Video narration
How can one transformer make voltage go up or down, like the one inside a phone charger? The answer starts with alternating current, or AC, flowing through the primary coil. That changing current creates a changing magnetic field in the core. The field reaches the secondary coil, where it induces an alternating voltage. This is electromagnetic induction: a changing magnetic field produces voltage in another coil. Now the outcome flips with the number of turns. If the secondary coil has more turns than the primary, its voltage is higher: that is a step-up transformer. Fewer secondary turns give a lower voltage: that is a step-down transformer. For an ideal transformer, voltage follows the turns ratio: Vs over Vp equals Ns over Np. So voltage rises with more secondary turns, but current falls, keeping energy approximately conserved. That's the answer: more coil turns step voltage up, fewer turns step it down.