IB Physics Magnetism: Common Mistakes and Fixes

IB Physics magnetism questions are rarely difficult because of algebra alone. Most lost marks come from using the wrong angle, confusing field and force directions, ignoring the sign of a charge, or applying a memorized equation without checking the physical situation. The most effective fix is to study worked past-paper video solutions step by step, then reproduce the method independently rather than merely watching the calculation.

This guide explains the main IB Physics magnetism common mistakes, why they occur, and the routines that prevent them under exam conditions. It covers magnetic fields, forces on charges and currents, circular motion, field diagrams, magnetic flux, and induction.

Where magnetism fits in IB Physics

In the current IB Diploma Programme Physics course, first assessed in 2025, magnetic fields are studied within Theme D: Fields. Magnetism connects closely to mechanics, electric current, circular motion, energy, and electromagnetic induction, so exam questions often require ideas from more than one part of the course.

IB questions reward more than equation recall. You must identify the relevant model, represent directions accurately, substitute consistent units, and interpret the result physically. The official IB Physics subject page provides the current course overview, while RevisionDojo's IB Physics resources can support topic-by-topic practice.

The recurring IB Physics magnetism mistakes

Common mistake	Why it loses marks	Reliable fix
Using the wrong hand rule	The force or field direction is reversed	Write what each finger represents before using the rule
Treating electron motion as conventional current	Electrons and conventional current point in opposite directions	Reverse the electron velocity before finding the current direction
Forgetting the sine factor	The calculated force is too large unless the vectors are perpendicular	Mark the angle between the two vectors explicitly
Assuming a magnetic field changes speed	Magnetic force is incorrectly treated as doing work	Check whether the force is perpendicular to velocity
Confusing flux angle definitions	Using the plane instead of its normal gives sine instead of cosine	Draw a normal to the surface
Combining equations mechanically	The resulting expression may not match the stated conditions	Explain why each equation applies before substitution
Ignoring signs and units	Magnitudes may be correct but directions or powers of ten are wrong	Separate magnitude, direction, and unit checks

Mistake 1: Applying a hand rule without defining the vectors

Students often perform a remembered hand gesture before deciding which physical quantities are involved. This is risky because different classroom conventions exist, while the underlying vector relationships remain fixed.

For a moving positive charge,

$$\vec F=q\vec v\times\vec B.$$

The magnetic force is perpendicular to both the velocity and magnetic field. First identify velocity, magnetic field, and force on the diagram. Apply the right-hand rule to a positive charge, then reverse the resulting force if the charge is negative.

For a current-carrying conductor, use the direction of conventional current in

$$F=BIL\sin\theta.$$

A worked video solution is useful here because it makes the orientation process visible. Pause before the presenter gives the direction, determine it yourself, and then compare the reasoning rather than only the final arrow.

Mistake 2: Confusing electron flow with conventional current

Conventional current points in the direction positive charge would move. In a metal, electrons drift in the opposite direction, so an electron-motion arrow cannot automatically be used as the current arrow.

Use this sequence:

1. Identify whether the diagram shows positive charges, electrons, or conventional current.
2. If it shows electrons, reverse that direction to obtain conventional current.
3. Use conventional current when finding the force on a wire or the magnetic field around it.
4. If finding the force on one electron, use its actual velocity and reverse the positive-charge result because $q<0$.

This distinction matters especially in questions involving wires, beams of charged particles, or crossed electric and magnetic fields.

Mistake 3: Using the wrong angle in magnetic-force equations

In both

$$F=qvB\sin\theta$$

and

$$F=BIL\sin\theta,$$

$\theta$ is the angle between the velocity or current direction and the magnetic field. It is not automatically the angle printed most prominently on the diagram.

If motion is parallel or antiparallel to the field, $\sin\theta=0$, so there is no magnetic force. If the vectors are perpendicular, $\sin\theta=1$, giving the maximum force. Before substituting, draw the two relevant vectors from a common point and label the included angle.

A useful check is to test the limiting cases. Any expression predicting a maximum magnetic force for motion parallel to the field must be wrong.

Mistake 4: Saying that a magnetic field changes a particle's speed

A magnetic force acting alone is perpendicular to the instantaneous velocity. Therefore,

$$P=\vec F\cdot\vec v=0,$$

so the field does no work on the particle. It can change the direction of momentum, but not the particle's kinetic energy or speed.

When a charged particle enters a uniform field perpendicular to its motion, the magnetic force supplies the centripetal force:

$$qvB=\frac{mv^2}{r}, \qquad r=\frac{mv}{|q|B}.$$

Use (|q|) when calculating the positive radius. Use the sign of the charge separately to determine the direction of curvature. OpenStax's explanation of charged-particle motion in magnetic fields provides a helpful independent treatment of this model.

Mistake 5: Using circular-motion formulas without checking the conditions

The expression $r=mv/(|q|B)$ applies when the velocity is perpendicular to a uniform magnetic field and the magnetic force is the relevant centripetal force. If the particle has a velocity component parallel to the field, that component remains unchanged while the perpendicular component produces circular motion, resulting in a helical path.

Do not begin by quoting the radius formula. A stronger solution shows the model:

1. State that the magnetic force supplies the centripetal force.
2. Write $|q|vB=mv^2/r$ for perpendicular motion.
3. Cancel one factor of $v$.
4. Rearrange and substitute SI units.
5. Determine curvature separately using the charge sign.

This derivation reduces formula-selection errors and can earn method credit even if the arithmetic later goes wrong.

Mistake 6: Drawing magnetic fields incorrectly

Magnetic-field lines show the direction a north test pole would move. Around a long straight wire, the lines form concentric circles centered on the wire. Their spacing represents field strength qualitatively, while their direction follows from the conventional current.

Students commonly draw radial lines, forget arrowheads, or reverse the direction because they use electron flow. For a long straight conductor,

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

so the field becomes weaker as distance $r$ increases. A complete diagram should show the wire, current direction, circular field lines, and clear arrowheads.

The standard page notation is also important:

• Dot, $\odot$: vector coming out of the page, like the tip of an arrow.
• Cross, $\otimes$: vector going into the page, like the tail feathers of an arrow.

Mistake 7: Confusing magnetic field with magnetic flux

Magnetic field strength $B$ is measured in tesla, whereas magnetic flux $\Phi$ is measured in webers. For a uniform field through a flat area,

$$\Phi=BA\cos\theta,$$

where $\theta$ is the angle between the magnetic field and the normal to the surface, not the surface itself.

If the field is perpendicular to the surface, it is parallel to the normal and the flux is maximum. If the field lies in the plane of the surface, the flux is zero. Always draw a dashed normal before choosing the angle.

For a coil with $N$ turns, flux linkage is $N\Phi$. Faraday's law concerns the rate of change of flux linkage:

$$\mathcal{E}=-N\frac{\Delta\Phi}{\Delta t}.$$

The negative sign represents Lenz's law, meaning the induced effect opposes the change that produces it. It does not mean that the numerical magnitude of the emf must be reported as negative in every question.

Mistake 8: Substituting before explaining the physics

Magnetism questions often combine several equations. For example, a particle accelerated through a potential difference and then entering a magnetic field may require

$$qV=\frac12mv^2$$

followed by

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

Writing equations without stating their roles makes it easier to combine incompatible models. First explain that electrical work becomes kinetic energy, then explain that the magnetic force supplies centripetal force in the field region.

The RevisionDojo Physics Questionbank is most useful when practice is organized by subtopic. For each question, write a one-line model statement before doing any algebra.

How to review worked video solutions effectively

Watching a solution passively can create familiarity without recall. Use per-question IB Physics past-paper video solutions as a correction tool through this sequence:

1. Attempt the question under timed conditions.
2. Mark the exact line where your reasoning became uncertain.
3. Watch only until the presenter reaches that step.
4. Pause and predict the next equation, direction, or diagram.
5. Finish the problem independently.
6. Record the error as a short rule, such as “flux angle is measured from the normal.”
7. Repeat a similar question several days later without notes.

Use IB Physics Study Notes to repair a conceptual gap and Physics Flashcards for definitions, symbols, and limiting cases. Jojo AI can help explain why a particular equation applies, but your final practice should still require a complete handwritten solution without prompts.

A reliable exam routine for magnetism questions

Before calculating, annotate the diagram with $\vec B$, $\vec v$, conventional current, charge sign, and any surface normal. Then state the governing physical relationship in words.

During the calculation:

• Convert all quantities to SI units.
• Keep charge magnitude and charge sign conceptually separate.
• Use the angle between the correct pair of vectors.
• Retain sufficient significant figures until the final answer.
• Include a unit and, where relevant, a direction.

After calculating, test the result physically. A stronger field should produce a smaller particle radius at fixed momentum, parallel motion should give zero magnetic force, and magnetic force alone should not change kinetic energy.

Conclusion

The most common IB Physics magnetism mistakes are systematic rather than random. Students lose marks by mishandling directions, charge signs, angles, field diagrams, circular-motion assumptions, and the distinction between magnetic flux and magnetic field.

The solution is a consistent routine: label vectors, state the physical model, derive before substituting, and check the result against limiting cases. RevisionDojo's questionbank, study notes, flashcards, and especially per-question past-paper video solutions can help turn each error into a repeatable correction.

Sources and referenced URLs

• International Baccalaureate: Diploma Programme Physics
• OpenStax: Motion of a Charged Particle in a Magnetic Field
• RevisionDojo IB Physics resources
• RevisionDojo IB Physics Questionbank
• RevisionDojo IB Physics past papers and video solutions
• RevisionDojo IB Physics Study Notes
• RevisionDojo IB Physics Flashcards