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6.2.4Chemical Reactions

Transformer Equations — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 21 of 21 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Section 1 of 5Key Equations

Two equations link the voltages, turns and currents of an ideal transformer.

Turns ratio equation
  • Turns ratio equation Vp/Vs = Np/Ns, where V is potential difference in volts and N is the number of turns on each coil.
Power equation
  • Power equation Vp × Ip = Vs × Is for an ideal, 100% efficient transformer, where current I is measured in amperes (A).
Ideal transformer
  • Ideal transformer An ideal transformer has no energy losses, so the input power equals the output power.
Efficiency
  • Efficiency Efficiency = (output power ÷ input power) × 100%, with both powers measured in watts (W).

Section 2 of 5Solving Transformer Problems

A consistent method avoids mixing up the primary and secondary quantities.

1
Identify which coil is the primary and which is the secondary, and list the quantities that are known.
2
Rearrange the turns-ratio equation to make the unknown potential difference or number of turns the subject.
3
Use the power equation Vp × Ip = Vs × Is to find an unknown current when the transformer is ideal.
4
Check the answer: a step-up transformer gives a larger output voltage and a smaller output current.
5
Give the answer with its unit, such as volts (V) or amperes (A), to a sensible number of significant figures.

Section 3 of 5Efficiency and Losses

Questions usually assume an ideal transformer unless an efficiency is stated.

Efficiency and Losses
Real transformer
  • Real transformer Energy is lost as heat in the core and coils, so the output power is less than the input power.
Ideal assumption
  • Ideal assumption Questions usually assume 100% efficiency unless a percentage is given, so the power equation can be used directly.
Efficiency below 100%
  • Efficiency below 100% If efficiency is less than 100%, the input power Vp × Ip is greater than the output power Vs × Is.
Power unit
  • Power unit Power in watts (W) is the product of potential difference in volts and current in amperes.

Section 4 of 5Worked Example

Question

  • A step-down transformer has 2000 turns on its primary coil and 100 turns on its secondary coil.
  • The primary is connected to a 230 V supply.
  • The secondary current is 4.0 A.
  • Assuming the transformer is 100% efficient, calculate (a) the secondary voltage and (b) the primary current.

  1. (a) Use Vp/Vs = Np/Ns, so Vs = Vp × Ns/Np = 230 × 100/2000.
  2. (a) Vs = 230 × 0.05 = 11.5 V.
  3. (b) Use Vp × Ip = Vs × Is, so Ip = Vs × Is/Vp = 11.5 × 4.0/230.
  4. (b) Ip = 46/230 = 0.20 A.

Answer (a) 11.5 V. (b) 0.20 A.

Section 5 of 5Exam Checklist & Tips

6.2.4 – Key Points for the Exam

  • Turns ratio The ratio Np/Ns, which determines the ratio of the potential differences in an ideal transformer.
  • Ideal transformer A transformer with 100% efficiency, so that input power equals output power.
  • Efficiency The useful output power as a percentage of the input power.
  • Primary potential difference (Vp) The potential difference across the primary coil, in volts.

Common Mistakes

  • Inverting the turns ratio, writing Vp/Vs = Ns/Np rather than Vp/Vs = Np/Ns.
  • Placing the currents in the wrong positions in the power equation, such as writing Vs × Ip instead of Vp × Ip.
  • Forgetting efficiencya value below 100% means the input power is greater than the output power.
  • Writing the answer without units, or giving far more significant figures than the data justify.

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Transformer Golden Equations Physics Standard

Transformer Turns Ratio & Efficiency

Ratio: Vp / Vs = Np / Ns = Is / Ip

  • Step-up reduces grid thermal loss (P = I2R).
  • Laminated core prevents eddy currents.
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