For IB Physics SL and HL, sketch magnetic field lines by determining the field direction, then drawing smooth lines with arrowheads and spacing that represents field strength. The pattern depends on the source.
A field line shows the direction in which the north pole of a small compass points. Lines closer together indicate a stronger field. They form continuous closed loops and never cross, because the field has only one direction at any point.
| Source | Field-line pattern and direction |
|---|---|
| Bar magnet | Draw curved lines leaving the north pole and entering the south pole. Inside the magnet, they return south to north. Place lines closest near the poles. |
| Straight current-carrying wire | Draw concentric circles centred on the wire. Use the right-hand grip rule: point your right thumb along conventional current; curled fingers show the field direction. |
| Circular coil | Draw curved lines through the centre. Fields from different wire sections reinforce inside, creating a short bar-magnet-like pattern. Curl your fingers with the current; your thumb points towards the coil's north pole. |
| Solenoid | Draw nearly straight, parallel, equally spaced lines inside to show an approximately uniform field. Outside, draw curved lines leaving the north end and entering the south end. |
A dot represents current coming out of the page, while a cross represents current going into the page. Apply the grip rule to determine whether the circular arrows are anticlockwise or clockwise.
Common misconception: Magnetic field lines begin at north and end at south. This is incorrect: unlike electric field lines, magnetic field lines continue through the magnet and close on themselves.
Exam technique: In a D.2 Draw response, include correct arrow direction, non-crossing lines, and spacing that shows relative field strength. For a solenoid, show the strong, approximately uniform internal field.
