Two coils that never touch, wound on one iron ring: drive an AC voltage into the first and a different voltage comes out of the second. What sets the output voltage — the field, the frequency, the load, or just how many times each coil is wound?
▶ Launch the interactive simulationWind a primary of N₁ turns and a secondary of N₂ turns on a shared core so the SAME flux Φ(t) threads both. An AC primary makes V₁ = N₁ dΦ/dt; the secondary sees V₂ = N₂ dΦ/dt. The lab traces the shared flux Φ(t) against V₁(t) and V₂(t) (a scaled copy), and an ideal power balance V₁I₁ = V₂I₂ fixes the current. A real grid step-up — N₁=100, N₂=10000, fed a 4 kV station line — is read with 2% noise on its secondary peak.
V₂/V₁ = N₂/N₁ — output voltage is set ONLY by the turns ratio (the flux cancels), independent of frequency or load: a step-up coil makes hundreds of kV from a few kV. But it is no free lunch — power conservation V₁I₁ = V₂I₂ steps the current the OTHER way, I₂/I₁ = N₁/N₂, and DC (dΦ/dt = 0) transforms nothing. Inverting a real 4 kV→400 kV (×100) step-up recovers the ratio ≈ 100, and at 100× the voltage the line current is 100× smaller so the I²R loss falls 10⁴× (625 kW → 62.5 W on a 1 MW, 10 Ω line) — the reason the grid is high-voltage AC