Transformer and transmission

In an ideal transformer two coils are inductively coupled:

• Same magnetic flux in both primary and secondary coil, the field lines are guided by magnetic material (iron core).
  


• Energy is conserved: power input by primary coil equals to power output of secondary coil.

In reality the iron core is heated due to eddy currents and hysteresis. This can be avoided by laminating the core into individually electrically insulated thin layers.

Look how the transformer acts in step-up or step-down (voltage increased/decresed) mode.

Energy transmission in power lines suffer from losses due to resistance of the cables. Since power dissipated is proportional to current squared it is convenient to use low current but high voltage in the transmission over long distances.