AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The National Grid: potential difference across primary coil x current in primary coil = potential difference across secondary coil x current in secondary coil
The published AQA 8464 Combined Science specification includes this relationship for Higher Tier.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The published AQA 8464 Combined Science specification includes this relationship for Higher Tier. For a 100% efficient transformer, the electrical power entering the primary coil equals the electrical power leaving the secondary coil. Since power equals potential difference multiplied by current, Vₚ × Iₚ = Vₛ × Iₛ.
For example: If Vₚ = 230 V, Iₚ = 4 A and Vₛ = 460 V, then Iₛ = (230 × 4) ÷ 460 = 2 A. A step-up transformer raises potential difference and lowers current for the same power; this lower current reduces thermal energy dissipation in National Grid cables.
What earns the marks
- States the ideal-transformer relationship Vₚ × Iₚ = Vₛ × Iₛ.
- Identifies Vₚ and Iₚ as primary-coil quantities and Vₛ and Iₛ as secondary-coil quantities.
- Substitutes values with volts and amperes consistently before rearranging for the unknown.
- Explains that increasing potential difference reduces current when transmitted power is unchanged.
- Links lower transmission current to reduced thermal energy dissipation in cables.
Common mistakes and how to put them right
Pairing a primary potential difference with a secondary current.
Put it right by keeping Vₚ with Iₚ and Vₛ with Iₛ.
Assuming a step-up transformer increases both potential difference and current.
Put it right by using equal input and output power for an ideal transformer.
Dividing before writing the full equality.
Put it right: Write VₚIₚ = VₛIₛ, substitute, then isolate the required quantity.
Exam tips
- Write VₚIₚ = VₛIₛ before substituting so each value stays attached to the correct coil.
- Check the direction: a step-up transformer should give a lower secondary current for the same power.
Try the lesson
The start of the The National Grid: potential difference across primary coil x current in primary coil = potential difference across secondary coil x current in secondary coil lesson, made from checked teaching notes for this specification point.
Have a go first
Which equation correctly links the potential difference and current in the primary and secondary coils of a 100% efficient transformer?
Listen
The published AQA 8464 Combined Science specification includes this relationship for Higher Tier. For a 100% efficient transformer, the electrical power entering the primary coil equals the electrical power leaving the secondary coil. Since power equals potential difference multiplied by current, Vₚ × Iₚ = Vₛ × Iₛ. For example, if Vₚ = 230 V, Iₚ = 4 A and Vₛ = 460 V, then Iₛ = (230 × 4) ÷ 460 = 2 A. A step-up transformer raises potential difference and lowers current for the same power; this lower current reduces thermal energy dissipation in National Grid cables. A strong answer earns marks for these points. States the ideal-transformer relationship Vₚ × Iₚ = Vₛ × Iₛ. Identifies Vₚ and Iₚ as primary-coil quantities and Vₛ and Iₛ as secondary-coil quantities. Substitutes values with volts and amperes consistently before rearranging for the unknown. Explains that increasing potential difference reduces current when transmitted power is unchanged. A common slip: Pairing a primary potential difference with a secondary current. Instead, keeping Vₚ with Iₚ and Vₛ with Iₛ. Exam tip: Write VₚIₚ = VₛIₛ before substituting so each value stays attached to the correct coil.
Key words
Say what each word means before you turn the card.
Ideal transformer power balance
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Potential difference and current
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National Grid efficiency
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See every definition
- Ideal transformer power balance
- For a 100% efficient transformer, the electrical power entering the primary coil equals the electrical power leaving the secondary coil.
- Potential difference and current
- Power equals potential difference multiplied by current, so Vₚ × Iₚ = Vₛ × Iₛ for a 100% efficient transformer.
- National Grid efficiency
- A step-up transformer raises potential difference and lowers current for the same power, and this lower current reduces thermal energy dissipation in National Grid cables.
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