Describe an activity to show the magnetic field around a straight current-carrying conductor. State the rule that gives the direction of the field.

1. 1 · Open with the idea. A current in a conductor produces a magnetic field around it, as Oersted found in 1820.
2. 2 · Describe the activity.
   • Pass a thick copper wire vertically through a horizontal cardboard.
   • Connect a battery, a key and a rheostat in series with it.
   • Sprinkle iron filings, switch on and tap the card.
3. 3 · Draw the set-up. [Vertical wire through a card, current arrow upwards, concentric circles with anticlockwise arrows, a compass on one circle]
4. 4 · Write the observations.
   • Filings form concentric circles.
   • Reversing the current reverses the compass needle.
   • More current gives a stronger field; more distance gives a weaker one.
5. 5 · State the rule. Right-hand thumb rule: thumb along the current, curled fingers give the direction of the field lines.
6. 6 · Real life. A compass held near a cable carrying a large direct current can give a wrong reading.
7. 7 · Close in one line. A straight current makes circular field lines whose direction follows the right-hand thumb rule.

Scoring tip: Show arrows on the wire and on the circles. 'Concentric circles' is the key phrase examiners look for.