Theme D — Fields
D.4.5 AC Generators — IB Physics HL Revision Notes & Topical Questions
5 min read 9 parts Lesson 39 of 39 1 quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
D.4.5 AC Generators — IB PHYSICS HL notes: Explain how a rotating coil produces an alternating e.m.f.
- Conversion
- Basic action
- Alternator
- Slip rings
- Centre-reading meter
- Cutting field lines
Part 1 of 7Principle of the a.c. generator
Conversion
- Conversion a generator converts mechanical energy into electrical energy as alternating current
Basic action
- Basic action a coil rotated in a magnetic field by an external force has an e.m.f. induced in it
Alternator
- Alternator a rectangular coil spinning in a uniform field
Part 2 of 7Construction
Coil
- Coil a rectangular coil rotates in a uniform field
Slip rings
- Slip rings two metal rings fixed to the coil
Brushes
- Brushes metal brushes press on the slip rings and give a continuous connection to the meter or circuit
Centre-reading meter
- Centre-reading meter its pointer deflects both ways, showing the current alternates
Part 3 of 7Why the current alternates
Cutting field lines
- Cutting field lines as the coil rotates, opposite sides cut the field lines and e.m.f. is induced in each
Direction
- Direction the induced potential difference repeatedly changes direction, so the current alternates
Continued rotation
- Continued rotation this continues as long as the coil turns in the same direction
Part 4 of 7Flux linkage and e.m.f. of a rotating coil
1Flux linkage
- NΦ = BAN cos θ with θ = ωt, so NΦ = BAN cos ωt
2Angular speed
- ω = Δθ/Δt in rad s−1
3Induced e.m.f.
- ε = BANω sin ωt = ε0 sin ωt
4Maximum e.m.f.
- ε0 = BANω
5Phase
- Phase the e.m.f. varies sinusoidally and is 90° out of phase with the flux linkage
Part 5 of 7Maximum and zero positions
| Coil position | Flux linkage | E.m.f. |
|---|---|---|
| Plane perpendicular to the field | Maximum | Zero |
| Plane parallel to the field | Zero | Maximum |
- Reason
- the e.m.f. is the rate of change of flux linkage
Part 6 of 7Effect of rotation speed
Peak e.m.f.
- Peak e.m.f. it is directly proportional to ω
Frequency
- Frequency f = ω/2π so it is proportional to ω
Doubling ω
- Doubling ω doubles both the amplitude and the frequency, halving the period
Ways to increase ε0
- Ways to increase ε0 a stronger field, more turns, a larger area or faster rotation
Part 7 of 7More from the specification
1Faraday's Law of Electromagnetic Induction
- Faraday's law states that the induced electromotive force (voltage) in a conductor coil is directly proportional to the rate of change of magnetic flux linkage, expressed by the equation ε = -N(ΔΦ/Δt).
2AC Generator Principles and Flux Angle
- In an alternating current generator, a rotating coil cuts through magnetic field lines of flux density B;
- as the coil rotates through angle θ = ωt, it induces an alternating sinusoidal voltage detectable with an AC voltmeter, described by ε = BANω sin(ωt).
3Transformer Voltage Relationship
- An alternating voltage applied to the primary coil of a transformer creates a changing magnetic flux in the core that induces an alternating voltage in the secondary coil according to the ideal equation Vs / Vp = Ns / Np.
4Faraday's Law of Electromagnetic Induction
- Faraday's law states that the induced electromotive force (voltage) in a conductor coil is directly proportional to the rate of change of magnetic flux linkage, expressed by the equation ε = -N(ΔΦ/Δt).
5AC Generator Principles and Flux Angle
- In an alternating current generator, a rotating coil cuts through magnetic field lines of flux density B;
- as the coil rotates through angle θ = ωt, it induces an alternating sinusoidal voltage detectable with an AC voltmeter, described by ε = BANω sin(ωt).
6Transformer Voltage Relationship
- An alternating voltage applied to the primary coil of a transformer creates a changing magnetic flux in the core that induces an alternating voltage in the secondary coil according to the ideal equation Vs / Vp = Ns / Np.
7Constant speed
- Speed that does not change.
8Electromagnetic induction
- Production of an e.m.f. by relative movement of a conductor and a magnetic field.
9Induction
- Charging an object without touching it, giving it the opposite sign to the charged object.
10Conductor
- Material that allows current to flow.
11Density
- Mass per unit volume.
12Voltmeter
- Instrument that measures p.d., connected in parallel.
13Primary
- Input coil.
14Secondary
- Output coil.
15Mutual induction
- Induction of an e.m.f. in one circuit by a change of current in a neighbouring circuit.
16Closed circuit
- Complete loop with a source in which current can flow
Key terms
| Term | Meaning |
|---|---|
| Alternator | A rotating coil in a magnetic field connected through slip rings that produces alternating current. |
| Slip rings | Metal rings fixed to the coil that keep a continuous connection with the external circuit via brushes. |
| Peak e.m.f. | ε0 = BANω, the maximum induced e.m.f. |
| Angular speed | The rate of change of angular displacement, ω = 2πf. |
| Alternating current | Current that repeatedly changes direction. |
Avoid these mistakes
1
Confusing when the e.m.f. and the flux linkage are maximum — they are 90° out of phase.
2
Using degrees in sin ωt with the calculator in radians mode.
3
Saying doubling the speed doubles only the frequency — it doubles the amplitude too.
4
Forgetting the number of turns N in the peak e.m.f.
5
Believing slip rings reverse the current — the current alternates because the induced e.m.f. changes direction.
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