Year 7 Science · Unit 3 · Lesson 15
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Learning Goals
1. Describe two different ways to map the invisible magnetic field around a bar magnet. For each one, say what you would do and what you would see.
Cause and effect: why an iron core helps
Fill each empty box. Each box is caused by the one before it, and each row carries on from the end of the row above.
Plan an investigation: more cells, stronger magnet?
Plan a fair test: does connecting 1, 2 or 3 cells change how strong the electromagnet is? Equipment: a nail wrapped in insulated wire, three 1.5 V cells, leads and a box of paper clips.
Variables
Method (3 steps)
One safety rule, and why
Results table
| Number of cells | Trial 1 | Trial 2 | Trial 3 | Average clips |
|---|---|---|---|---|
| 1 | ||||
| 2 | ||||
| 3 |
Read the data: turns of wire
Scenario: winding the coil
Jai wound wire onto an iron nail, 10 turns at a time. After each extra 10 turns he tested how many paper clips the tip of the nail could lift. Nothing else about his set-up changed.
| Number of turns | 10 | 20 | 30 | 40 |
|---|---|---|---|---|
| Clips picked up | 2 | 5 | 7 | 10 |
Source: sample class results.
2. Draw a line graph of Table 1, with the number of turns on the horizontal axis. Add a line of best fit.
3. Describe the trend in Table 1, and explain it using the idea of current-carrying wire. Then say why this counts as a fair test.
4. At a Newcastle scrapyard, the operator lowers a large flat electromagnet onto a wrecked car, flicks it on, swings the car over the crusher and flicks it off. Explain how the magnet grabs and releases the car, why a permanent magnet would be useless here, and one way engineers could make it lift heavier cars.
Question 1
Iron filings: sprinkle on paper over the magnet and tap (1). Plotting compass: move it round the magnet, marking the needle direction at each place (1). What you see, from either method (1): the filings line up in curves, crowded at the poles; or the needle direction changes around the magnet. [3 marks]
Cause and effect: why an iron core helps
Box 1: the current in the coil makes a magnetic field, like a bar magnet’s (1). Box 2: the coil’s field makes the iron nail inside it magnetic (1). Box 3: the total magnetic field is much stronger than the coil’s field on its own (1). [3 marks]
Plan an investigation: more cells, stronger magnet?
Change: number of cells, 1 to 3 (1). Measure: clips picked up (1). Same (two): coil and turns; nail; type of clip; way of dipping the tip (1). Method: connect 1 cell, dip the tip, count the clips; repeat with 2 and 3 cells, repeating each test (1). Safety: low-voltage cells only, never mains; or disconnect between tests as the wire gets warm (1). [5 marks]
Read the data: turns of wire
2. Axes labelled “Number of turns” (x) and “Clips picked up” (y), with even scales (1); all four points plotted correctly (1); a straight line of best fit (1). [3 marks]
3. More turns, more clips: from 2 clips at 10 turns to 10 clips at 40 turns (1). More turns put more current-carrying wire around the core, so the magnetic field is stronger (1). Only the number of turns changed; the nail, cell and clips stayed the same (1). [3 marks]
Question 4
Marking guide, 1 mark each: switching the current on makes the coil magnetic, so it grabs the car; switching the current off makes the magnetism disappear, so the car drops; the car body is steel, mostly iron, a magnetic material; a permanent magnet is always on and cannot be switched off; so it could not let go of the car; one way to make it stronger, with a reason: more turns, more current or a bigger iron core, as each gives a stronger magnetic field. [6 marks]
Model answer: When the operator switches the current on, the coil becomes magnetic and grabs the car, because the car body is steel, which is mostly iron. When the current is switched off, the magnetism disappears and the car drops. A permanent magnet is always on, so it could never let go of the car. Engineers could wind more turns of wire on the coil, because more current-carrying wire makes a stronger magnetic field.
Common error: changing turns and cells in the same test; only one variable may change.