Compare conductor movement when current reverses
Keep the imposed magnetic field fixed and compare equal-magnitude opposite currents in the same movable-rod arrangement.
Section P4.5.4 · Force on a current-carrying conductor
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Distinguish conventional current from electron motion.
- Identify controlled variables.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which direction should be recorded for this comparison?
What you will learn
- Keep the imposed magnetic field fixed and compare equal-magnitude opposite currents in the same movable-rod arrangement.
- Predict relative reversal while keeping the actual conductor current and imposed field explicit.
Reverse one electrical condition
Use the same supervised demonstration, keeping the pole arrangement, rod and supports fixed. Compare current in one direction with current reversed at the same magnitude. The magnetic force reverses direction. A freely moving rod can therefore move in the opposite sense, provided friction and other forces allow the effect to be observed.
A supplied table may say current A gives motion toward one marked side and reversed current gives motion toward the other. That is a relative reversal result. Core learners do not need to assign an absolute page direction using the Supplement motor rule. Keep the stated observation labels instead of inventing a new viewing convention.
Compare force without confusing source setting and current
A changed supply connection establishes reversal only if the current through the investigated rod really reverses. Record or state the conventional-current direction; the same apparent lead position in a redrawn circuit is not sufficient. Equal source control settings may not keep current magnitude equal when contact resistance or temperature changes.
Compare matched short observations and current-off references. If the rod sticks after reversal, support friction may prevent visible movement despite an opposite magnetic force. This written comparison does not replace practical demonstration or teacher review.
Sources for this lesson
Pause and explain
The field is fixed and current alone reverses. Which prediction follows?
Worked example
With a fixed field, current A gives a supplied rod motion toward side X. Current alone is reversed. Predict the magnetic-force direction relative to its original direction.
Show the worked solution
- Keep the pole gap and mechanical geometry fixed.
- Reverse conventional current through the rod.
- The magnetic force reverses.
Answer Opposite to the original magnetic-force direction.
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.
Plan a matched current-direction reversal comparison.
Give me a hint
Change direction while retaining the field and mechanical setup.
Compare my reasoning
- Record the initial current and motion sense.
- Reverse rod current at equal magnitude.
- Retain pole arrangement and compare the motion sense.
Opposite current produces opposite force under the matched conditions.
Look for these in your work
- Identify the one changed direction.
- Keep the observation convention.
The rod sticks after current reversal. Explain why this does not prove the force remained unchanged.
Give me a hint
Separate the interaction from the mechanical response.
Compare my reasoning
- Current reversal reverses the magnetic force.
- Friction can balance a changed force.
- Check supports and suitable observation conditions.
A support constraint can mask the reversed force.
Look for these in your work
- State the reversal result.
- Explain the role of friction.
A report swaps source leads and reverses the magnet at the same time, then says it tested current reversal alone. Critique it.
Give me a hint
Count the independently changed directions.
Compare my reasoning
- The rod current changes.
- The imposed field also changes.
- A current-only comparison must keep the magnet arrangement fixed.
Two direction changes do not isolate current reversal.
Look for these in your work
- Identify both changes.
- Propose the single-change comparison.
Common mistakes
- Reverse actual rod current.
- Keep field and geometry unchanged.
- Do not treat sticking as proof of zero force.
What can you explain now?
What happens to conductor magnetic force when only current direction reverses?
Compare with the explanation
Its direction reverses
Under unchanged imposed field and geometry, reversing current reverses the magnetic force.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Predict relative reversal while keeping the actual conductor current and imposed field explicit.
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.