Generators and Transformers (Higher Tier)

This lesson covers AC generators, the alternating voltage they produce, and transformers (including the transformer equation). This is Higher Tier only for AQA GCSE Combined Science Trilogy (8464), specification section 6.7.2.

The AC Generator (Alternator)

An AC generator (also called an alternator) converts kinetic energy into electrical energy using electromagnetic induction.

Components

Component	Function
Rectangular coil	Rotates in the magnetic field; the conductor in which voltage is induced
Permanent magnets	Provide the external magnetic field
Slip rings	Two continuous rings that rotate with the coil; maintain connection to the external circuit
Carbon brushes	Stationary contacts that press against the slip rings; connect to the external circuit
Axle	Allows the coil to rotate

graph TD
    subgraph "AC Generator Components"
        Coil["Rotating coil"] --- Magnets["Permanent magnets (N and S)"]
        Coil --- SR["Slip rings (rotate with coil)"]
        SR --- CB["Carbon brushes (stationary)"]
        CB --- Circuit["External circuit / load"]
    end

How the AC Generator Works

The coil is rotated (by an external force — e.g. wind, water, steam) inside a magnetic field.
As the coil rotates, its sides cut through the magnetic field lines.
By electromagnetic induction, a potential difference is induced across the ends of the coil.
If the coil is connected to a complete circuit (via slip rings and brushes), a current flows.
Because the coil continuously rotates, each side alternately moves up through the field then down — so the induced current repeatedly changes direction.
This produces alternating current (AC).

When Is the Voltage Maximum and Zero?

This is one of the most important (and most commonly misunderstood) concepts:

Coil Position	Induced Voltage	Explanation
Coil parallel to the field (sides cutting through field lines at greatest rate)	Maximum	The coil sides are moving perpendicular to the field lines, cutting the maximum number of field lines per second
Coil perpendicular to the field (sides moving parallel to field lines)	Zero	The coil sides are momentarily moving parallel to the field lines, cutting zero field lines

Exam Tip: Students frequently get this wrong. Remember: voltage is maximum when the coil is parallel to the magnetic field (flat, like a pancake in the field). At this position the sides of the coil are slicing across the field lines at the fastest rate. The voltage is zero when the coil is perpendicular to the field (edge-on to the poles) because the sides are moving along the field lines, not across them.

The AC Voltage Waveform

The output of an AC generator is a sinusoidal (sine wave) voltage:

graph LR
    subgraph "AC Generator Output — One Full Rotation"
        A["0° — coil perpendicular to field — V = 0"] --> B["90° — coil parallel to field — V = max (+)"]
        B --> C["180° — coil perpendicular — V = 0"]
        C --> D["270° — coil parallel — V = max (−)"]
        D --> E["360° — coil perpendicular — V = 0 (one full cycle)"]
    end

The voltage oscillates between positive and negative peak values.
One full rotation of the coil = one complete cycle of AC.

Increasing the Output

Change	Effect on Peak Voltage	Effect on Frequency
Spin the coil faster	Increases	Increases
Use a stronger magnet	Increases	No change
Use more turns on the coil	Increases	No change
Use a larger coil	Increases	No change

Motor vs Generator — Full Comparison

Feature	DC Motor	AC Generator
Energy conversion	Electrical → kinetic	Kinetic → electrical
Input	Electrical current	Mechanical rotation
Output	Rotational motion	Alternating current (AC)
Contact mechanism	Split-ring commutator	Slip rings
Principle	Motor effect ( $F = BIl$ )	Electromagnetic induction
What rotates	Coil (driven by current)	Coil (driven by external force)

Exam Tip: The key hardware difference between a motor and a generator is the contact mechanism. A motor uses a split-ring commutator (two halves); a generator uses slip rings (two complete rings). This is a favourite AQA comparison question.

Transformers

A transformer changes the voltage of an alternating current (AC) supply. It consists of two coils of wire wound on the same iron core.

Structure

Part	Name
Input coil	Primary coil
Output coil	Secondary coil
Iron core	Links the two coils magnetically

How a Transformer Works

An alternating current flows through the primary coil.
This produces a changing magnetic field in the iron core.
The changing magnetic field passes through the secondary coil.
By electromagnetic induction, a potential difference is induced in the secondary coil.
If the secondary coil is connected to a circuit, an alternating current flows.

Key Point: Transformers only work with AC (alternating current). A constant DC current would produce a constant magnetic field — no change means no induction.

Types of Transformer

Type	Turns Relationship	Voltage Change	Use
Step-up	$n_s > n_p$	$V_s > V_p$ (voltage increases)	Power stations → National Grid
Step-down	$n_s < n_p$	$V_s < V_p$ (voltage decreases)	National Grid → homes (230 V)

The Transformer Equation

$\frac{V_s}{V_p} = \frac{n_s}{n_p}$

Where:

$V_p$ = potential difference across the primary coil (V)
$V_s$ = potential difference across the secondary coil (V)
$n_p$ = number of turns on the primary coil
$n_s$ = number of turns on the secondary coil

Exam Tip: This equation is on the AQA equation sheet. You may also see it written as $\frac{V_p}{V_s} = \frac{n_p}{n_s}$ . Both are equivalent. Make sure you can rearrange it to find any of the four quantities.

Worked Examples

Example 1: Step-Up Transformer

Q: A transformer has 200 turns on the primary coil and 1000 turns on the secondary coil. The input voltage is 12 V. Calculate the output voltage.

A: $\frac{V_s}{V_p} = \frac{n_s}{n_p}$ $\frac{V_s}{12} = \frac{1000}{200}$ $V_s = 12 \times 5 = 60 \text{ V}$

This is a step-up transformer because $V_s > V_p$ and $n_s > n_p$ .

Example 2: Step-Down Transformer

Q: A step-down transformer reduces 400 V to 20 V. The primary coil has 4000 turns. How many turns are on the secondary coil?

A: $\frac{V_s}{V_p} = \frac{n_s}{n_p}$ $\frac{20}{400} = \frac{n_s}{4000}$ $n_s = \frac{20 \times 4000}{400} = 200 \text{ turns}$

Example 3: Power Conservation (Ideal Transformer)

For an ideal (100% efficient) transformer, input power = output power:

$V_p \times I_p = V_s \times I_s$

Q: A transformer steps up 25 V to 250 V. The current in the primary coil is 10 A. Calculate the current in the secondary coil (assume 100% efficiency).

A: $V_p I_p = V_s I_s$ $25 \times 10 = 250 \times I_s$ $I_s = \frac{250}{250} = 1.0 \text{ A}$

Note: when voltage increases, current decreases (and vice versa) in an ideal transformer. Power is conserved.

Transformers in the National Grid

The National Grid transmits electricity from power stations to homes and businesses across the country.

graph LR
    subgraph "National Grid"
        PS["Power Station (25 kV)"] --> SU["Step-Up Transformer"]
        SU -->|"275–400 kV, low current"| TL["Transmission Lines"]
        TL --> SD["Step-Down Transformer"]
        SD -->|"230 V for homes"| H["Homes and Businesses"]
    end

Why Use High Voltage for Transmission?

Power loss in cables = $P = I^2R$ (power dissipated as heat).
Stepping up the voltage means the current decreases (for the same power).
Lower current means less energy wasted as heat in the cables.
This makes transmission much more efficient.

Stage	Voltage	Current	Purpose
Power station output	~25 kV	High	Generated by the alternator
After step-up transformer	275–400 kV	Low	Efficient long-distance transmission
After step-down transformer	230 V	As needed	Safe for domestic use

Common Mistakes

Mistake	Correction
"Transformers work with DC"	Transformers only work with AC
"Step-up transformers increase power"	They increase voltage but decrease current — power is conserved (ideal)
"Voltage is max when coil is perpendicular to the field"	Voltage is max when coil is parallel to the field
Confusing slip rings with split-ring commutator	Slip rings = generator (AC); split-ring commutator = motor (DC)
"High voltage transmission is dangerous so we use low voltage"	High voltage is used for efficiency; step-down transformers reduce it for safety at homes

Summary

An AC generator rotates a coil in a magnetic field to induce an alternating voltage.
The voltage is maximum when the coil is parallel to the field and zero when perpendicular.
Generators use slip rings (not a commutator).
A transformer changes AC voltage using two coils on an iron core.
$\frac{V_s}{V_p} = \frac{n_s}{n_p}$ — step-up ( $n_s > n_p$ ) or step-down ( $n_s < n_p$ ).
For an ideal transformer: $V_p I_p = V_s I_s$ .
The National Grid uses step-up transformers for efficient transmission and step-down transformers for safe domestic supply.

Exam Tip (AQA 8464 — Higher): Generator and transformer questions are Higher Tier favourites. Always be clear about when voltage is maximum/zero in a generator, explain why transformers need AC, and be able to use and rearrange both the transformer equation and the power equation. Remember that the motor uses a commutator while the generator uses slip rings.

Generators and Transformers (Higher Tier)

Generators and Transformers (Higher Tier)

The AC Generator (Alternator)

Components

How the AC Generator Works

When Is the Voltage Maximum and Zero?

The AC Voltage Waveform

Increasing the Output

Motor vs Generator — Full Comparison

Transformers

Structure

How a Transformer Works

Types of Transformer

The Transformer Equation

Worked Examples

Example 1: Step-Up Transformer

Example 2: Step-Down Transformer

Example 3: Power Conservation (Ideal Transformer)

Transformers in the National Grid

Why Use High Voltage for Transmission?

Common Mistakes

Summary

Important Specification Note

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