What magnetism equations should I know for IB Physics?

The main force equations are F = qvB sin θ for a moving charge and F = BIL sin θ for a current-carrying conductor. For circular particle motion, combining magnetic force with centripetal force gives r = mv/(qB) when velocity is perpendicular to the field. You may also need the force per unit length between parallel currents, depending on the question. HL students must use Φ = BA cos θ, ε = -NΔΦ/Δt, and ε = BvL for induction. These relationships appear in the data booklet, but you must understand their conditions and determine directions independently.

How do I determine the direction of magnetic force on an electron?

First determine the force direction that would act on a positive charge moving in the stated velocity direction. Use the relationship represented by F = qv × B, keeping the velocity and magnetic-field directions clearly marked. Because an electron has negative charge, reverse the positive-charge result. Do not reverse the magnetic field or velocity as an extra step, since doing so can reverse the answer twice. A useful final check is that the magnetic force must be perpendicular to both the velocity and the magnetic field.

Why does a magnetic field not change the speed of a charged particle?

The magnetic force on a moving charge acts perpendicular to its instantaneous velocity. A perpendicular force changes the direction of the velocity vector but not its magnitude. The force therefore does no work on the particle because there is no force component along the displacement. In a uniform field with perpendicular entry, the force acts as a centripetal force and produces circular motion. The particle's velocity changes because direction changes, even though its speed and kinetic energy remain constant.

Is electromagnetic induction included in IB Physics SL?

Under the current IB Physics course first assessed in 2025, D.4 Induction has no Standard Level content. Magnetic fields and magnetic forces still appear at SL through D.2 and D.3, so SL students cannot omit magnetism as a whole. HL students additionally study magnetic flux, Faraday's law, Lenz's law, motional emf, and specified applications of induction. Students should follow the level designation in the current guide rather than topic lists created for the previous syllabus. If uncertain, confirm the scope with your teacher and the current subject guide.

How should I explain Lenz's law in an exam answer?

Begin by identifying whether magnetic flux through the circuit is increasing or decreasing and in which direction. Then state that the induced current produces a magnetic field that opposes that change in flux. Use a field-direction rule to determine the resulting current direction if the question requests it. Avoid saying only that the induced current “opposes the magnetic field,” because the original field may remain present while its magnitude changes. For a fuller explanation, connect the opposition to energy conservation and the external work required to maintain the change.

What is the best way to practise IB Physics magnetism questions?

Attempt a focused question before consulting notes, even if you are unsure of the direction or equation. Then watch a complete per-question video solution and compare each decision rather than checking only the final value. Record whether the mistake involved the charge sign, direction rule, angle, physical model, algebra, or written explanation. Redo the same question from a blank page and follow it with a similar question under timed conditions. RevisionDojo's Physics Questionbank and worked video solutions make this attempt-watch-correct-repeat method easier to apply consistently.

IB Physics: How to Answer Magnetism Questions

RevisionDojo Team

•7/31/2026•14 min read

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IB Physics: How to Answer Magnetism Questions

RevisionDojo Team

•7/31/2026•14 min read

IB Physics magnetism questions become much more manageable when you separate each problem into four decisions: identify the object experiencing the force, determine the relevant directions, select the correct equation, and explain the physical result. The same structures recur across multiple-choice, data-based, short-answer, and extended-response questions.

The fastest way to improve is not to reread magnetism notes repeatedly. Attempt a question without help, compare your method with a complete worked solution, record the exact error, and then solve a similar question. RevisionDojo's IB Physics Questionbank and Physics video solutions support this attempt-watch-correct-repeat cycle.

Where magnetism appears in IB Physics

Under the current course, first assessed in 2025, magnetism is mainly found within Theme D: Fields:

D.2 Electric and magnetic fields develops magnetic fields and interactions involving currents.
D.3 Motion in electromagnetic fields covers forces on moving charges and current-carrying conductors, including circular motion.
D.4 Induction covers magnetic flux, Faraday's law, Lenz's law, and applications. The official guide identifies D.4 as HL-only content.

Magnetism can be assessed on Paper 1A through multiple-choice questions, on Paper 1B through data-based problems, and on Paper 2 through short-answer or extended-response questions. The official IB subject brief gives Paper 1 a weighting of 36% and Paper 2 a weighting of 44%, with the scientific investigation contributing the remaining 20%.

The official IB Physics specimen papers are particularly useful because they demonstrate the structure and expected response style of the current assessment model. They do not predict future questions, but they are an authoritative guide to format.

The equations you must know how to use

The Physics data booklet supplies the principal equations, but it does not decide which one applies or determine a vector direction for you. Its equations are presented as magnitude relationships, so signs and directions must be handled separately.

Situation	Relationship	Essential condition
Moving charge in a magnetic field	F = qvB sin θ	θ is between velocity and magnetic field
Current-carrying wire in a magnetic field	F = BIL sin θ	L is the wire length within the field
Charged particle moving in a circle	r = mv/(qB)	Velocity is perpendicular to a uniform field
Parallel current-carrying wires	F/L = μ₀I₁I₂/(2πr)	Long, parallel conductors
Magnetic flux through a coil	Φ = BA cos θ	θ is between B and the normal to the coil
Induced emf	ε = -NΔΦ/Δt	HL induction
Motional emf	ε = BvL	Conductor moves perpendicular to the field

Two angle conventions cause frequent errors. In a force equation, θ is the angle between B and the velocity or current. In magnetic flux, θ is the angle between B and the normal to the area, not between the field and the plane of the coil.

A reliable method for answering magnetism questions

Step 1: Identify what is experiencing the force

Ask whether the object is a moving charged particle, a current-carrying wire, or a coil undergoing changing magnetic flux. This immediately narrows the possible principles.

A stationary charge experiences no magnetic force. A moving charge parallel to the field also experiences no magnetic force because sin 0° = 0.

Step 2: Mark every direction on the diagram

Translate symbols before calculating:

A dot, ⊙, means out of the page.
A cross, ⊗, means into the page.
Conventional current runs in the direction positive charge would move.
An electron's motion is opposite to conventional current.

For a positive charge, determine the direction of the force using the relationship represented by F = qv × B. If the charge is negative, reverse the resulting force direction. For a conductor, Fleming's left-hand rule can be used with field, conventional current, and force, provided you apply the rule consistently.

Step 3: Check the angle before substituting

Do not assume sin θ = 1 merely because a diagram looks roughly perpendicular. State the angle or identify the perpendicular condition explicitly.

This check also provides a quick reasonableness test:

Parallel motion gives zero force.
Perpendicular motion gives maximum force.
An intermediate angle gives a force between zero and the maximum.

Step 4: Show the physics before the arithmetic

For a calculation, write the equation, substitute values with SI units, and give the final unit. For a vector quantity, include the direction when the question requires it.

For an explanation, connect the cause to the result. For example: “The magnetic force remains perpendicular to the velocity, so it changes the direction of the velocity but not its magnitude. The particle therefore follows a circular path at constant speed.”

Worked example: charged particle in a magnetic field

A proton enters a uniform magnetic field of strength 0.20 T at 3.0 × 10⁶ m s⁻¹. Its velocity is perpendicular to the field. Determine the radius of its path.

The magnetic force supplies the centripetal force:

qvB = mv²/r

Rearranging:

r = mv/(qB)

Using m = 1.67 × 10⁻²⁷ kg and q = 1.60 × 10⁻¹⁹ C:

r = (1.67 × 10⁻²⁷)(3.0 × 10⁶) / [(1.60 × 10⁻¹⁹)(0.20)] = 0.157 m

To an appropriate number of significant figures, r = 0.16 m. Notice that the speed does not decrease: the force is perpendicular to the displacement, so the magnetic field does no work on the proton.

If the particle were an electron with the same entry direction, its path would curve in the opposite direction because its charge is negative. Its radius would also differ because electron and proton masses are not equal.

Worked example: force on a current-carrying wire

A 0.080 m section of wire carries a current of 4.0 A perpendicular to a 0.50 T magnetic field. Calculate the magnetic force.

Because θ = 90°, sin θ = 1:

F = BIL = (0.50)(4.0)(0.080) = 0.16 N

The numerical answer is incomplete if the question asks for the force as a vector. Use the field and conventional-current directions to state whether the force is upward, downward, into the page, or out of the page.

If the wire were rotated until it became parallel to the magnetic field, the force would become zero. This follows from the angle dependence rather than from any change in current or field strength.

How to answer induction questions at HL

Induction questions usually test three separate ideas: calculating flux, finding the magnitude of induced emf, and determining its direction. Treat these as separate stages.

For flux, use Φ = BA cos θ and check whether θ refers to the normal. For emf, calculate the rate of change of flux linkage using |ε| = N|ΔΦ/Δt|. For direction, apply Lenz's law: the induced current creates a magnetic effect that opposes the change in flux, not necessarily the original magnetic field itself.

Suppose the north pole of a magnet approaches a coil. The magnetic flux through the coil increases, so the induced current produces a field that resists that increase. The near face of the coil therefore behaves like a north pole, opposing the approaching magnet. This resistance is consistent with energy conservation because external work is required to continue moving the magnet.

RevisionDojo's D.4 Induction resources and guide to Lenz's law can be used to review these steps before attempting HL questions.

Recurring question types and traps

Question type	Productive approach	Common trap
Direction of force	Establish B and positive-charge or current direction first	Forgetting to reverse the force for an electron
Force magnitude	Use the correct sine relationship	Using the complementary angle
Circular motion	Equate magnetic and centripetal forces	Assuming magnetic force changes speed
Field around a wire	Apply the right-hand grip rule	Confusing current direction with electron flow
Parallel wires	Decide attraction or repulsion before calculating	Reversing the same-direction rule
Magnetic flux	Use the angle to the area normal	Measuring from the plane of the coil
Lenz's law	Oppose the change in flux	Opposing the field in every situation
Explanation	Give a cause-and-effect chain	Repeating the observation without explaining it

Long parallel wires carrying currents in the same direction attract; currents in opposite directions repel. Learn this as a consequence of each wire producing a field that exerts a force on the other, rather than as an isolated phrase.

Command terms and mark-winning presentation

The current Physics guide defines calculate as obtaining a numerical answer while showing relevant working. Explain requires reasons or causes, while show requires the steps in a calculation or derivation.

Apply those distinctions directly:

State: give the direction or relationship briefly.
Calculate: show equation, substitution, result, and unit.
Determine: obtain the definite answer from the supplied information.
Explain: connect field, force, motion, and outcome.
Show: begin from an appropriate known relationship and include the algebraic steps.
Sketch: include the correct shape, direction, and relevant labels.

A two-mark explanation usually requires more than naming a law. “By Lenz's law” is rarely a complete causal account by itself. State what is changing and how the induced field or current opposes that change.

The most efficient magnetism revision cycle

Worked video solutions are especially effective for magnetism because they reveal decisions that a static final answer can hide: hand-rule setup, angle selection, diagram annotation, and equation choice. Use the following cycle:

Attempt one question under a short time limit.
Commit to a direction and method before checking anything.
Watch the per-question worked solution in the RevisionDojo Physics video library.
Classify the error as conceptual, directional, algebraic, or presentational.
Redo the question without viewing the solution.
Attempt a related problem from the D.2 electric and magnetic fields Questionbank or D.3 motion in electromagnetic fields resources.

This is faster than passive rereading because it trains retrieval and method selection under exam-like conditions. Jojo AI can help clarify why a direction or equation was wrong, but you should still produce your own complete solution before requesting feedback.

Conclusion

Successful answers to IB Physics magnetism questions combine accurate direction reasoning, a correctly selected equation, explicit working, and an explanation matched to the command term. The most common losses come from charge signs, angle conventions, confusion between speed and velocity, and incomplete applications of Lenz's law.

Revise by attempting questions first and then studying how each one is worked through. RevisionDojo's Questionbank, topic resources, and per-question Physics video solutions are the most relevant tools for building that repeatable method.