Year 9 Science · Unit 3 · Lesson 1
Challenge Worksheet
Learning Goals
1. Energy is described as “the capacity to do work or produce change”. Explain why energy is not a substance, and what people really mean when they say a battery’s energy has been “used up”.
2. A student wrote an energy chain for each situation. Tick each chain that is correct. For each wrong one, write the correct chain.
| Situation | Student’s energy chain | ✓ if correct, or the correct chain |
|---|---|---|
| A bushfire burns a gum tree | thermal → chemical → light + sound | |
| A solar panel charges a phone | light → electrical → chemical | |
| A coal power station makes electricity | chemical → electrical → kinetic → thermal | |
| A skateboarder rolls up a ramp and slows down | kinetic → the energy disappears | |
| An electric heater warms a room | electrical → thermal |
3. Beside the heater chain, the student wrote “transformation only”. Explain why warming a room with an electric heater involves a transformation and a transfer.
Investigation: how hot do bike brakes get?
A class rode the same bike, with the same rider, along a flat path at four different speeds. Each time, the rider stopped using only the rear disc brake. A thermometer probe measured the temperature of the brake disc before and after each stop, and the disc was left to cool to room temperature before the next run.
| Speed before braking (km/h) | Temperature rise of the disc (°C) |
|---|---|
| 10 | 1.1 |
| 20 | 4.3 |
| 30 | 10.1 |
| 40 | 17.4 |
Source: sample class results.
4. Describe the energy transformation that happens during each stop, and state where the bike’s kinetic energy ends up.
5. When the speed doubled from 10 to 20 km/h, how many times bigger was the temperature rise? Work out the same thing for 20 to 40 km/h. What do your answers suggest about how the bike’s kinetic energy changes when its speed doubles?
10 to 20 km/h times
20 to 40 km/h times
6. Name two variables the class kept the same, and explain why each one had to be controlled.
7. A coal-fired power station turns about 33% of the chemical energy in its coal into electrical energy. A rooftop solar panel turns about 20% of the light that hits it into electrical energy. For every 100 J that goes into each one, how much energy is wasted?
Coal J
Solar panel J
8. A student claims: “Rooftop solar panels waste less energy than a coal-fired power station, because they have fewer energy transformations.” Evaluate the claim, using the energy chain for each source and your answers to Question 7.