What it shows
A small compass placed near a bar magnet points along the field at that spot. Joining many such points draws the field lines, which loop from the north pole round to the south pole and crowd together where the field is strongest.
The science
A magnetic field is invisible but it has a direction at every point, and a compass needle shows that direction. Near a bar magnet the field comes out of the north pole, curves round through the space outside and goes back into the south pole. By placing the compass, marking where its needle points, moving to that mark and repeating, you trace a line that follows the field. Where the lines are close together, near the poles, the field is strongest; where they spread out, it is weak.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A bar magnet
- A small plotting compass (available at stationery shops) or the needle compass from the project above
- A sheet of white paper and a pencil
- Tape to hold the magnet
- A second bar magnet for the extension
Safety
For this project: Keep magnets away from phones, cards and small children.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Tape the magnet to the middle of the paper and draw round it, marking which end is north.
Place the compass near the north pole and put a pencil dot at the tip of the needle that points away from the magnet.
Move the compass so its tail sits on the dot you just made and mark a new dot where the tip now points.
Keep going dot by dot until the line reaches the south pole or runs off the paper, then join the dots into a smooth line.
Start again from a different spot around the north pole and trace another line; make six or eight lines all round the magnet.
Mark an arrow on each line showing the direction the needle pointed, from north pole to south pole.
Look at where the lines are crowded and where they are spread out and label the strong and weak regions.
For the extension, place two magnets end to end with like poles facing and map the region between them.
What you should see
A sheet showing curved lines looping from the north pole to the south pole of the magnet, close together near the poles and spread out further away, with arrows showing the direction of the field.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Which way do the field lines go outside the magnet?
From the north pole round to the south pole.
2What does it mean when the lines are close together?
The field is strong there, which is near the poles.
3Can two field lines cross?
No, because the compass can point in only one direction at any spot.
4What is a compass needle itself?
A small magnet that turns to line up with the field it is in.
Take it further
Sprinkle iron filings collected with a magnet from sand on a paper over the magnet, and compare the pattern with your compass map.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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