TransformersAQA GCSE Physics: Revision notes

Section 1

What is the structure of a transformer?

A transformer consists of three essential components:

Primary coil: the input coil connected to the alternating current (AC) power supply
Secondary coil: the output coil where the transformed voltage is produced
Iron core: a soft iron cylinder around which both coils are wound, which significantly increases the efficiency of magnetic flux linkage between the coils

The iron core is crucial because it provides a path of low reluctance for the magnetic field, ensuring that nearly all magnetic flux from the primary coil passes through the secondary coil. Both coils are insulated from the iron core and from each other to prevent unwanted current flow.

Key termsprimary coilsecondary coiliron coremagnetic flux linkage

Think of it like this

Think of the iron core as a bridge between two islands (the coils): without it, the magnetic signal would scatter into empty space; with it, nearly all the signal crosses from one island to the other.

Section 2

How does a transformer work using electromagnetic induction?

Transformers operate on the principle of electromagnetic induction:

An alternating current flows through the primary coil, creating a continuously changing magnetic field around it
This changing magnetic field threads through the iron core, concentrating the flux
The changing magnetic flux passes through the secondary coil, inducing an electromotive force (EMF) in it
The magnitude of the induced EMF depends on the rate of change of magnetic flux and the number of turns in the secondary coil

This process occurs without any physical connection between the coils—only magnetic flux transfers energy from primary to secondary. The transformer only works with alternating current because direct current produces a constant (non-changing) magnetic field that cannot induce an EMF.

Key termselectromagnetic inductionmagnetic fieldelectromotive forcemagnetic fluxalternating current

Exam tip

Examiners expect you to explain that the changing magnetic field is essential—state that a constant field (from DC) produces no induction.

Common mistake

Students often forget to mention that transformers only work with AC. Always state this explicitly when discussing how they function.

Section 3

What is the transformer turns ratio equation and how is it used?

The relationship between voltages and the number of turns in a transformer is described by the turns ratio equation:

$\frac{V_1}{V_2} = \frac{n_1}{n_2}$

Where:

$V_1$ = voltage across the primary coil (input voltage)
$V_2$ = voltage across the secondary coil (output voltage)
$n_1$ = number of turns in the primary coil
$n_2$ = number of turns in the secondary coil

Key principle: The voltage ratio equals the turns ratio. This means:

If the secondary has more turns than the primary ( $n_2 > n_1$ ), the output voltage is greater than the input voltage
If the secondary has fewer turns than the primary ( $n_2 < n_1$ ), the output voltage is less than the input voltage

This equation applies to ideal transformers (those with 100% efficiency and no energy losses).

Key termsturns ratio equationprimary voltagesecondary voltagenumber of turnsideal transformer

Example

A transformer has 200 turns in the primary coil and 1000 turns in the secondary coil. If the input voltage is 12 V, find the output voltage. Using V₁/V₂ = n₁/n₂: 12/V₂ = 200/1000. Rearranging: V₂ = (12 × 1000)/200 = 60 V.

Exam tip

When rearranging the turns ratio equation, clearly show each algebraic step—examiners award marks for method, not just the final answer.

Section 4

How does the power equation apply to ideal transformers?

For an ideal transformer, energy is conserved, meaning the electrical power input equals the electrical power output:

$V_1 I_1 = V_2 I_2$

Where:

$V_1$ = primary voltage (volts)
$I_1$ = primary current (amperes)
$V_2$ = secondary voltage (volts)
$I_2$ = secondary current (amperes)

Important relationship: When voltage increases (step-up), current decreases proportionally, and vice versa. This is expressed by the current ratio equation:

$\frac{I_1}{I_2} = \frac{n_2}{n_1}$

Note that the current ratio is inverted compared to the turns ratio—the secondary current is larger when the secondary has fewer turns.

Real transformers have slightly lower efficiency due to energy losses (heat from resistance in the coils, eddy currents in the core, and hysteresis losses), so $V_1 I_1 > V_2 I_2$ in practice.

Key termspower equationprimary currentsecondary currentcurrent ratioenergy losseseddy currentshysteresis

Example

A step-up transformer has 100 primary turns and 500 secondary turns. The primary voltage is 120 V and primary current is 10 A. Find the secondary voltage and current. V₂ = V₁(n₂/n₁) = 120 × (500/100) = 600 V. I₂ = I₁(n₁/n₂) = 10 × (100/500) = 2 A. Check: V₁I₁ = 120 × 10 = 1200 W; V₂I₂ = 600 × 2 = 1200 W. ✓

Exam tip

Always verify your answer using the power equation—if V₁I₁ ≠ V₂I₂ for an ideal transformer, you've made an error.

Section 5

What is the difference between step-up and step-down transformers?

Transformers are classified by their effect on voltage:

Property	Step-up Transformer	Step-down Transformer
Secondary turns vs primary	More turns in secondary ( $n_2 > n_1$ )	Fewer turns in secondary ( $n_2 < n_1$ )
Output voltage vs input	Higher voltage ( $V_2 > V_1$ )	Lower voltage ( $V_2 < V_1$ )
Turns ratio	Turns ratio > 1	Turns ratio < 1
Current effect	Current decreases ( $I_2 < I_1$ )	Current increases ( $I_2 > I_1$ )
Example application	National Grid transmission (11 kV → 132 kV)	Home use (230 V → 12 V for appliances)

Key insight: The type of transformer is determined solely by the ratio of secondary to primary turns. A step-up transformer increases voltage but reduces current; a step-down transformer decreases voltage but increases current. Power remains constant in an ideal transformer, regardless of type.

Key termsstep-up transformerstep-down transformerturns ratiovoltage transformation

Think of it like this

Think of a step-up transformer like a bicycle gear change—fewer teeth on the input gear drive more teeth on the output gear, making the output spin more slowly but with greater force; voltage increases but current decreases.

Section 6

Why are transformers essential in the National Grid?

Transformers are critical for efficient power transmission across the National Grid:

The power transmission problem:

Power stations generate electricity at moderate voltages (around 11 kV)
Electricity must travel long distances through transmission cables to reach homes and businesses
As current travels through cables, energy is lost as heat according to the equation: $P_{loss} = I^2 R$ , where $R$ is the cable resistance

How transformers solve this:

At power stations: A step-up transformer increases voltage from 11 kV to 132 kV (or higher), which reduces the current for transmission
Effect of reduced current: Since power loss is proportional to $I^2$ , reducing current dramatically reduces energy losses during transmission
At local substations: A step-down transformer reduces voltage from 132 kV to 33 kV, then to 11 kV, then to 230 V for domestic use
Final step: Small transformers reduce voltage to levels safe for household appliances (e.g., 12 V for doorbells)

Efficiency gain: By transmitting at high voltage and low current, the National Grid minimises energy losses, making electricity distribution more economical and sustainable. This is why the formula $P_{loss} = I^2 R$ is central to understanding the Grid's design.

Key termsNational Gridpower transmissionstep-up transformerstep-down transformertransmission lossescable resistance

Exam tip

Examiners expect you to explain the $I^2 R$ relationship and emphasise that reducing current (by stepping up voltage) has a squared effect on reducing losses—this is the key reason transformers are so important to the Grid.

Example

If current is reduced from 100 A to 10 A (a factor of 10), power loss decreases by a factor of 100 (10²). This massive reduction in losses is why high-voltage transmission is used.

Must Know

Structure: A transformer has a primary coil (input), secondary coil (output), and iron core that links the magnetic flux between coils
Induction principle: Transformers work by electromagnetic induction—a changing AC current in the primary coil creates a changing magnetic field that induces an EMF in the secondary coil; they only work with AC, not DC
Turns ratio equation: $V_1/V_2 = n_1/n_2$ —the voltage ratio equals the turns ratio; use this to calculate unknown voltages
Power conservation: For an ideal transformer, $V_1 I_1 = V_2 I_2$ —power in equals power out; voltage and current have an inverse relationship (when voltage increases, current decreases)
Step-up vs step-down: Step-up transformers have more secondary turns, increase voltage, and decrease current; step-down transformers have fewer secondary turns, decrease voltage, and increase current
National Grid efficiency: Transformers minimise transmission losses by stepping up voltage (reducing current) for long-distance transmission, then stepping down voltage for local use; this is critical because power loss is proportional to $I^2 R$ , so reducing current has a squared effect on reducing losses

Key termsprimary coilsecondary coiliron coreelectromagnetic inductionturns ratio equationpower equationstep-up transformerstep-down transformerNational Gridtransmission losses

That's the notes covered.

Carry on to the next subtopic.