Check beam direction reversals and parallel motion limits
Separate reversal of charge, field and velocity from a parallel-motion case and from changes in beam speed.
Section 4.5.4 · Force on a current-carrying conductor
Exam scope and assessment notes
Extended only (Supplement) · Physics 0972, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked. Human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Use charge-aware beam directions.
- Distinguish velocity direction from speed.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the ideal magnetic force for motion parallel to the field?
What you will learn
- Separate reversal of charge, field and velocity from a parallel-motion case and from changes in beam speed.
- Apply beam reversal rules and parallel limits without confusing magnetic deflection with energy gain.
Change only a named direction or sign
In the supplied perpendicular beam model, reversing field alone reverses the instantaneous magnetic force. Reversing particle motion alone also reverses force; reversing both restores the original force direction when charge stays fixed. Changing charge sign alone reverses it again. Label each change explicitly rather than comparing two drawings with different views.
A particle moving parallel or antiparallel to the imposed field has zero ideal magnetic deflecting force at that instant. A stationary particle also has no magnetic force from this mechanism. A neutral particle has no charge-based magnetic deflection in the stated simple model; other interactions are outside this account.
Direction changes do not imply magnetic speeding-up
For the ideal magnetic force considered here, force is perpendicular to particle velocity, so it bends the path rather than increasing speed through work along the motion. The beam’s speed or energy may still be changed by other interactions, such as an electric field; those must not be attributed to the magnetic field alone.
This lesson asks for directions and limiting cases, not numerical orbit radii or a particle-force formula. Determine the instantaneous force before describing the next part of the path. A later force arrow need not point in the same page direction as the entry arrow because velocity direction can change.
Pause and explain
Both particle motion and field reverse while charge sign stays fixed. What happens to force direction?
Worked example
A rightward positive beam in an inward field initially has upward force. Reverse field alone, then reverse field and particle motion together.
Show the worked solution
- Use the original charge sign.
- Field reversal alone makes force downward.
- Adding velocity reversal restores the original upward force.
Answer Field alone: down; field and motion together: up.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
For the supplied initial upward-force beam, reverse only the field.
Give me a hint
Use a single reversal in the fixed reference frame.
Compare my reasoning
- Keep motion rightward and charge positive.
- Change field from inward to outward.
- The force changes from upward to downward.
Downward.
Look for these in your work
- Fix the unchanged conditions.
- State the relative reversal.
A positive particle moves upward in an upward magnetic field. State the ideal magnetic force.
Give me a hint
Check the relative directions before applying perpendicular axes.
Compare my reasoning
- Motion and field are parallel.
- The ideal magnetic deflecting force is zero.
- Do not invent an inward or outward force.
Zero.
Look for these in your work
- Use the parallel condition.
- Do not replace the stated geometry.
An ideal beam bends in a magnetic region. A report says this proves the magnet increased its kinetic energy. Critique it.
Give me a hint
Separate velocity direction from speed.
Compare my reasoning
- Magnetic force acts perpendicular to instantaneous motion.
- It can change direction without increasing speed.
- Energy gain would require another energy-transfer mechanism.
Bending alone does not show a magnetic increase in kinetic energy.
Look for these in your work
- Distinguish speed and velocity.
- Retain the ideal magnetic-only condition.
Common mistakes
- Retain the charge sign.
- Name one or two changed vectors.
- Do not infer speed increase from bending.
What can you explain now?
Can a uniform magnetic field alone speed up this ideal deflecting beam?
Compare with the explanation
No
The magnetic force is perpendicular to its motion and does not do work along that motion.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Apply beam reversal rules and parallel limits without confusing magnetic deflection with energy gain.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.