Explain motor reversals and energy transfers
Predict changes when field or supply current reverses and separate the motor’s driving moment from power and final speed.
Extended only (Supplement); Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Use current/field reversal rules.
- Balance supplied energy transfers.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Does reversing current guarantee an instantaneous rotor-speed reversal?
What you will learn
- Predict changes when field or supply current reverses and separate the motor’s driving moment from power and final speed.
- Predict simple motor driving reversals and conserve the supplied source energy without equating moment, speed and power.
Use the fixed motor geometry
With the same simple motor geometry and commutation arrangement, reversing the supply current reverses the driving force pair and turning tendency. Reversing the imposed magnetic field alone does the same. Reversing both restores the original driving sense. Describe the tendency in a fixed reference view; an observer walking around the motor does not reverse its physical rotation.
Actual motion may first slow before reversing because the rotor has inertia and the load can oppose it. A direction prediction does not guarantee an instantaneous speed change. The simplified comparison assumes the stated field and current directions really change while geometry and switching conditions remain suitable.
Follow the source energy account
A maintained motor takes electrical energy from its source and transfers energy into mechanical work on a load, heating and other stated pathways. In a supplied interval, 30 J input, 18 J load work and 12 J other transfers balance the account. The field magnet does not create free mechanical energy.
Turning effect is a moment, power is energy per time, and final speed depends on source, load and losses. Do not identify these quantities just because they all affect useful motor operation. This lesson uses specified simple operating states rather than instructions to alter real mains-connected motors.
Pause and explain
Field and current both reverse with geometry fixed. What happens to driving sense?
Worked example
A supplied motor takes 30 J electrically and performs 18 J of load work in one interval. Account for the remaining transfer and state what happens to driving sense if current alone reverses.
Show the worked solution
- Other transfer is 30 − 18 = 12 J.
- The source supplies the energy account.
- Current-only reversal reverses the driving turning tendency.
Answer 12 J other transfers; reversed driving tendency.
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.
Compare driving senses for current-only, field-only and both reversals.
Give me a hint
Apply the two independent force-pair reversals in one view.
Compare my reasoning
- Current alone reverses the driving tendency.
- Field alone also reverses it.
- Both reversals restore the original sense.
Opposite, opposite, original.
Look for these in your work
- Name the changed factors.
- Keep the view fixed.
A supplied motor receives 50 J and transfers 32 J as load work, with no net stored-energy change. Find other transfers.
Give me a hint
Use the same interval and system boundary.
Compare my reasoning
- Input is 50 J.
- Load transfer is 32 J.
- Other transfers are 50 − 32 = 18 J.
18 J.
Look for these in your work
- Keep the energy boundary.
- State the no-net-storage condition.
A report says a stronger driving moment proves greater final speed and that the permanent magnet supplies all load work. Correct it.
Give me a hint
Separate mechanical response and energy source.
Compare my reasoning
- Final speed depends on load and operating conditions.
- The electrical source supplies energy.
- The magnet establishes the field rather than an unlimited energy account.
Neither the speed claim nor the free-magnet-energy claim follows.
Look for these in your work
- Explain the speed limitation.
- Identify the energy source.
Common mistakes
- Fix the view and changed factor.
- Attribute energy to the source.
- Do not equate moment with speed.
What can you explain now?
What supplies a maintained motor’s delivered energy?
Compare with the explanation
The electrical source
The source transfers electrical energy; the magnet provides the field condition rather than free output energy.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Predict simple motor driving reversals and conserve the supplied source energy without equating moment, speed and power.
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.