15.6 The Alternating-Current Generator

A free GCSE Physics lesson for AQA (8463) from OBA School: higher tier, physics only. Alternators and dynamos, graphs of induced p.d. against time, microphones and loudspeakers. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• (Higher tier) describe how a simple alternator is made and explain why it produces an alternating p.d.
• (Higher tier) explain where in each turn the induced p.d. is at its peak and where it is zero
• (Higher tier) draw and interpret graphs of p.d. against time for an alternator and a dynamo, and find frequency from them
• (Higher tier) explain how a moving-coil microphone uses the generator effect
• (Higher tier) explain how a moving-coil loudspeaker or headphone uses the motor effect

Key words

• Alternator: An alternating-current (a.c.) generator: a coil turned in a magnetic field, connected through slip rings.
• Dynamo: A direct-current (d.c.) generator: like an alternator but with a split-ring commutator instead of slip rings.
• Slip rings: Two separate rings on the axle, each joined to one end of the coil, so the coil stays connected to the same brush all the time.
• Peak p.d.: The biggest value of the induced p.d., positive or negative.
• Frequency: The number of cycles each second, in hertz (Hz). f = 1 ÷ T where T is the time for one cycle.

The alternator

A simple alternator is a rectangular coil turned steadily in a uniform magnetic field. As the coil turns, its sides cut through the field lines, so a p.d. is induced. Each end of the coil is joined to its own slip ring; carbon brushes press on the rings and connect the spinning coil to the outside circuit.

Each side of the coil moves up for half a turn and then down for half a turn. So the induced p.d. reverses every half-turn: the output is an alternating p.d. and the current through the meter changes direction twice in every turn.

The output graph

• Peak p.d. when the plane of the coil is parallel to the field: the sides move straight across the field lines, cutting them fastest.
• Zero p.d. when the plane of the coil is perpendicular to the field: the sides move along the field lines for an instant and cut none.
• One cycle of the graph = one full turn of the coil, so the frequency of the a.c. equals the number of turns per second.

• turn the coil faster; Peak p.d. bigger; Frequency higher
• stronger magnet; Peak p.d. bigger; Frequency no change
• more turns on the coil / bigger coil area; Peak p.d. bigger; Frequency no change

Worked example: frequency of a generator

A coil turns 1200 times per minute. Find the frequency and the period of the a.c.
turns per second = 1200 ÷ 60 = 20, so f = 20 Hz
T = 1 ÷ f = 1 ÷ 20 = 0.050 s

Answer: 20 Hz; period 0.050 s (50 ms)

The dynamo

A dynamo is built like the alternator, but the ends of the coil are joined to a split-ring commutator instead of two slip rings. Every half-turn, just as the p.d. in the coil is about to reverse, the commutator swaps the connections to the outside circuit. So the output p.d. never changes direction: it rises from zero to a peak and back to zero twice in every turn.

Key fact: Slip rings → a.c. output (alternator). Split-ring commutator → d.c. output (dynamo).

Microphones and loudspeakers

A moving-coil microphone and a moving-coil loudspeaker have the same design: a light coil attached to a diaphragm or cone, sitting in the narrow gap of a strong cylindrical magnet.

• Sound waves (pressure variations) make the diaphragm vibrate.; Moving-coil loudspeaker (motor effect) An alternating current from an amplifier flows in the coil.
• The coil vibrates in the magnetic field, cutting field lines.; Moving-coil loudspeaker (motor effect) The current in the field feels a force (motor effect).
• An alternating p.d. is induced in the coil.; Moving-coil loudspeaker (motor effect) The current keeps reversing, so the force keeps reversing.
• The p.d. varies in the same way as the sound, with the same frequency: sound is turned into an electrical signal.; Moving-coil loudspeaker (motor effect) The coil and cone vibrate at the frequency of the current, pushing the air and making sound waves.

Exam tip: Microphone: sound in → electrical signal out (generator effect). Loudspeaker: electrical signal in → sound out (motor effect).

Common mistakes

• Saying the p.d. is biggest when the coil is perpendicular to the field. It is zero there; it is biggest when the coil is parallel to the field.
• Saying a stronger magnet raises the frequency. Only the speed of rotation changes the frequency.
• Saying a microphone uses the motor effect. A microphone generates a p.d. (generator effect); a loudspeaker uses the motor effect.
• Forgetting to change ms to s, or turns per minute to turns per second.

Revision questions

Try these, then check your answers below each one.

Question 1. (Higher tier) Which part does an alternator (a.c. generator) use to connect its coil to the outside circuit? (Choose from: Two slip rings and brushes, A split-ring commutator and brushes, A diode, A transformer.)

Answer: Two slip rings and brushes

Working: An alternator uses slip rings, so each end of the coil stays connected to the same brush and the output alternates.

Question 2. (Higher tier) At what position is the p.d. induced in an alternator coil at its peak? (Choose from: When the plane of the coil is parallel to the magnetic field, When the plane of the coil is perpendicular to the magnetic field, When the coil is not moving, At every position equally.)

Answer: When the plane of the coil is parallel to the magnetic field

Working: When the coil is parallel to the field, its sides move at right angles to the field lines and cut them fastest, giving the peak p.d.

Question 3. (Higher tier) An alternator coil turns 3600 times per minute. What is the frequency of the alternating p.d.?

Answer: 60 Hz

Working: 3600 ÷ 60 = 60 turns per second, so f = 60 Hz.

Related lessons

• Previous lesson: 15.5 The Generator Effect
• Next lesson: 15.7 Transformers
• In this chapter: 15.1 Magnetic Fields
• In this chapter: 15.2 Magnetic Fields of Electric Currents
• In this chapter: 15.3 Electromagnets in Devices

Continue in the full course

This lesson is section 15.6 (AGCPH1506) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.