Transformers

This lesson covers transformers — devices that change the voltage of an AC supply — as required by the Edexcel GCSE Physics specification (1PH0), Topic 8: Magnetism and Electromagnetism. You need to understand the structure and types of transformers, the transformer equation, power conservation (Higher tier), and the role of transformers in the National Grid.

---

What Is a Transformer?

A transformer is a device that changes the voltage of an alternating current (AC) supply. Transformers only work with AC — they do not work with direct current (DC).

Structure of a Transformer

A transformer consists of three main parts:

Component	Description
Primary coil	The input coil — connected to the AC supply
Secondary coil	The output coil — connected to the device being powered
Iron core	A soft iron core that links the two coils; it transfers the changing magnetic field from the primary to the secondary coil

graph LR
    A["AC Supply"] --> B["Primary Coil<br/>(Np turns)"]
    B --> C["Soft Iron Core<br/>(transfers magnetic field)"]
    C --> D["Secondary Coil<br/>(Ns turns)"]
    D --> E["Output Voltage"]

    style A fill:#e74c3c,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#95a5a6,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#8e44ad,color:#fff

How a Transformer Works

1. An alternating current flows through the primary coil.
2. This produces a changing magnetic field in the iron core (the field alternates as the current alternates).
3. The iron core carries this changing magnetic field to the secondary coil.
4. The changing magnetic field induces an alternating voltage across the secondary coil (by electromagnetic induction).
5. If the secondary coil is part of a complete circuit, an alternating current flows.

Why Transformers Only Work with AC

• Transformers rely on a changing magnetic field to induce a voltage in the secondary coil.
• AC produces a continuously changing current (and therefore a changing magnetic field).
• DC produces a constant current and a constant magnetic field — there is no change, so no voltage is induced in the secondary coil.

Exam Tip: "Why do transformers not work with DC?" is a very common exam question. The answer is: DC produces a constant magnetic field in the iron core, and electromagnetic induction requires a changing magnetic field to induce a voltage. No change = no induction = no output voltage.

---

Types of Transformer

Step-Up Transformer

A step-up transformer increases the voltage:

• The secondary coil has more turns than the primary coil.
• Output voltage > Input voltage.
• The current in the secondary coil is less than in the primary coil (to conserve power).

Step-Down Transformer

A step-down transformer decreases the voltage:

• The secondary coil has fewer turns than the primary coil.
• Output voltage < Input voltage.
• The current in the secondary coil is greater than in the primary coil (to conserve power).

Comparison

Feature	Step-Up	Step-Down
Turns on secondary vs primary	Ns > Np	Ns < Np
Output voltage vs input	Higher	Lower
Output current vs input	Lower	Higher
Use	Power stations → National Grid	Substations → homes

---

The Transformer Equation

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

Vp / Vs = Np / Ns

Where:

• Vp = voltage across the primary coil (in volts, V)
• Vs = voltage across the secondary coil (in volts, V)
• Np = number of turns on the primary coil
• Ns = 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 50 V. Calculate the output voltage.

Vp / Vs = Np / Ns

50 / Vs = 200 / 1000

50 / Vs = 0.2

Vs = 50 / 0.2

Vs = 250 V

This is a step-up transformer (Ns > Np, Vs > Vp).

Worked Example 2

A step-down transformer converts 230 V to 12 V. The secondary coil has 60 turns. How many turns does the primary coil have?

Vp / Vs = Np / Ns

230 / 12 = Np / 60

19.17 = Np / 60

Np = 19.17 × 60

Np = 1150 turns

Worked Example 3

A transformer has 400 turns on the primary and 100 turns on the secondary. If the input voltage is 240 V, what is the output voltage?

Vp / Vs = Np / Ns

240 / Vs = 400 / 100

240 / Vs = 4

Vs = 240 / 4

Vs = 60 V

Exam Tip: When using the transformer equation, check whether you are told it is step-up or step-down. If the output voltage should be higher, Ns must be greater than Np (and vice versa). This is a quick way to check your answer makes sense.

---

Power in Transformers (Higher Tier)

For a 100% efficient transformer, the power input to the primary coil equals the power output from the secondary coil:

Vp × Ip = Vs × Is

Where:

• Vp = primary voltage (V)
• Ip = primary current (A)
• Vs = secondary voltage (V)
• Is = secondary current (A)

What This Means

• If a transformer steps up the voltage, the current must step down (and vice versa) to keep the power the same.
• No transformer is perfectly efficient in practice — some energy is lost as heat in the coils and the iron core.

Worked Example 4 (Higher)

A 100% efficient transformer steps up 25 V to 500 V. The current in the primary coil is 10 A. Calculate the current in the secondary coil.

VpIp = VsIs

25 × 10 = 500 × Is

250 = 500 × Is

Is = 250 / 500

Is = 0.50 A

Worked Example 5 (Higher)

A transformer has a primary voltage of 230 V and secondary voltage of 11,500 V. The secondary current is 2.0 A. Assuming 100% efficiency, calculate the primary current.

VpIp = VsIs

230 × Ip = 11,500 × 2.0

230 × Ip = 23,000

Ip = 23,000 / 230

Ip = 100 A

Exam Tip (Higher): In transformer calculations, if they tell you the transformer is "100% efficient" or "ideal", use VpIp = VsIs. If a step-up transformer increases voltage by a factor of 10, the current decreases by a factor of 10.

---

Transformers and the National Grid

The National Grid is the network of cables and transformers that distributes electricity from power stations to homes and businesses across the country.

Why Transformers Are Essential

Electricity from power stations must travel long distances through cables. These cables have resistance, which causes energy to be lost as heat (wasted thermal energy).

The power lost in the cables is given by:

P = I²R

Where P = power lost (W), I = current (A), R = resistance of the cables (Ω).

The Key Insight