Electromagnetism · Experiment

Transformer

Two coils share one iron core, coupled only by magnetic flux. The turns ratio sets the voltage ratio — step it up or step it down — while the power flowing through stays the same.

primary V₁ (N₁ turns)secondary V₂ (N₂ turns)shared flux Φ in the core

Controls

Turns ratio N₂/N₁—
Output V₂ = (N₂/N₁)·V₁—
Currents I₁ / I₂—
Power in = power out—
Step-up ×2.00: V₂ = (N₂/N₁)·V₁ — but the primary draws 2× the current. Power in = power out.
About this experiment ›

What you are looking at

An iron core links two separate coils: the violet primary (driven by an AC source) and the green secondary (feeding a load). No wire connects them — only the gold magnetic flux circulating in the core. The graph tracks the two voltages in real time.

How flux coupling works

The AC current in the primary magnetizes the core, and the iron guides almost all of that flux Φ through the secondary too. Faraday's law then acts on each coil in proportion to its turns:
V₁ = N₁·dΦ/dt   V₂ = N₂·dΦ/dt
Same dΦ/dt for both — so dividing the equations kills it:
V₂ / V₁ = N₂ / N₁
More secondary turns than primary and the voltage is stepped up; fewer and it's stepped down. Notice this only works because the flux keeps changing — feed a transformer DC and dΦ/dt = 0: nothing comes out (and the primary overheats).

No free lunch: the current trades the other way

An ideal transformer passes power straight through, so what the voltage gains the current must lose:
V₁·I₁ = V₂·I₂  ⇒  I₂ / I₁ = N₁ / N₂
Step 6 V up to 12 V and the secondary delivers only half the current the primary draws. The stats panel shows both sides of the ledger balancing exactly.

Why the grid depends on this

Transmission losses go as I²R in the cables — so the grid steps voltage up (hundreds of kV) to push the same power with tiny current over long lines, then steps it back down near your home. This single trick — and the fact that transformers need AC — is the main reason the power grid is AC.

Things to try

Set N₂ = N₁ for a 1:1 isolation transformer. Slide N₂ up and watch V₂ grow while I₁ climbs to pay for it. Halve the load resistance and both currents double while the voltages stand still — the transformer passes through whatever power the load demands.