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6.2.3Chemical Reactions

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

5 min read 5 parts Lesson 10 of 21 Unit quiz

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

6.2.3 Transformer Calculations
Turns ratio
  • For an ideal transformer, Vp ÷ Vs = Np ÷ Ns, where V is the voltage and N is the number of turns on each coil.
Power equation
  • For an ideal (100% efficient) transformer the input power equals the output power, so Vp × Ip = Vs × Is.
Units
  • Potential difference is measured in volts (V), current in amperes (A) and power in watts (W).
Rearranging
  • The turns-ratio equation can be rearranged to find any unknown, for example Vs = Vp × Ns ÷ Np.

Section 2 of 5Calculation Steps

1
Identify which coil is the primary and which is the secondary, and write down the known values with their units.
2
Use the turns ratio to find the unknown voltage or the unknown number of turns.
3
Use Vp × Ip = Vs × Is to find the unknown current when a current is required.
4
Check that the result is consistent with the type of transformer, such as a step-down transformer having a lower secondary voltage.

Section 3 of 5Common Scenarios

Common Scenarios
Step-down
  • A step-down transformer has more primary turns than secondary turns, so the output voltage is lower and the output current is higher.
Efficiency
  • Real transformers lose some energy as heat, so the output power is slightly lower than the input power.
Current on the step-down side
  • The current rises on the step-down side, because the same power is delivered at a lower voltage.
Checking answers
  • For an ideal transformer, the input and output power should be equal, which is a useful check on any answer.

Section 4 of 5Worked Example

Question

  • A transformer has 2000 turns on the primary coil and 100 turns on the secondary coil.
  • The primary is connected to a 230 V supply and draws 0.20 A.
  • Assuming 100% efficiency, calculate (a) the secondary voltage and (b) the secondary current.

  1. (a) Vs = Vp × Ns ÷ Np = 230 V × 100 ÷ 2000.
  2. Vs = 23 000 ÷ 2000 = 11.5 V.
  3. (b) Input power = Vp × Ip = 230 V × 0.20 A = 46 W.
  4. Is = power ÷ Vs = 46 W ÷ 11.5 V = 4.0 A.

Answer

  • The secondary voltage is 11.5 V and the secondary current is 4.0 A;
    • this is a step-down transformer.

Section 5 of 5Exam Checklist & Tips

6.2.3 – Key Points for the Exam

  • Turns ratio The ratio of the number of turns on the secondary coil to the number on the primary coil.
  • Ideal transformer A transformer with no energy losses, so input power equals output power.
  • Efficiency The percentage of input energy or power that is transferred to useful output.
  • Input power The power supplied to the primary coil.
  • Output power The power delivered by the secondary coil.

Common Mistakes

  • Placing the turns ratio the wrong way up, so the voltage ratio is reversed.
  • Forgetting that the current ratio is inverse to the voltage ratio, so current changes in the opposite direction.
  • Using the lengths of the coils instead of their numbers of turns.
  • Giving answers without units, such as 11.5 instead of 11.5 V.

Last updated:

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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