P = V × I. To transmit a given power P at high voltage → LOW current needed.
P_lost = I² × R (power lost in cable resistance).
Low current → much less power wasted in transmission cables.
EXAMPLE:
Transmit 100 MW at 1000 V vs 1,000,000 V:
At 1000 V: I = 100,000 A → P_loss = 100,000² × R (enormous).
At 1,000,000 V: I = 100 A → P_loss = 100² × R (10,000 times less).
ENERGY LOSSES IN REAL TRANSFORMERS:
Eddy currents in the iron core — reduced by using laminated core.
Resistance heating in coil wires — use thick copper wire.
Magnetic flux leakage — tight winding, soft iron core.
Real transformers: ~95–99% efficient.
⚠️ Common Mistake
Transformers only work with AC — not DC. DC produces constant flux → no change → no induced emf. Step-up transformer increases VOLTAGE but DECREASES current (power is conserved). National Grid uses high voltage to REDUCE current → reduces I²R power losses in transmission cables.
📐 Variables
VₚPrimary voltage (Vₚ) is measured in volts (V)
VₛSecondary voltage (Vₛ) is measured in volts (V)
NₚNumber of primary turns (Nₚ) is measured in ()
NₛNumber of secondary turns (Nₛ) is measured in ()
Transformer: AC in primary → alternating flux in iron core → emf induced in secondary. Turns ratio: Vs/Vp = Ns/Np. Step-up: Ns > Np. Power conservation: Vp×Ip = Vs×Is. National Grid: step-up to ~400 kV for transmission (low current → less I²R loss), step-down for consumers. AC only — DC gives constant flux, no induction.
🎯 Matching Activity — Transformer Equations
Match each transformer scenario to the correct calculation. — drag the symbols on the right to match the component names on the left.
Step-up transformer
Drop here
Step-down transformer
Drop here
National Grid transmission
Drop here
Power conservation
Drop here
Step-up to ~400 kV → low current → low I²R losses in cables
Vp × Ip = Vs × Is — higher voltage means lower current
Ns > Np → output voltage greater than input voltage
Ns < Np → output voltage less than input voltage
⚽ FIFA Worked Examples
Transformer Voltage
A transformer has 200 primary turns and 800 secondary turns. Input voltage = 230 V. Find the output voltage.
F
Vs/Vp = Ns/Np
I
Vp = 230 V, Np = 200, Ns = 800
F
Vs = Vp × (Ns/Np) = 230 × (800/200) = 230 × 4
A
Vs = 920 V (step-up transformer)
⭐ Higher Tier Only
HT only — use the turns ratio equation to calculate transformer voltages and turns. Apply power conservation to find current in transformer circuits. Explain why the National Grid uses high voltage transmission and calculate power losses at different voltages.
🔬 Triple Science Only
Transformers (HT only, physics only) — not in Combined Science.
🎯 Test Yourself
Question 1 of 2
1. Why can't transformers work with direct current (DC)?
2. Why does the National Grid transmit electricity at very high voltages?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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🤖 Ask Mr Badmus AI
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