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Grade 9 Intermediate About 20 min Version 1 Awaiting review Premium

Magnetic fields: bar magnet and current-carrying wire

The space around a magnet where it pushes or pulls on other magnets is its magnetic field. We draw it with field lines that run from the N pole 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 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

  1. Place the compass 1 cm and then 8 cm from the N pole of the bar magnet, recording the field strength.
  2. Switch to the straight wire with a current of 2 A out of the page and record where the compass points.
  3. 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.