Both beakers stood on the bench in a room at 22 °C.
| Time (min) | 100 mL beaker (°C) | 400 mL beaker (°C) |
|---|---|---|
| 0 | 80 | 80 |
| 5 | 66 | 75 |
| 10 | 56 | 70 |
| 15 | 48 | 66 |
| 20 | 42 | 62 |
Source: sample class results.
4. Draw the cooling curves for the 100 mL and 400 mL beakers on the same grid, with minutes along the bottom. Label which curve is which.
5. (a) How do the two curves differ, and why do both level off? Predict what each beaker will read 3 hours later, using the term thermal equilibrium. (b) By the time both beakers reach room temperature, which one will have given more heat to the room altogether? Explain.
Scenario: an esky at the beach
A family packs drinks and ice into a foam esky and leaves it on the sand in the sun for a day at the beach. The air is 32 °C. They open the lid often to grab drinks.
6. Explain how heat gets into the esky and why the ice melts in the end. Say where conduction, convection and radiation happen, and suggest two ways to make the ice last longer.