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4.5.6 The Transformer

Electricity and magnetism · IGCSE Physics

4.5.6 The Transformer — IGCSE Physics Notes

Exam years: 2025–2027 Topic: Electricity and magnetism Lesson 38 of 48

4.5.6 The Transformer

Definition

A transformer is an electrical device consisting of two or more coils of wire used to transfer energy by means of a changing magnetic field.

Construction

  • It has two coils of insulated wire wound around a soft iron core.
  • The coil connected to the alternating voltage supply is the primary coil.
  • The coil connected to the output is the secondary coil.
  • The two coils are not electrically connected — they are linked magnetically through the iron core.

Working Principle

  • When an alternating voltage is applied to the primary coil, an alternating current flows through it.
  • This creates a changing magnetic field in the soft iron core.
  • The changing magnetic flux through the core induces an alternating voltage in the secondary coil.
  • This process is based on electromagnetic induction.
  • The transformer does not work with direct current (D.C.), since a constant current does not change magnetic flux and therefore cannot induce e.m.f.

Types of Transformers

TypeConditionEffect
Step-up Transformer Nₛ > Nₚ Vₛ > Vₚ → increases voltage, decreases current.
Step-down Transformer Nₛ < Nₚ Vₛ < Vₚ → decreases voltage, increases current.

Transformer Equation

Vₚ / Vₛ = Nₚ / Nₛ

  • Vₚ → voltage across primary coil
  • Vₛ → voltage across secondary coil
  • Nₚ → number of turns on primary coil
  • Nₛ → number of turns on secondary coil

Efficiency of an Ideal Transformer

For 100% efficiency: Vₚ Iₚ = Vₛ Iₛ

Hence: Iₛ / Iₚ = Vₚ / Vₛ

Input power = Output power (neglecting energy losses).

Energy Losses in Transformers

  • Heating of coils due to current (minimized by using low-resistance copper wire).
  • Eddy current loss in the iron core (reduced by using laminated cores).
  • Magnetisation and vibration losses causing sound and heat.

Uses of Transformers

  • Step-up transformers are used at power stations to raise voltage for efficient long-distance transmission.
  • Step-down transformers are used at distribution points and in devices (e.g., chargers, adaptors) to lower voltage for safe use.

Advantages of High Voltage Transmission

  • Reduces current in cables for the same power transfer.
  • Less current → less heating → lower energy loss in cables.
  • Results in higher efficiency of the power grid.

Important Notes

  • A transformer does not change the frequency of the current.
  • Both step-up and step-down transformers work only with alternating current.

Disadvantages of Using Transformers (Social and Environmental)

  • Increased dependence on electricity and automation reduces manual job opportunities.
  • Higher electricity demand leads to more fossil fuel consumption and air pollution.
  • Increased risk of electrocution in domestic and industrial systems.

Example Calculation

Given: Step-up transformer, 100% efficient

Vₚ = 100 V, Vₛ = 200 V, Iₚ = 0.4 A, find Iₛ

Vₚ Iₚ = Vₛ Iₛ → Iₛ = (Vₚ × Iₚ) / Vₛ = (100 × 0.4) / 200 = 0.2 A

Exam Focus Points

  • Always state that transformers work on electromagnetic induction.
  • Step-up: increases voltage, decreases current. Step-down: decreases voltage, increases current.
  • Use Vₚ / Vₛ = Nₚ / Nₛ and Vₚ Iₚ = Vₛ Iₛ accurately in calculations.
  • For high-efficiency transmission, link high voltage to low current and reduced heating losses.
  • State energy loss minimisation methods — copper windings, laminated core, soft iron core.

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