QCE Physics - Unit 3 - Electromagnetism

Magnetic Fields | QCE Physics

Learn magnetic fields for QCE Physics, including field-line direction, wires, solenoids, magnetic force on charges and current-carrying conductors.

Updated 2026-06-17 - 4 min read

Magnetic fields describe the region where moving charges, current-carrying conductors and magnets can experience magnetic effects. The QCE skill is not only knowing formulas. You also need direction rules, field-line sketches and data interpretation.

Field-line conventions

Magnetic field lines show direction and relative strength. Around a bar magnet, field lines leave the north pole and enter the south pole outside the magnet. Where lines are closer together, the field is stronger.

Around a straight current-carrying wire, field lines form concentric circles. Use the right-hand grip rule for conventional current: thumb points with current, fingers curl in the magnetic-field direction. For a solenoid, curl the fingers of your right hand in the direction of conventional current around the coil; your thumb points toward the solenoid's north pole. The field inside a solenoid is stronger and more uniform, and the solenoid behaves like a bar magnet.

If more than one source creates a magnetic field at the same point, combine the field contributions as vectors. Fields in the same direction add; fields in opposite directions subtract; perpendicular fields must be resolved or drawn as a vector resultant.

Sketches should include:

• arrows showing field direction
• closer lines where the field is stronger
• clear labels for current direction or magnet poles
• circular field lines around straight wires
• near-uniform internal field lines for a solenoid

Field strength near wires and solenoids

For a long straight current-carrying wire:

$ B = \frac{\mu_0 I}{2\pi r} $

The field increases with current and decreases with distance from the wire. This is an inverse-distance relationship, not inverse-square.

For a long solenoid:

$ B = \mu_0 nI $

where $n$ is turns per metre. More turns per metre and larger current create a stronger field inside the solenoid.

Magnetic force on a wire

A current-carrying conductor in an external magnetic field experiences a force:

$ F = BIL\sin\theta $

where $\theta$ is the angle between the current direction and the magnetic field. The force is greatest when the current is perpendicular to the field and zero when current is parallel to the field.

The direction is perpendicular to both current and field. Use the right-hand palm rule or a consistent equivalent direction rule for conventional current. Keep this separate from the right-hand grip rule: grip rules give magnetic-field direction around currents, while palm rules give force direction on a current or charge in an external field.

Magnetic force on a moving charge

A moving charge in a magnetic field experiences:

$ F = qvB\sin\theta $

The force is perpendicular to the velocity and magnetic field. If the velocity is perpendicular to a uniform magnetic field, the force can act as a centripetal force, producing circular motion:

$ qvB = \frac{mv^2}{r} $

This is why charged particles curve in magnetic fields. The direction of curvature depends on the sign of the charge.

Data interpretation

The syllabus specifically names data relating to force on a conductor and magnet strength at different distances. If a wire of fixed length is held perpendicular to a uniform magnetic field:

$ F = BLI $

So a graph of $F$ against $I$ should be linear, and the gradient is $BL$. If $L$ is known, the gradient can be used to estimate $B$.

For magnet strength against distance, expect field strength to decrease as distance increases. The exact model can depend on magnet geometry, so avoid forcing every magnet dataset into a perfect inverse-square law unless the question supports it.

Worked example

Quick check

Sources

• QCAA Physics subject page
• QCAA Physics 2025 syllabus
• QCAA Physics 2025 formula and data book
• OpenStax University Physics Volume 2: Magnetic Force on a Current-Carrying Conductor
• OpenStax University Physics Volume 2: Solenoids and Toroids