Magnetic Force Common Exam Traps

Overview

Magnetic Force is a high-scoring topic, but many marks are lost through predictable mistakes involving:

• direction rules
• angle usage
• particle charge sign
• circular motion logic
• attraction or repulsion of wires
• confusing magnetic and electric effects

This page collects the most common traps and how to avoid them.

Main topic: Magnetic Force

Definition

An exam trap is a predictable mistake caused by wrong direction handling, missing angle factors, sign errors, or confusion about what magnetic force can and cannot do.

Why It Matters

Magnetic-force questions are often straightforward once direction, geometry, and force properties are interpreted correctly.

Key Representations

Trap 1: Wrong Fleming’s Left-Hand Rule

Mistake

Using the fingers in the wrong order.

Correction

Use:

• first finger = magnetic field
• second finger = current or positive-charge velocity
• thumb = force

For current direction, use conventional current.

Trap 2: Forgetting $\sin \theta$

Mistake

Using:

$$F=BIL$$

or

$$F=B|q|v$$

for all cases.

Correction

Correct general forms:

$$F=BIL\sin \theta$$ $$F=Bqv\sin \theta$$

where $\theta$ is the angle between current or velocity and the magnetic field.

Trap 3: Using the Wrong Angle

Mistake

Using angle to the page, horizontal, or some other diagram angle.

Correction

Always use the angle between:

• current direction and field, or
• velocity direction and field

Trap 4: Electron Force Direction Error

Mistake

Applying the left-hand rule directly to electrons.

Correction

Use the left-hand rule for a positive charge first, then reverse the direction for a negative charge.

Trap 5: Thinking Magnetic Force Changes Speed

Mistake

Assuming a particle speeds up or slows down in a magnetic field.

Correction

Magnetic force is perpendicular to velocity.

So:

• direction changes
• speed remains constant
• kinetic energy is unchanged

Trap 6: Forgetting Force Is Perpendicular

Mistake

Drawing force along field lines or along velocity.

Correction

Magnetic force acts perpendicular to both:

• magnetic field
• current or velocity

Trap 7: Wrong Circular-Radius Proportionality

Mistake

Thinking a faster particle has a smaller radius.

Correction

From:

$$r=\frac{mv}{B|q|}$$

radius increases with:

• larger $m$
• larger $v$

radius decreases with:

• larger $B$
• larger $q$

Trap 8: Mixing Proton and Electron Paths

Mistake

Assuming the same curvature direction.

Correction

Particles with opposite charges curve in opposite directions.

Trap 9: Same or Opposite Current Confusion

Mistake

Forgetting whether wires attract or repel.

Correction

• same-direction currents attract
• opposite-direction currents repel

Trap 10: Forgetting Both Wires Feel Force

Mistake

Only assigning force to one wire.

Correction

Each wire exerts an equal and opposite force on the other.

Newton’s third law applies.

Trap 11: Mixing Electric and Magnetic Forces

Mistake

Using magnetic-force direction along field lines.

Correction

Electric Force

Acts along the electric field:

$$F=qE$$

Magnetic Force

Acts perpendicular:

$$F=Bqv\sin \theta$$

Related topic: Electric Fields

Trap 12: Velocity-Selector Logic Error

Mistake

Forgetting the forces oppose each other.

Correction

For undeflected motion:

$$qE=qvB$$

So:

$$v=\frac{E}{B}$$

Only one speed passes straight through.

Trap 13: Using the Wrong Current Direction

Mistake

Using electron drift direction in conductor-force questions.

Correction

Use conventional current unless the question explicitly states otherwise.

Trap 14: Confusing Magnetic Field with Magnetic Force

Mistake

Treating $B$ as force.

Correction

$B$ is magnetic flux density, not force.

Force depends on motion or current inside the field.

Quick Self-Check Checklist

Before submitting:

• Did I use Fleming’s left-hand rule correctly?
• Did I include $\sin \theta$?
• Is the angle between velocity or current and field?
• Did I reverse direction for an electron?
• Is force perpendicular to motion?
• Should speed remain constant?
• Same current means attract?
• Opposite current means repel?
• Is this electric force or magnetic force?

Rapid Correction Table

If You See…	Check…
Wrong direction	Fleming rule and charge sign
Zero force unexpectedly	Is velocity parallel to field?
Wrong magnitude	Missing $\sin \theta$?
Wrong particle path	Radius relation and sign
Wire interaction wrong	Same vs opposite currents
Particle speeding up	Magnetic force does no work

Summary

Most Magnetic Force errors come from:

1. wrong direction rules
2. missing $\sin \theta$
3. forgetting negative-charge reversal
4. misunderstanding perpendicular force
5. mixing electric and magnetic effects

Master these, and the topic becomes highly scorable.

Links

• Magnetic Force
• Charged Particles in Magnetic Fields
• Force Between Parallel Currents
• Magnetic Fields
• Circular Motion
• Electric Fields
• Vectors