Year 7 Science · Unit 3 · Lesson 14
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Learning Goals
1. A bushwalker uses a compass to find north. Explain how the compass needle works, and why this is an example of a non-contact force.
Order the steps: when does the clip jump?
Ruby tests how close a magnet must be before a paper clip moves. Number her steps from 1 (first) to 5 (last).
| Step (1–5) | What Ruby does |
|---|---|
| Stop as soon as the clip first moves, and read the magnet’s distance from the clip on the ruler. | |
| Lay a ruler flat on the bench and put a steel paper clip at the 0 cm mark. | |
| Put the clip back on 0 cm, repeat two more times, and average the distances. | |
| Place the bar magnet at the 20 cm mark, with the same pole facing the clip every time. | |
| Slide the magnet slowly towards the clip, a little at a time. |
2. Priya rubs a balloon on her hair, and it picks up small pieces of paper. She says: “The balloon has become a magnet.” Explain why Priya is wrong, and describe one quick test that shows the difference.
Read the data: Maya’s paper sheets
Scenario: clips through paper
Maya lays one sheet of paper over a pile of steel paper clips and lowers a bar magnet onto the sheet. She counts how many clips the magnet lifts. She repeats this with 2, 3 and 4 sheets, so the gap between the magnet and the clips gets bigger each time.
| Number of sheets | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Clips lifted | 6 | 4 | 2 | 0 |
Source: sample class results.
3. By how many clips does the count fall for each extra sheet? What does this trend show about how a magnet’s pull changes with distance?
4. Name the one variable Maya changes, what she measures, and one thing she must keep the same.
5. Leo repeats Maya’s test, but for 3 and 4 sheets he swaps to a bigger magnet. He lifts 5 clips and 3 clips. Leo says: “Distance hardly matters.” Explain why his results cannot support this conclusion.
6. In Ruby’s test, nothing happens while the magnet is far away. Then, suddenly, the clip jumps the last centimetre to the magnet. Ruby says: “The magnet switched on when it got close.” Evaluate Ruby’s idea.
Question 1
The needle is a tiny magnet that spins freely (1). It lines up with Earth’s magnetic field, so its north-seeking end points roughly north (1). Nothing touches it: Earth’s field acts across a gap, a non-contact force (1). [3 marks]
Order the steps: when does the clip jump?
Reading down the table as printed, the correct step numbers are 4, 1, 5, 2, 3. In order: ruler and clip at 0 cm; magnet at 20 cm; slide slowly; stop and read when the clip moves; reset, repeat and average.
Question 2
Rubbing charges the balloon, so the force is electrostatic, caused by charge (1). Magnetism comes from materials such as iron, nickel and cobalt; rubbing cannot make a balloon magnetic (1). Test: a real magnet does not lift the paper; or the balloon cannot lift a steel clip the way a magnet does (1). [3 marks]
Question 3
It falls by 2 clips for each extra sheet: 6, 4, 2, then 0 (1). Each sheet adds distance, so the magnetic pull gets weaker as distance increases (1). [2 marks]
Question 4
Changes: the distance (number of sheets) (1). Measures: clips lifted (1). Same (any one): magnet; kind of clip; type of paper; way of lowering the magnet (1). [3 marks]
Question 5
Leo changed two variables, the distance and the magnet (1). The bigger magnet may pull harder, so he cannot tell which caused his results: not a fair test (1). [2 marks]
Question 6
Marking guide, 1 mark each: Ruby is wrong, nothing switches on; the non-contact force acts at every distance; it weakens very quickly with distance; far away it is too weak to overcome friction on the clip; once the clip moves it gets closer, so the pull grows and it speeds up; this feeds on itself, so the last centimetre is almost instant. [6 marks]
Model answer: Ruby is wrong. The magnet pulls at every distance, but the pull weakens very quickly with distance. Far away it cannot overcome friction on the clip. Close enough, the pull wins and the clip moves; getting closer makes the pull stronger, which pulls it faster and closer still, so the last centimetre happens almost at once.
Common error: saying the force “stops” beyond a range; it just becomes too small to move the clip.