Transformers [H]

Transformers are devices that change the potential difference (voltage) of an alternating current supply. They are essential for the efficient transmission of electricity through the National Grid. This is Higher tier content from AQA GCSE Physics specification 4.7.3.

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What Is a Transformer?

A transformer is a device that changes the potential difference (voltage) of an AC supply. It consists of:

• A primary coil — the input coil connected to the AC supply.
• A secondary coil — the output coil connected to the device or load.
• An iron core — links the two coils magnetically.

The primary and secondary coils are not electrically connected — they are linked only through the magnetic field in the iron core.

graph LR
    subgraph "Transformer"
        AC["AC Supply"] --> PC["Primary Coil (n_p turns)"]
        PC -->|"Changing magnetic field in iron core"| CORE["Soft Iron Core"]
        CORE -->|"Induces p.d. in secondary"| SC["Secondary Coil (n_s turns)"]
        SC --> LOAD["Load / Output"]
    end

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How a Transformer Works

Step-by-Step Explanation

1. An alternating current flows through the primary coil.
2. This creates an alternating magnetic field in the iron core (because the current is constantly changing direction).
3. The iron core carries (channels) this alternating magnetic field to the secondary coil.
4. The alternating magnetic field passing through the secondary coil is a changing magnetic field.
5. By the generator effect, this changing magnetic field induces an alternating potential difference across the secondary coil.
6. If the secondary coil is connected to a complete circuit, an alternating current flows.

Step	Physics
AC in primary coil	Creates alternating magnetic field
Iron core	Channels the magnetic field to the secondary coil
Changing field in secondary coil	Generator effect induces an alternating p.d.
Secondary coil connected to circuit	AC flows in the secondary circuit

Exam Tip: Transformers only work with AC, NOT DC. This is because a constant (DC) current would produce a constant magnetic field, and a constant field does not induce a p.d. The field must be CHANGING for electromagnetic induction to occur.

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Why Must the Supply Be AC?

Supply Type	Magnetic Field in Core	Induced p.d. in Secondary
AC	Alternating (constantly changing)	Yes — alternating p.d. induced
DC	Constant (not changing)	No — no changing field, no induced p.d.

A DC supply would only induce a momentary p.d. when it is first switched on or off (because only at those moments does the field change). For continuous operation, AC is essential.

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Step-Up and Step-Down Transformers

Step-Up Transformer

A step-up transformer increases the voltage. It has more turns on the secondary coil than on the primary coil.

Feature	Value
Secondary turns	More than primary turns
Output voltage	Higher than input voltage
Output current	Lower than input current (to conserve power)

Step-Down Transformer

A step-down transformer decreases the voltage. It has fewer turns on the secondary coil than on the primary coil.

Feature	Value
Secondary turns	Fewer than primary turns
Output voltage	Lower than input voltage
Output current	Higher than input current (to conserve power)

graph TD
    subgraph "Step-Up Transformer"
        A["Primary: fewer turns"] -->|"V increases"| B["Secondary: more turns"]
    end
    subgraph "Step-Down Transformer"
        C["Primary: more turns"] -->|"V decreases"| D["Secondary: fewer turns"]
    end

Exam Tip: "Step-up" means voltage goes UP, so the secondary needs MORE turns. "Step-down" means voltage goes DOWN, so the secondary needs FEWER turns. The number of turns determines the voltage ratio.

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The Transformer Equation

The relationship between the voltages and the number of turns is:

V_p / V_s = n_p / n_s

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

Worked Example 1

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.

V_p / V_s = n_p / n_s

12 / V_s = 200 / 1000

12 / V_s = 0.2

V_s = 12 / 0.2 = 60 V

The output voltage is 60 V. This is a step-up transformer (voltage increased from 12 V to 60 V).

Worked Example 2

A step-down transformer has an input voltage of 230 V and an output voltage of 11.5 V. The primary coil has 4600 turns. How many turns are on the secondary coil?

V_p / V_s = n_p / n_s

230 / 11.5 = 4600 / n_s

20 = 4600 / n_s

n_s = 4600 / 20 = 230 turns

The secondary coil has 230 turns.

Exam Tip: Always check your answer makes sense. If it is a step-up transformer, V_s should be larger than V_p and n_s should be larger than n_p. If it is a step-down transformer, the opposite should be true.

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Power in Transformers

If a transformer is 100% efficient (an ideal transformer), the power input equals the power output:

V_p x I_p = V_s x I_s

Where:

• V_p = primary voltage (V)
• I_p = primary current (A)
• V_s = secondary voltage (V)
• I_s = secondary current (A)

This means:

• If a step-up transformer increases the voltage, the current must decrease (so that power is conserved).
• If a step-down transformer decreases the voltage, the current must increase.

Worked Example 3

An ideal transformer has an input of 230 V and 2.0 A. The output voltage is 11.5 V. Calculate the output current.

V_p x I_p = V_s x I_s

230 x 2.0 = 11.5 x I_s

460 = 11.5 x I_s

I_s = 460 / 11.5 = 40 A

The output current is 40 A.

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Transformers and the National Grid

Transformers are essential for the efficient transmission of electricity through the National Grid.

The Problem

Power stations generate electricity at relatively low voltages (e.g., 25,000 V). Transmitting this over long distances through power lines would result in significant energy loss as heat in the cables (due to the resistance of the wires).

The Solution

Stage	Transformer Used	Voltage	Purpose
Power station	—	~25,000 V	Electricity generated
Step-up transformer	Step-up	~400,000 V	Increases voltage for transmission
Transmission lines	—	~400,000 V	High voltage, low current reduces energy loss
Step-down transformer	Step-down	~230 V	Reduces voltage to safe level for homes

Why High Voltage Reduces Energy Loss