D.2.10—Magnetic field lines

Interpret a magnetic field line

Magnetic field lines show the local direction of the magnetic field; a compass north pole or a suitable test direction follows the arrow convention. Unlike isolated electric field lines, magnetic field lines form continuous closed loops.

Use the right-hand rule

Around a long straight current-carrying wire, the field lines are concentric circles centred on the wire. Point the right thumb in the conventional current direction; curled fingers give the magnetic-field direction. If electrons move into the page, conventional current is out of the page, so reverse the electron-motion direction before applying the rule.

Source	Magnetic-field pattern	Direction rule
Bar magnet	closed loops; outside from north to south	arrows return through the magnet
Straight wire	concentric circles around the wire	right thumb = conventional current; curled fingers = field
Circular coil	loops combine into a field through the coil centre along its axis	curl fingers with current; thumb gives axial field
Air-core solenoid	nearly parallel, uniform lines inside; bar-magnet-like return field outside	curl fingers with coil current; thumb gives the solenoid’s north end

Common trap

Do not use electron motion as though it were conventional current, and do not draw magnetic field lines starting or ending on an isolated magnetic pole.

How it is assessed

Questions determine field direction at a point near one or more current-carrying wires.

Command terms

What is / What is the direction

What earns marks

Convert electron motion to conventional current when needed, apply the right-hand rule, and identify the magnetic-field direction from the local circular or closed-loop pattern.

Watch for

Applying the right-hand rule directly to electron motion instead of conventional current, or reversing the field direction around the wire.

Representative question

[Maximum number: 1]

Two parallel wires carry equal currents in the same direction out of the paper. Which diagram shows the magnetic field surrounding the wires?

D.2.10—Magnetic field lines

Read Magnetic Field Lines

D.2.10 Exam Analysis

Assessment in practice

Question 1