OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Electromagnetism: (e) simple a.c. generator
A simple a.c. generator is a coil rotating at constant angular speed in a uniform magnetic field.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 6 of 8
Study notes
From the checked teaching notes for this specification point.
The idea
A simple a.c. generator is a coil rotating at constant angular speed in a uniform magnetic field. As the coil turns, the angle between the coil plane and the field changes, so the flux linkage NΦ = NBA cos θ varies sinusoidally. By Faraday's law the induced e.m.f. equals the negative rate of change of flux linkage, giving a sinusoidal output that reverses each half turn. Slip rings and brushes connect the rotating coil to the external circuit without reversing the connections, so the current alternates. Increasing N, B, A or the rotation frequency increases the peak e.m.f.; the output frequency equals the rotation frequency.
What earns the marks
- A simple a.c. generator consists of a coil rotating in a uniform magnetic field.
- The flux linkage varies as NΦ = NBA cos θ, where θ is the angle between the field and the normal to the coil.
- The induced e.m.f. is the negative rate of change of flux linkage, so the output is sinusoidal and reverses direction.
- Slip rings and brushes transfer the alternating current to the external circuit.
- Peak e.m.f. increases with N, B, A and angular speed; output frequency equals rotation frequency.
Common mistakes and how to put them right
Believing the e.m.f. is largest when the coil is perpendicular to the field.
Put it right: In fact the rate of change of flux linkage is then zero, so the e.m.f. is zero.
Confusing slip rings with a split-ring commutator.
Put it right: A split-ring commutator would give a d.c. output, not a.c.
Thinking the output frequency is twice the rotation frequency.
Put it right: It equals the rotation frequency.
Ignoring the negative sign in Faraday's law, which indicates the direction of the induced e.m.f. opposes the change.
Exam tips
- Sketch the sinusoidal e.m.f. against time and mark where the coil plane is parallel and perpendicular to the field.
- Use NΦ = NBA cos θ and differentiate to show the sinusoidal form when needed.
- State clearly that slip rings allow the current to alternate rather than being rectified.
Try the lesson
The start of the Electromagnetism: (e) simple a.c. generator lesson, made from checked teaching notes for this specification point.
Listen
A simple a.c. generator is a coil rotating at constant angular speed in a uniform magnetic field. As the coil turns, the angle between the coil plane and the field changes, so the flux linkage NΦ = NBA cos θ varies sinusoidally. By Faraday's law the induced e.m.f. equals the negative rate of change of flux linkage, giving a sinusoidal output that reverses each half turn. Slip rings and brushes connect the rotating coil to the external circuit without reversing the connections, so the current alternates. Increasing N, B, A or the rotation frequency increases the peak e.m.f.; the output frequency equals the rotation frequency. A strong answer earns marks for these points. A simple a.c. generator consists of a coil rotating in a uniform magnetic field. The flux linkage varies as NΦ = NBA cos θ, where θ is the angle between the field and the normal to the coil. The induced e.m.f. is the negative rate of change of flux linkage, so the output is sinusoidal and reverses direction. Slip rings and brushes transfer the alternating current to the external circuit. Exam tip: Sketch the sinusoidal e.m.f. against time and mark where the coil plane is parallel and perpendicular to the field.
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