Year 7 Science · Unit 3 · Lesson 18
Challenge Worksheet
Learning Goals
Evaluate the claim
Someone claims...
"Electric motors are about 90% efficient while petrol engines are only about 25% efficient. This means electric cars must always be better for the environment than petrol cars. Anyone driving a petrol car is wasting energy and harming the planet. Australia should ban petrol cars immediately."
(a) What part of this claim is supported by the science you have learned in this lesson? Be specific — quote the efficiency figures.
(b) What hidden assumptions or missing information make this claim misleading or incomplete? Identify at least two factors the claim ignores. (Hint: think about where the electricity for the EV comes from, and how batteries are made.)
(c) What evidence or extra information would you need to decide whether electric cars are truly always better for the environment in Australia? List at least two pieces of evidence.
1. A student claims: "A reverse-cycle air conditioner must be more than 100% efficient — it delivers more heat energy than the electricity it uses!" Explain why this claim sounds correct but is actually misleading. Use the idea of energy conservation in your answer.
2. An inventor claims they have built a "perpetual motion machine" — a device that runs forever with no energy input. Using what you know about efficiency and energy, explain clearly why this is impossible.
Wrap Up
In one sentence, what was the main idea of this lesson?