Induced Potential and the Generator Effect [H]

The generator effect is the reverse of the motor effect — instead of a current causing motion, motion causes a current. In this lesson you will learn about electromagnetic induction, Faraday's law, Lenz's law at GCSE level, and how generators and dynamos work. This is Higher tier content from AQA GCSE Physics specification 4.7.3.

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What Is the Generator Effect?

The generator effect (also called electromagnetic induction) occurs when an electrical conductor moves through a magnetic field, or when a magnetic field changes around a conductor. A potential difference (voltage) is induced across the ends of the conductor, and if the conductor is part of a complete circuit, a current is induced.

Key principle: A changing magnetic field induces a potential difference in a conductor.

Exam Tip: The key word is "changing." If the magnetic field is constant and nothing is moving, no potential difference is induced. There must be relative motion between the conductor and the magnetic field, or the field must be changing in strength.

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Ways to Induce a Potential Difference

There are several ways to induce a potential difference:

Method	Description
Moving a wire through a magnetic field	The wire cuts through field lines, inducing a p.d.
Moving a magnet into or out of a coil	The changing field through the coil induces a p.d.
Changing the magnetic field around a conductor	E.g., switching an electromagnet on/off near a coil
Rotating a coil in a magnetic field	The coil continuously cuts field lines (this is how a generator works)

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Factors Affecting the Induced Potential Difference

The size of the induced potential difference can be increased by:

Factor	Effect
Increasing the speed of movement	More field lines cut per second = greater p.d.
Using a stronger magnet	Greater magnetic flux density = more field lines to cut
Increasing the number of turns on the coil	Each turn contributes to the total p.d.
Increasing the area of the coil	More field lines pass through the coil

Exam Tip: If asked how to increase the induced p.d., give specific answers: "move the magnet faster," "use a stronger magnet," or "use a coil with more turns." These are the three most common correct answers.

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Direction of the Induced Current

The direction of the induced current depends on:

• The direction of motion of the wire or magnet.
• The direction of the magnetic field.

If you reverse the direction of motion, the induced current reverses. If you reverse the magnetic field, the induced current also reverses.

Fleming's Right-Hand Rule [H]

To find the direction of the induced current, use Fleming's right-hand rule:

Hold your right hand with the thumb, first finger and second finger at right angles:

Finger	Represents
Thumb	Direction of Motion (of the conductor)
First finger	Direction of the Field (N to S)
Second finger	Direction of the induced Current (conventional)

graph TD
    subgraph "Fleming’s Right-Hand Rule"
        T["THUMB = Motion of conductor"]
        F["FIRST FINGER = Field direction (N to S)"]
        C["SECOND FINGER = Induced current direction"]
        T --- F
        F --- C
    end

Exam Tip: LEFT hand = MOTOR effect (current causes motion). RIGHT hand = GENERATOR effect (motion causes current). Make sure you use the correct hand for each situation.

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The Magnet and Coil Experiment

A classic demonstration of electromagnetic induction:

1. Connect a coil of wire to a sensitive galvanometer (a meter that detects small currents and shows direction).
2. Push a bar magnet into the coil — the galvanometer needle deflects, showing an induced current.
3. Hold the magnet stationary inside the coil — the needle returns to zero (no changing field, no induced p.d.).
4. Pull the magnet out of the coil — the needle deflects in the opposite direction.
5. Push the magnet in faster — the deflection is larger (greater induced p.d.).
6. Use a stronger magnet — the deflection is larger.
7. Use a coil with more turns — the deflection is larger.

Action	Galvanometer Reading
Magnet pushed IN slowly	Small deflection
Magnet pushed IN quickly	Large deflection
Magnet held still inside coil	Zero (no deflection)
Magnet pulled OUT slowly	Small deflection (opposite direction)
Magnet pulled OUT quickly	Large deflection (opposite direction)
Reversed magnet pushed IN	Deflection in opposite direction to original

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Lenz's Law (Qualitative Understanding)

While Lenz's law is not explicitly named on the AQA GCSE specification, you should understand the principle:

The induced current always flows in a direction that opposes the change that caused it.

For example:

• When a north pole is pushed into a coil, the induced current flows in a direction that makes the coil act as a north pole facing the magnet — this repels the magnet, opposing its motion.
• When the north pole is pulled out, the induced current makes the coil act as a south pole — this attracts the magnet, opposing its withdrawal.

This is a consequence of the conservation of energy — the induced current cannot aid the motion, because that would create energy from nothing.

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The AC Generator (Alternator)

An AC generator (alternator) converts kinetic energy into electrical energy by rotating a coil in a magnetic field.

Components

Component	Role
Rectangular coil	Rotates in the magnetic field; p.d. is induced in it
Permanent magnets	Provide the external magnetic field
Slip rings	Two complete rings connected to the ends of the coil; rotate with it
Carbon brushes	Press against the slip rings; connect the rotating coil to the external circuit

graph TD
    subgraph "AC Generator (Alternator)"
        N["N pole"] -.->|"Magnetic field"| S["S pole"]
        COIL["Rotating coil (between magnets)"] --> SR1["Slip Ring 1"]
        COIL --> SR2["Slip Ring 2"]
        SR1 --> B1["Brush 1"]
        SR2 --> B2["Brush 2"]
        B1 --> LOAD["External Circuit / Load"]
        B2 --> LOAD
    end

How It Works

1. The coil rotates in the magnetic field.
2. As it rotates, the sides of the coil cut through the magnetic field lines.
3. This induces a potential difference across the ends of the coil.
4. The slip rings rotate with the coil, and the brushes maintain contact, allowing the induced p.d. to be transferred to the external circuit.
5. Because the coil rotates continuously, the direction of the induced p.d. reverses every half turn — producing alternating current (AC).

The Output of an AC Generator