7.3.2 Uses of the generator effect (HT)
The generator effect
Generator effect
The generator effect is the inducing of a potential difference when a conductor moves relative to a magnetic field.
- A coil rotating in a magnetic field feels the generator effect because its wires cut magnetic field lines.
- The induced potential difference causes a current only when the coil is part of a complete circuit.
- As the coil rotates, the rate and direction in which it cuts field lines change, so the size and polarity of the induced potential difference change.
Alternators generate alternating current
Alternator
An alternator is a generator that uses the generator effect to produce an alternating potential difference and, in a complete circuit, an alternating current.
Alternating potential difference
An alternating potential difference repeatedly changes polarity, producing an alternating current (ac) that repeatedly changes direction.
- A coil rotates between the poles of a magnet.
- Each end of the coil is connected to a separate slip ring.
- Stationary brushes press against the slip rings and connect the coil to the external circuit.
- During one half-turn the coil sides move through the field one way, giving one polarity; during the next half-turn they move the opposite way, so the potential difference reverses.
- The slip rings keep a continuous connection without swapping it, so the output reverses polarity every half-turn, giving ac.
Dynamos generate direct current
Dynamo
A dynamo is a generator that uses the generator effect and a split-ring commutator to produce a direct potential difference and, in a complete circuit, a direct current.
Direct potential difference
A direct potential difference has only one polarity, producing a direct current (dc) that flows in only one direction.
- A dynamo also has a rotating coil in a magnetic field, but uses a split-ring commutator instead of two slip rings.
- The induced potential difference in the coil still reverses every half-turn.
- At the same time the commutator swaps the connections to the external circuit, keeping the polarity the same across it.
- The output is therefore direct, although its size changes as the coil rotates.
Potential difference against time graphs
- Both graphs plot time on the horizontal axis and potential difference on the vertical axis.
- An alternator graph rises above and below zero, with positive and negative sections, showing the polarity reverses (an alternating output).
- A dynamo graph stays on one side of zero and falls to zero between peaks without reversing, showing a direct output whose size changes.
- The curved shape occurs because the rate of cutting field lines changes: the potential difference is zero when the coil is not cutting lines and greatest when it cuts them fastest.
Question: An output graph rises from 0 V0\ \text{V}0 V to +6 V+6\ \text{V}+6 V, returns to 0 V0\ \text{V}0 V, falls to −6 V-6\ \text{V}−6 V and repeats. Identify the generator and explain the shape.
Answer: It is an alternator, because the potential difference has positive and negative values, so its polarity reverses. As the coil rotates, the direction in which its sides cut the field reverses every half-turn, reversing the induced potential difference, and the slip rings connect the coil to the circuit without swapping the connections, giving an alternating output.
If a split-ring commutator replaced the slip rings, the negative sections would move to the positive side, giving a direct output because the commutator swaps the connections every half-turn.
- The generator effect induces a potential difference; a current flows only if the circuit is complete.
- Do not confuse slip rings (which give ac) with a split-ring commutator (which gives dc).
- A direct output need not be constant: a dynamo's potential difference changes in size but keeps the same polarity.
- Link the component to the output: a rotating coil feels the generator effect, the induced potential difference reverses every half-turn, then say what the rings do.
- Remember slip rings produce ac and a split-ring commutator produces dc, and for a graph check whether it crosses zero and changes polarity.
- What does the generator effect induce, and when does a current flow?
- Which component does an alternator use, and what output does it give?
- Which component does a dynamo use, and what output does it give?
- Why does an alternator graph have positive and negative sections?
- Why does a dynamo graph stay on one side of zero?
- Why is the induced potential difference zero at some points during a rotation?
