The Generator Effect
When an electrical conductor moves relative to a magnetic field, or when a magnetic field changes around a conductor, a potential difference (voltage) is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current will flow. This is known as the **Generator Effect**.
Fleming's Rules: Left-Hand vs. Right-Hand
Students frequently confuse Fleming's Left and Right Hand rules. Use this checklist to differentiate them instantly in your exams:
Fleming's LEFT-Hand Rule (Motors)
Used for the **Motor Effect** (when current and magnetic field exist, and you calculate the resulting **force/motion**).
• Thumb = Force (Motion)
• First Finger = Field (North to South)
• Second Finger = Current (Positive to Negative)
Fleming's RIGHT-Hand Rule (Generators)
Used for the **Generator Effect** (when motion and magnetic field exist, and you calculate the resulting induced **current**).
• Thumb = Motion (applied force)
• First Finger = Field (North to South)
• Second Finger = Induced Current
Alternators vs. Dynamos
Both devices turn mechanical rotation into electrical energy, but their outputs are different:
- Alternators (AC output): Use **slip rings** and brushes. As the coil rotates, the contacts do not swap sides, meaning the direction of the induced current reverses every half-turn, producing Alternating Current (AC).
- Dynamos (DC output): Use a **split-ring commutator**. This reverses the connection to the external circuit every half-turn, keeping the current flowing in one direction only, producing Direct Current (DC).
Worked Exam Example
Worked GCSE Example
Question: Describe how the output of a dynamo differs from an alternator, and explain the role of the commutator in achieving this difference.
Solution:
- The output of a dynamo is **direct current (DC)** (flowing in one direction), whereas an alternator produces **alternating current (AC)** (changing direction periodically).
- A dynamo achieves direct current using a **split-ring commutator**.
- As the coil rotates past $180^\circ$, the two halves of the split-ring commutator swap contact with the brushes. This reverses the connections to the external circuit at the exact moment the induced current direction in the coil reverses, ensuring that current in the external circuit always flows in the same direction.
Frequently Asked Questions
Lenz's Law states that an induced current always flows in a direction such that its magnetic field **opposes** the change that created it. For instance, if you push a North pole of a magnet into a coil, the coil will induce a North pole at its entrance to repel the magnet.
Transformers pass alternating current through a primary coil, which creates a continuously changing magnetic field in an iron core. This changing magnetic field cuts through a secondary coil, inducing an alternating potential difference across it.
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