Step 1: The core idea.
A transformer transfers electrical energy from one coil to another without any electrical connection between them, relying entirely on a shared, time-varying magnetic field. This coupling of two coils through a changing flux is called mutual induction, which is the working principle of the device.
Step 2: How the voltage changes.
The alternating primary current sets up an alternating flux in the soft-iron core. Since both coils are wound on the same core, the same flux threads each turn of the secondary, and the secondary voltage scales with the ratio of turns, \(E_s/E_p = N_s/N_p\).
Step 3: List the ways energy is wasted.
• Winding (copper) loss: Joule heating \(I^2R\) in the coil wires.
• Eddy-current loss: circulating currents induced in the core; minimised by laminating the core.
• Hysteresis loss: energy spent in reversing the core's magnetisation each cycle; minimised by using soft iron of low coercivity.
• Magnetic flux leakage: not all primary flux reaches the secondary.
• Acoustic/humming loss: vibration of the core produces sound.
\[\boxed{\text{Mutual induction; losses = copper + eddy + hysteresis + leakage + humming}}\]