A transformer raises or lowers voltage in AC circuits through mutual induction. Alternating current in the primary creates changing flux in the core, and that changing flux induces alternating emf in the secondary.

• It works on AC only because only changing current gives the changing flux needed for induction.
• There is no electrical connection between primary and secondary; they are linked magnetically through the core.
• Trap: thinking a transformer can operate on DC after the first instant in the same way it operates on AC.

For the ideal transformer, Vs by Vp = Ns by Np = Ip by Is = k and the power relation is Vs Is = Vp Ip. The flux per turn of each coil is taken as the same in the textbook model.

• Voltage ratio follows turn ratio directly, while current ratio is inverse to the turn ratio relation.
• The page also notes that primary and secondary voltages are in opposite phase.
• Trap: writing Is by Ip = Ns by Np instead of using the inverse current relation given on the page.

A step-up transformer increases voltage and decreases current with Ns greater than Np and k greater than 1. A step-down transformer decreases voltage and increases current with Ns less than Np and k less than 1.

• Step-up means higher secondary voltage but lower secondary current.
• Step-down means lower secondary voltage but higher secondary current.
• Trap: saying both voltage and current increase in a step-up transformer, which violates ideal power conservation.

Efficiency is eta = Pout by Pin into 100. The page lists copper loss, eddy-current loss, hysteresis loss, flux leakage, and humming losses, and gives a separate design fix for each major loss.

• Copper loss comes from I squared R heating and is reduced by thick copper wires.
• Eddy-current loss is reduced by laminated silicon-steel cores, and hysteresis loss is reduced by soft iron or permalloy core material.
• Trap: mixing core-related losses with winding-resistance loss and then applying the wrong remedy.

Transformers are used in voltage regulators, induction furnaces, welding, long-distance AC transmission, electrical power distribution, audio and radio-frequency applications, and impedance matching. The application always follows whether voltage must be stepped up or stepped down.

• Long-distance transmission prefers high voltage and low current to reduce line loss.
• Domestic and equipment-side uses usually require step-down action from transmission levels.
• Trap: memorising applications as a list without linking each one to whether higher or lower output voltage is needed.