The Transformer Equations
Part of National Grid & Transformers — GCSE Physics
This how it works covers The Transformer Equations within National Grid & Transformers for GCSE Physics. Revise National Grid & Transformers in Extra Topics for GCSE Physics with 13 exam-style questions and 12 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 4 of 12 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 4 of 12
Practice
13 questions
Recall
12 flashcards
⚙️ The Transformer Equations
There are two key equations for transformers, both of which can appear in exam questions.
Turns Ratio Equation (Voltage Ratio)
Vp / Vs = Np / Ns
Primary voltage / Secondary voltage = Primary turns / Secondary turns
- Vp = primary (input) voltage in volts (V)
- Vs = secondary (output) voltage in volts (V)
- Np = number of turns on primary coil
- Ns = number of turns on secondary coil
Step-up transformer: Ns > Np, so Vs > Vp — output voltage is higher than input voltage.
Step-down transformer: Ns < Np, so Vs < Vp — output voltage is lower than input voltage.
Power Equation (100% Efficiency)
Vp × Ip = Vs × Is
Power in = Power out (for an ideal 100% efficient transformer)
If voltage is stepped up, current is stepped down by the same factor, keeping power constant (in an ideal transformer). If voltage doubles, current halves.