Use the motor rule for force field and current directions
Apply perpendicular motor directions using conventional current and explicit into-page and out-of-page symbols.
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Read dot and cross symbols.
- Distinguish the motor effect from induction.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which current belongs in the motor direction rule?
What you will learn
- Apply perpendicular motor directions using conventional current and explicit into-page and out-of-page symbols.
- Assign motor-effect directions with conventional current, explicit page axes and parallel-case limits.
Use a motor convention for a supplied current
For the perpendicular motor effect, Fleming’s left-hand convention sets first finger along magnetic field, second finger along conventional current and thumb along force. Field in the pole gap is north to south. A dot means toward the viewer, out of the page; a cross means away, into the page. Draw and label these axes before using the rule.
For conventional current upward and field rightward, the force is into the page. For current out of the page and field rightward, force is upward. These examples use a right/up/out-of-page frame. Force is perpendicular to both current direction and field in the stated perpendicular arrangement.
Keep cause and convention distinct
This is the motor effect: a supplied current in an imposed field experiences a force. The generator direction rule concerns imposed conductor motion and induced e.m.f. or current. Do not use the same word force to mean both imposed generator motion and the magnetic reaction on a driven motor conductor.
If current and field are parallel, the ideal magnetic force on that straight section is zero; an orthogonal hand construction is then not appropriate. The rule specifies direction, not a numerical force magnitude, speed or guaranteed visible displacement. Other mechanical forces can balance it.
Sources for this lesson
Pause and explain
Current out of the page and field rightward give which force direction?
Worked example
Conventional current points upward and magnetic field points rightward. Give the force direction.
Show the worked solution
- Set first finger rightward for field.
- Set second finger upward for conventional current.
- The motor-rule thumb points into the page.
Answer Force into the page.
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.
Find force for upward current and rightward field.
Give me a hint
Name the three axes before using the motor rule.
Compare my reasoning
- Use the fixed right/up/out frame.
- Set field right and conventional current up.
- Force points into the page.
Into the page.
Look for these in your work
- State the current convention.
- Use a cross for the resulting force.
Force is upward and field is rightward in a perpendicular motor arrangement. Determine conventional current direction.
Give me a hint
Complete the ordered motor axes rather than guessing a page symbol.
Compare my reasoning
- The force and field are perpendicular.
- Outward conventional current gives upward force in this frame.
- Mark the current by a dot.
Conventional current out of the page.
Look for these in your work
- Solve for the missing axis.
- State the dot convention.
Current and field both point rightward in the stated ideal straight-section model. A student draws an upward force by forcing a perpendicular hand shape. Correct it.
Give me a hint
Check whether the supplied vectors are perpendicular.
Compare my reasoning
- Current and field are parallel.
- The ideal straight-section magnetic force is zero.
- An orthogonal hand shape changes the given geometry.
Zero ideal magnetic force for this parallel section.
Look for these in your work
- Retain the supplied directions.
- Distinguish zero force from a missing diagram label.
Common mistakes
- Use the left-hand motor convention.
- Use conventional current.
- Keep the page frame fixed.
What can you explain now?
What does a cross indicate for a perpendicular direction?
Compare with the explanation
Into the page
The cross represents the tail of an arrow pointing away from the viewer.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Assign motor-effect directions with conventional current, explicit page axes and parallel-case limits.
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.