Explain a rotating magnet generator with a fixed coil
Describe how a rotating field source changes the linking of a stationary receiving coil without unnecessary slip rings.
Section 4.5.2 · The a.c. generator
Extended only (Supplement); Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Explain induction in a stationary receiver.
- Distinguish an input field circuit and receiving output circuit.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Can a stationary receiving coil have induced e.m.f.?
What you will learn
- Describe how a rotating field source changes the linking of a stationary receiving coil without unnecessary slip rings.
- Decide whether contacts are needed by locating the rotating electrical terminals in the specified design.
Move the field source instead of the receiving coil
A simple a.c. generator can rotate a magnet or magnetic rotor while the receiving coil remains stationary. The magnetic field linking that fixed coil changes during rotation, producing induced e.m.f. Its sign reverses in the stated alternating two-pole arrangement. Mechanical input drives the rotor and supplies delivered electrical energy.
Because the output coil is fixed, its ends can connect directly to a stationary load without sliding output contacts. Slip rings and brushes are needed where the required electrical connection crosses between rotating and stationary parts, not merely because a device is called a generator.
State the particular design
A permanent-magnet rotor has no electrical excitation circuit to feed. A rotating electromagnet may require a separate supply to its field winding and can use additional connections for that purpose. That excitation circuit is distinct from the stationary receiving-coil output.
For this simple two-pole model, one revolution gives one electrical cycle. The actual graph depends on magnetic geometry and rotation conditions; not every rotating-magnet design produces a perfect sine wave. A fixed receiver still needs a closed output path for sustained load current.
Pause and explain
Why can the specified fixed output coil use fixed output leads?
Worked example
Does a permanent-magnet rotor with a fixed output coil need slip rings in the output-coil path? Explain.
Show the worked solution
- The output coil does not rotate.
- Its ends can use fixed leads to the stationary load.
- No rotating-to-stationary output connection is required.
Answer No output slip rings are needed in that specified fixed-coil arrangement.
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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 permanent-magnet rotating-source generator with a stationary output coil.
Give me a hint
Identify the moving source, receiver and energy input.
Compare my reasoning
- Mechanical input turns the magnet.
- The field linking the fixed coil changes.
- The coil output e.m.f. can drive a closed load.
Rotating magnet → changing fixed-coil linking → induced output e.m.f.
Look for these in your work
- Name both moving and fixed parts.
- Identify mechanical input.
Compare output connections for a rotating coil and a fixed coil with a permanent-magnet rotor.
Give me a hint
Identify whether the receiving terminals cross a rotating boundary.
Compare my reasoning
- Rotating coil ends need sliding contacts to a stationary load.
- Fixed coil ends can use direct stationary leads.
- The output contact requirement follows the specified design.
Rotating-coil output uses ring–brush contacts; fixed-coil output need not.
Look for these in your work
- Compare receiving terminals.
- Avoid a universal contact rule.
A rotating electromagnet uses brushes to feed its field winding while the output coil is stationary. Explain why this does not contradict fixed-coil output leads.
Give me a hint
There are two different circuit functions.
Compare my reasoning
- The rotating field winding needs an excitation connection.
- The stationary output coil is a separate receiving circuit.
- Sliding excitation contacts do not make output slip rings compulsory.
Field excitation and fixed-coil output connections serve separate circuits.
Look for these in your work
- Identify both circuits.
- State which connection crosses the rotating boundary.
Common mistakes
- Specify which part rotates.
- Excitation and receiving output are different circuits.
- Do not impose sliding contacts on every design.
What can you explain now?
What changes to induce e.m.f. when the receiving coil is fixed?
Compare with the explanation
The field linking the coil
The rotating magnetic source changes linking even without moving the receiving conductor.
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Make it stick
Decide whether contacts are needed by locating the rotating electrical terminals in the specified design.
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.