Year 9 Science · Unit 3 · Lesson 2
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Learning Goals
1. State the law of conservation of energy, and give one example of energy being transformed.
Order the steps
Number the steps from 1 to 6 to show the order of energy changes in a hydroelectric scheme. Step 1 happens first.
| Order | Step |
|---|---|
| The generator changes the kinetic energy of the spinning turbine into electrical energy. | |
| Water stored high up in a dam has gravitational potential energy. | |
| Electricity travels along transmission lines to homes, and the wires warm up slightly. | |
| Fast-moving water strikes the turbine blades and makes them spin. | |
| Electrical energy runs appliances in homes, changing to light, sound, heat or kinetic energy. | |
| Water falls through large pipes, and its gravitational potential energy changes to kinetic energy. |
2. Complete the table to compare an open system with a closed system.
| Feature | Open system | Closed system |
|---|---|---|
| Can energy enter or leave? | ||
| Does the total energy inside stay constant? | ||
| One example | ||
| Does conservation of energy still hold? |
Investigation: how high does a ball bounce?
A class dropped a tennis ball from 100 cm onto a concrete floor. They filmed each drop in front of a metre ruler and measured the top of each bounce. They did three trials.
| Bounce | Trial 1 | Trial 2 | Trial 3 | Average |
|---|---|---|---|---|
| 1st | 58 | 55 | 58 | 57 |
| 2nd | 33 | 31 | 32 | |
| 3rd | 18 | 17 | 19 |
Source: sample class results. Averages are rounded to the nearest centimetre.
3. Complete the Average column. Then plot the average height against bounce number. Plot the 100 cm drop as bounce 0.
4. The ball’s gravitational potential energy is proportional to its height. What percentage of its energy does the ball keep from the drop to the 1st bounce, and from the 1st bounce to the 2nd?
Drop to 1st bounce %
1st to 2nd bounce %
5. Use your answers to predict the height of the 4th bounce. Then explain where the rest of the energy goes at each bounce, and why this does not break the law of conservation of energy.
6. A student says: “Our classroom is a closed system, so we can track all the energy in it.” Is the classroom a closed system? Justify your answer with two reasons.
7. A hydroelectric scheme turns only part of the water’s gravitational potential energy into electrical energy. Using your ordered steps and the law of conservation of energy, explain what happens to the energy that does not become electrical energy, and why this does not break the law.