Transformer
A device using electromagnetic induction to change AC voltage and current. For an ideal transformer with primary turns N_p and secondary turns N_s: V_s/V_p = N_s/N_p and I_p/I_s = N_s/N_p.
Formula
Vs/Vp = Ns/Np
Real World
The National Grid transmits electricity at up to 400 kV using step-up transformers at power stations, reducing current and therefore power loss (P = I²R) in the cables before step-down transformers reduce voltage for safe home use.
Exam Focus
In efficiency questions, use P = VI for both sides; never assume ideal unless told — always calculate power loss if resistance is given.
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