Magnetic fields: bar magnet and current-carrying wire
Map the field around a bar magnet with a plotting compass, then pass a current through a straight wire and see the circular field it makes.
Core concept
A magnetic field runs from N to S around a magnet; a current in a wire makes circular field lines whose direction follows the right-hand grip rule
Topic: Magnetism
The space around a magnet: An object that pulls iron and steel towards it. All key words where it pushes or pulls on other magnets is its magnetic field. We draw it with field lines that run from the N pole: One of the two ends of a magnet (north and south), where its pull is strongest. All key words round to the S pole; where the lines are close the field is strong. A plotting compass points along the field. In 1820 Ørsted discovered that an electric current: The flow of electricity through a wire, measured in amperes (A). All key words also makes a magnetic field: around a straight wire the field lines are circles. Point your right thumb along the current and your fingers curl the way the field goes (the right-hand grip rule). A bigger current, or being closer, means a stronger field.
What you will learn
- Draw the field pattern around a bar magnet.
- Describe the field around a straight current-carrying wire.
- Use the right-hand grip rule to find the direction of the field.
Before you start
- Know that a compass needle is a small magnet.
Materials
- Bar magnet, plotting compasses, iron filings
- Thick copper wire through a card
Equipment
- Low-voltage power supply (up to 4 A) with a switch
Procedure
- Place the compass 1 cm and then 8 cm from the N pole of the bar magnet, recording the field strength.
- Switch to the straight wire with a current of 2 A out of the page and record where the compass points.
- Reverse the current, then try 4 A.
How this simulation is simplified
- The bar magnet is 7 cm long; its field on the axis falls off as 1 ÷ distance³.
- The wire's field is B = 20 × I ÷ d microtesla (d in cm); the Earth's own field (about 50 µT) points north.