Compare conductor movement when the magnetic field reverses
Keep conductor current fixed and describe opposite magnetic-force direction when the imposed field alone is reversed.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-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.
Before you begin
- Read north and south pole labels.
- Separate a physical change from a viewpoint change.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is a physical field reversal here?
What you will learn
- Keep conductor current fixed and describe opposite magnetic-force direction when the imposed field alone is reversed.
- Explain single and double current/field reversals in one fixed observation frame.
Swap the magnetic condition alone
In the same movable-rod demonstration, retain the rod current and reverse the imposed field by changing the pole arrangement while preserving a suitable gap and conductor placement. The field in the gap runs from north to south; exchanging the poles reverses that direction. The magnetic force on the current-carrying rod then reverses.
Keep current magnitude and direction, mechanical supports and observation conditions matched. Turning the whole drawing or looking from the other side is not necessarily a physical field reversal. State the pole positions in one unchanged reference view.
Use a two-change check carefully
Reversing current alone reverses force; reversing field alone also reverses force. Reversing both together restores the original magnetic-force direction in the stated unchanged geometry. These are direction predictions, not claims that the rod must have the same speed or displacement in every trial.
A field-off or current-off observation supports the interpretation, but Earth’s field, nearby conductors and constraints can still act. Written comparisons show reasoning rather than supervised practical competence. Do not combine two changes and label the result a field-only experiment.
Sources for this lesson
Pause and explain
Current and field both reverse in the same geometry. What happens to force direction?
Worked example
A fixed-current rod is pushed toward side X by the specified pole gap. Swap north and south while keeping current unchanged. Then also reverse current. Compare force directions.
Show the worked solution
- Field reversal alone gives the opposite force.
- The additional current reversal reverses that result again.
- Both reversals together restore the initial force direction.
Answer Field alone: opposite; field and current together: original 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.
Describe a field-only reversal trial for the movable rod.
Give me a hint
Keep the rod current unchanged and state the pole arrangement.
Compare my reasoning
- Record the first north-to-south field direction.
- Exchange poles with suitable matched geometry.
- Compare the observed motion under unchanged current.
Field reversal alone reverses magnetic-force direction.
Look for these in your work
- Name the fixed current.
- Retain the same view.
Complete a relative-force table for original, current-only reversed, field-only reversed and both reversed states.
Give me a hint
Each independent reversal changes the force sign.
Compare my reasoning
- Use the initial force as the reference.
- Either single reversal gives the opposite direction.
- Both reversals restore the original direction.
Original, opposite, opposite, original.
Look for these in your work
- Include all four states.
- Do not confuse a double change with an isolated test.
A student turns the page upside down and says the experiment reversed its field. Explain the error.
Give me a hint
Separate representation from the actual pole arrangement.
Compare my reasoning
- The page rotation changes labels in the observer’s view.
- The physical magnet may be unchanged.
- A reversal claim requires changed field direction in a fixed reference.
Changing the view alone does not reverse the physical field.
Look for these in your work
- Name the reference view.
- Describe the required physical change.
Common mistakes
- Fix the viewing convention.
- Swap actual poles, not only the drawing.
- Separate single and double reversals.
What can you explain now?
Which way does the field point in the pole gap?
Compare with the explanation
North to south
The imposed external field direction is from the north pole toward the south pole.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain single and double current/field reversals in one fixed observation frame.
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.