IB Physics magnetism centres on a small set of highly testable ideas: representing magnetic fields, finding force magnitude and direction, analysing charged-particle motion, and explaining interactions between currents. To earn marks consistently, you must connect diagrams, vector directions, equations, and physical reasoning rather than treating each formula as an isolated fact.
In the current course, first assessed in 2025, magnetism appears mainly in D.2 Electric and magnetic fields and D.3 Motion in electromagnetic fields. The D.3 content is common to SL and HL, while D.4 Induction is additional higher-level content. This guide explains the core magnetism needed for exams and shows how to turn it into mark-scoring working.
Where magnetism appears in IB Physics
The official course organizes magnetism across related parts of Theme D. Questions can therefore combine magnetic fields with circular motion, electric fields, energy, or experimental data.
| Syllabus area | What you should be able to do | Level |
|---|---|---|
| D.2 Electric and magnetic fields | Sketch and interpret fields around magnets, wires, coils, and solenoids | SL and HL |
| D.3 Motion in electromagnetic fields | Calculate forces on moving charges and currents; analyse particle paths; explain forces between parallel wires | SL and HL |
| D.4 Induction | Apply magnetic flux, Faraday's law, motional emf, and Lenz's law | HL only |
Under the current assessment model, Paper 1A contains multiple-choice questions, Paper 1B contains data-based questions, and Paper 2 contains short-answer and extended-response questions. Magnetism may consequently appear as a rapid direction question, a calculation, a field-line interpretation, or one stage of a longer problem.
Magnetic fields and field-line diagrams
A magnetic field is a region in which a magnet, moving charge, or current can experience a magnetic force. The field is represented by the vector B, measured in tesla (T).
Magnetic field lines communicate both direction and relative strength:
- At any point, B is tangent to the field line.
- Outside a bar magnet, arrows run from north to south.
- Magnetic field lines form closed loops rather than beginning or ending at isolated poles.
- Lines drawn closer together represent a stronger field.
- Field lines never cross because the field cannot have two directions at one point.
- A uniform field is shown by straight, parallel, equally spaced lines.
For a straight current-carrying wire, field lines are concentric circles centred on the wire. Apply the right-hand grip rule: point your right thumb along the conventional current, and your curled fingers show the magnetic-field direction.
For a solenoid, curl your fingers in the direction of conventional current around the turns. Your thumb points along the field inside the solenoid and toward its north-seeking end. Examiners may ask you to add arrows, identify a pole, or predict the effect of reversing the current.
Remember that conventional current runs in the direction positive charge would move. It is opposite to electron drift in a metal, which is a frequent source of reversed answers.
Magnetic force equations you must understand
| Situation | Magnitude equation | Angle used |
|---|---|---|
| Charge moving through a magnetic field | **F = | q |
| Straight current-carrying conductor | F = BIL sin θ | Between conventional current and field |
| Parallel current-carrying wires | F/L = μ₀I₁I₂/(2πr) | Here, r is wire separation |
| Perpendicular circular particle motion | **r = mv/( | q |
The sine factor is not optional. Force is maximum when the motion or current is perpendicular to the field and zero when it is parallel.
Finding the direction of magnetic force
For a positive charge, the force follows the vector relationship F = q(v × B). Use a consistent right-hand rule to determine the direction perpendicular to both v and B. For a negative particle, such as an electron, reverse the direction obtained for a positive charge.
For a wire, use the direction of conventional current, not electron movement. Whatever hand convention your teacher uses, label the three vectors first and check that the resulting force is perpendicular to both current and field.
In two-dimensional diagrams:
- ⊙ means out of the page, like the tip of an arrow.
- ⊗ means into the page, like the tail feathers of an arrow.
A direction question should usually end with an unambiguous statement such as “the force acts into the page,” not merely a hand-rule sketch.
Charged particles in magnetic fields
When a particle moves perpendicular to a uniform magnetic field, the magnetic force is always perpendicular to its instantaneous velocity. It changes the direction of the velocity but not its magnitude, so the particle moves in a circle at constant speed.
Set magnetic force equal to centripetal force:
|q|vB = mv²/r
Therefore:
r = mv/(|q|B)
This relationship generates several common exam deductions:
- Increasing mass or speed increases the radius.
- Increasing charge magnitude or field strength decreases the radius.
- Reversing the sign of charge reverses the curvature.
- The magnetic field does no work, so kinetic energy and speed remain constant.
If velocity has both perpendicular and parallel components, the perpendicular component creates circular motion while the parallel component remains unchanged. The resulting path is a helix around the field direction.
Worked exam-style example
A proton travels at 3.0 × 10⁶ m s⁻¹ perpendicular to a 0.20 T magnetic field. Using proton mass 1.67 × 10⁻²⁷ kg and charge magnitude 1.60 × 10⁻¹⁹ C:
r = mv/(qB)
r = (1.67 × 10⁻²⁷)(3.0 × 10⁶) / [(1.60 × 10⁻¹⁹)(0.20)]
r = 0.157 m ≈ 0.16 m
A complete solution states the equation, substitutes SI values, gives the unit, and addresses direction separately if requested. The RevisionDojo circular-motion practice is useful because these questions often combine D.3 magnetism with A.2 mechanics.
Forces on currents and parallel wires
A current-carrying conductor in an external magnetic field experiences a force because its moving charge carriers experience magnetic forces. For a straight section in a uniform field:
F = BIL sin θ
Here, L is only the length inside the field. A common error is to use the wire's total length even when the diagram shows that only part of it lies between the magnetic poles.
Two parallel currents also exert forces on one another. Each wire produces a magnetic field at the position of the other wire, giving:
F/L = μ₀I₁I₂/(2πr)
Currents in the same direction attract, while currents in opposite directions repel. In an explanation question, state the field produced by one wire, determine its direction at the second wire, and then use the current-field force rule.
How examiners phrase magnetism questions
Command terms indicate how much reasoning you must show.
| Command term | What a strong response does |
|---|---|
| State | Gives a concise fact or direction without unnecessary derivation |
| Determine | Uses the information provided to obtain a result, normally with working |
| Calculate | Selects an equation, substitutes correctly, and includes a unit |
| Explain | Links cause and effect using magnetic-force principles |
| Show that | Presents enough algebra and substitution to reach the supplied result |
| Sketch | Shows the required shape, direction, labels, and relevant field features |
A typical “explain” response might say: “The magnetic force is perpendicular to the particle's velocity, so it acts as a centripetal force. It changes the velocity's direction but does no work, meaning the speed and kinetic energy remain constant.” Each sentence supplies a distinct marking point.
Common mistakes that lose marks
- Using electron flow instead of conventional current in a wire-force rule.
- Forgetting to reverse the force direction for a negative charge.
- Using cosine instead of sine, or using the angle to the normal rather than the angle between the stated vectors.
- Claiming that a magnetic field increases a particle's speed.
- Writing F = mv²/r as an additional force instead of the resultant radial-force condition.
- Omitting arrows on a field diagram.
- Using the entire wire length rather than the length inside the field.
- Confusing B, measured in tesla, with magnetic flux Φ, measured in weber.
Always perform a perpendicularity check: magnetic force must be perpendicular to both the field and the charge velocity or conventional current. If it is not, your direction is wrong.
An efficient revision method
Start with a one-page map of the field patterns, hand rules, and equations. Then complete mixed questions in which you must decide whether the relevant object is a charge, wire, or pair of wires.
Use the IB Physics Study Notes for concept review, then move to the Topic D Fields Questionbank. The broader IB Physics Questionbank helps you practise magnetism alongside circular motion and electric fields.
Most importantly, compare your method with worked solutions rather than checking only the final number. Open the Past Papers area from the RevisionDojo IB Physics hub to watch per-question video walkthroughs and see how diagrams, equations, substitutions, and explanations are converted into marks. The Topic D video collection and Physics video library provide further concept support.
Once individual skills are reliable, attempt a timed set or a Physics predicted paper. Use the official-style IB Physics data booklet resource during practice so that equation selection becomes automatic. Jojo AI can then help identify whether an error came from physics, direction, algebra, or exam communication.
Conclusion
IB Physics magnetism becomes manageable when you master field patterns, distinguish the three principal force equations, and treat direction as seriously as magnitude. The most important physical insight is that magnetic force acts perpendicular to motion, allowing it to curve a charged particle without changing its speed.
RevisionDojo can support the sequence from understanding to application: review the notes, practise Topic D questions, and then study the per-question past paper video solutions. Finish with timed Questionbank or predicted-paper practice so that the method is secure under exam conditions.
Sources and referenced URLs
- Official IB Physics subject page
- Official IB Physics curriculum update
- Official IB Physics subject brief
- Official IB Physics specimen papers and markschemes
- Current IB Physics guide, first assessment 2025
- OpenStax charged-particle motion in a magnetic field
- OpenStax magnetic force on a current-carrying conductor
- RevisionDojo IB Physics hub and past paper videos
- RevisionDojo IB Physics Study Notes
- RevisionDojo IB Physics Questionbank
- RevisionDojo Topic D Fields Questionbank
- RevisionDojo circular-motion Questionbank
- RevisionDojo Topic D Fields videos
- RevisionDojo Physics video library
- RevisionDojo Physics predicted papers
- RevisionDojo IB Physics data booklet