Year 7 Science · Unit 3 · Lesson 15
Challenge Worksheet
Learning Goals
True or false? Fix the false ones
Circle T or F. If a statement is false, correct it on the line.
1. Field lines are drawn closest together where the magnetic field is strongest.
Correct it:
2. A plotting compass shows both the direction and the strength of the field at a point.
Correct it:
3. Outside a bar magnet, field lines point from the north pole to the south pole.
Correct it:
4. An electromagnet stays magnetic after the current is switched off.
Correct it:
5. Adding more cells makes an electromagnet weaker, because the current is shared between them.
Correct it:
1. Nadia stores her orienteering compass in a pencil case beside a strong fridge magnet. Inside the case, the needle points towards the fridge magnet rather than north. Explain why.
2. Winding extra turns and connecting extra cells strengthen the coil by one shared idea; a nail inside does it in a different way. Explain both.
Read the data: four electromagnets
Scenario: a messy set of tests
Four teams each made one electromagnet and tested it in one shared box of steel paper clips. They did not agree beforehand on what to vary, so their designs differ in more than one way.
| Magnet | Turns | Cells | Core | Clips |
|---|---|---|---|---|
| A | 20 | 1 | iron | 4 |
| B | 40 | 1 | iron | 8 |
| C | 40 | 2 | iron | 13 |
| D | 40 | 2 | none | 1 |
Source: made-up results for this worksheet.
3. Name the pair of electromagnets that fairly tests the effect of each change: (a) the number of turns, (b) the number of cells, (c) the iron core.
4. Divide the two results in your answer to (c). What does the answer tell you about iron?
Answer times
5. Zoe compares A and C and says: “Adding a second cell more than tripled the strength, from 4 clips to 13.” Evaluate Zoe’s claim, using the data in Table 1.
6. A student claims: “Electromagnets are always better than permanent magnets.” Evaluate this claim. Use two examples: a magnet that holds a note on a fridge door, and a magnet that grips steel parts on a factory line and must drop them many times a minute.
True or false
1. True. 2. False: it shows only the direction; how close the field lines are shows the strength. 3. True. 4. False: it is a temporary magnet; when the current stops, the magnetism disappears. 5. False: more cells push a bigger current, so it gets stronger.
Question 1
The needle lines up with the magnetic field where it is (1). Near the strong magnet, that magnet’s field is far stronger than Earth’s, so the needle follows it (1). [2 marks]
Question 2
The field comes from current in the coil (1). Turns and cells both add current-carrying wire in the same space: more wire, or more current per turn (1). Iron is magnetic: the coil’s field magnetises it and its field adds to the coil’s (1). [3 marks]
Question 3
(a) A and B (1). (b) B and C (1). (c) C and D (1). In each pair only one thing changes. [3 marks]
Question 4
13 ÷ 1 = 13 times (1). The iron core makes the electromagnet many times stronger than the same coil with no core (1). [2 marks]
Question 5
Unfair: A and C differ in turns and cells (1). Doubling the turns alone took A to B, 4 to 8 clips (1). The fair cell test is B and C, 8 to 13: stronger, but not tripled (1). [3 marks]
Question 6
Marking guide, 1 mark each: judgement (wrong: each suits different jobs); an electromagnet can be switched on and off; because its field comes from the current; a fridge note needs a magnet always on with no power: a permanent magnet; the factory line needs an electromagnet, switched off to release parts; stronger with more turns, more current or an iron core. [6 marks]
Model answer: The claim is wrong, because each kind of magnet suits a different job. An electromagnet can be switched on and off, because its field comes from the current. That suits the factory line: switching off drops each part at once, and more turns, more current and an iron core make it strong enough for heavy parts. A fridge note needs a magnet that holds all the time with no power, so a permanent magnet, which is always on, is better there.
Common error: comparing A and C and crediting one change for both.