What it shows
A small compass points along the magnetic field at each point, so plotting its direction step by step traces the field lines of a magnet. A wire carrying current also deflects the compass, showing that electric current produces a magnetic field (Oersted’s experiment).
The science
A magnetic field exerts a torque on a compass needle until it lies along the field direction, so the needle is a tiny field detector. Marking the needle’s two ends, moving the compass forward, and repeating joins up into a continuous line that leaves the north pole and enters the south pole; where lines are closer, the field is stronger. A straight wire carrying current produces circular field lines around itself, which is why a compass above the wire swings one way and a compass below it swings the other way, and reversing the current reverses both.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two bar magnets
- A small plotting compass (about Rs 30) or five cheap button compasses
- Large sheets of white paper and a pencil
- A 1 m length of thick insulated copper wire
- Two AA cells in a holder and a switch
- Iron filings in a salt shaker for the optional picture
Safety
For this project: Cells only, and close the switch only for a few seconds at a time because the wire shorts the cells. Keep iron filings away from eyes and off the magnet surface (wrap the magnet in paper).
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.
Place a bar magnet in the centre of a large sheet, draw its outline and mark N and S.
Put the compass near the north pole, mark dots at both ends of the needle, then move the compass so its tail is on the previous head dot, and repeat until you reach the south pole or the edge of the paper.
Start new lines from different points around the north pole until you have 10 to 12 lines, and draw them smoothly with arrows.
Repeat on a fresh sheet with two magnets, north facing north, and find the neutral point where the compass cannot decide a direction.
Repeat with north facing south to show the lines running straight across the gap.
Stretch the wire straight north-south, lay the compass directly under it, close the switch briefly and record the deflection direction; then put the compass above the wire and compare.
Reverse the cells and show the deflection reverses; make a table of compass position, current direction and deflection direction.
As a final picture, place paper over a magnet and sprinkle filings gently, tap the paper, and photograph or sketch the pattern.
What you should see
Three field maps with lines leaving north and entering south, a clear neutral point between like poles, and a compass that deflects opposite ways above and below a current-carrying wire and reverses with the current.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Where is the magnetic field of a bar magnet strongest?
At the poles, where the field lines are closest together.
2Why do field lines never cross?
The field has only one direction at each point, so a compass could not point two ways at once.
3What did Oersted discover?
That a current-carrying wire produces a magnetic field around it.
4Which rule gives the field direction around a straight wire?
The right-hand thumb rule: thumb along the current, fingers curl in the direction of the field.
Take it further
Map the field of a coil of 20 turns with current flowing and compare it with the bar magnet 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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