Year 7 Science · Unit 3 · Lesson 14
Challenge Worksheet
Learning Goals
1. A student cuts a bar magnet in half, hoping to get one piece that is only a north pole and one that is only a south pole. Describe what really happens, and what happens if a half is cut again.
Pole puzzle
Scenario: two magnets with no labels
The pole markings have worn off two old magnets. Magnet 1 has ends called A and B; magnet 2 has ends called C and D.
2. Name the pole at each end, A, B, C and D. For each one, say which clue or rule you used. Then predict what happens when end B is brought near end D.
Read the data: whose result is more reliable?
Scenario: two groups, one magnet
Two groups used the same bar magnet and the same kind of paper clip. They slid the magnet slowly towards the clip along a ruler and recorded the distance at which the clip first moved. Kai’s group did the test once. Lin’s group did it four times.
| Trial | Kai’s group (cm) | Lin’s group (cm) |
|---|---|---|
| 1 | 3.1 | 2.4 |
| 2 | not done | 2.6 |
| 3 | not done | 4.1 |
| 4 | not done | 2.5 |
Source: made-up results for this worksheet.
3. One of Lin’s trials does not fit the others. Name it, then calculate the average of Lin’s other three trials.
Average cm
4. Whose result is more reliable, Kai’s or Lin’s? Explain, and suggest one thing that could have caused the unusual trial.
Plan an investigation: a compass near a magnet
A compass needle points north. When a bar magnet is placed beside the compass, the needle turns away from north. Plan a fair test of this question: does the distance between the magnet and the compass change how far the needle turns? You have a compass with a degree scale, a bar magnet and a ruler.
Hypothesis
If the distance is increased, then the needle will …, because …
Variables
Method (3 steps)
Results table
| Distance (cm) | Trial 1 (°) | Trial 2 (°) | Trial 3 (°) | Average (°) |
|---|---|---|---|---|
| 5 | ||||
| 10 | ||||
| 15 | ||||
| 20 |
5. A recycling centre sends a mixture of steel food cans, aluminium drink cans and plastic bottles along a conveyor belt. A strong magnet hangs just above the belt. The manager says: “The magnet will pull out all the metal, so only plastic will be left on the belt.” Evaluate the manager’s claim.
Question 1
Each half becomes a complete magnet with its own N and S poles (1). Cutting again gives more complete magnets: poles always come in pairs (1). [2 marks]
Pole puzzle
2. A is north: a hanging magnet’s north-seeking pole points north (1). B is south (accept with A). D is north: A repels D, like poles repel (1). C is south: A attracts C, unlike poles attract (1). B (S) near D (N): unlike, so they attract (1). [4 marks]
Read the data: whose result is more reliable?
3. Trial 3, 4.1 cm (1). (2.4 + 2.6 + 2.5) ÷ 3 = 2.5 cm (1). [2 marks]
4. Lin’s (1). Her repeats agree closely (2.4 to 2.6 cm) and expose the unusual trial; Kai’s single value cannot be checked (1). Cause (any one): bench bumped; clip not at 0 cm; ruler misread (1). [3 marks]
Plan an investigation: a compass near a magnet
Hypothesis: if the distance is increased, the needle will turn less, because the magnetic force gets weaker with distance (1). Change: the distance from magnet to compass; measure: the angle the needle turns from north (1). Keep the same (two): magnet; compass; pole facing the compass; side the magnet is on; no other magnets or steel nearby (1). Method: set the compass flat and check that it points north; place the magnet at 5 cm and read the angle; repeat at 10, 15 and 20 cm, repeating each distance (2: 1 angle from north at each distance, 1 same direction and pole). [5 marks]
Question 5
Marking guide, 1 mark each: judgement (the claim is wrong: not all the metal is removed); steel cans are pulled out because steel is mostly iron, a magnetic material; aluminium cans are metal but not magnetic, so they stay on the belt with the plastic; plastic is not magnetic and stays; magnetism is a non-contact force, so the magnet lifts the steel cans across the gap; the force weakens quickly with distance, so the magnet must hang close to the belt. [6 marks]
Model answer: The manager is wrong. The magnet removes the steel cans, as steel is mostly iron. Aluminium is a metal but not magnetic, so those cans stay on the belt. Plastic is not magnetic, so the bottles stay on the belt too. Magnetism is a non-contact force, so it lifts steel cans across the gap, but it weakens quickly with distance, so the magnet must hang close.
Common error: “metal” taken to mean “magnetic”.