1. Discuss why the primary of a power transformer would have excessive current if the secondary were short-circuited. Why would there be no voltage across the secondary if the primary is open?
2. Explain how a transformer with a 1:1 turns ratio and isolated secondary can be used to reduce the chance of electric shock from a 120-Vac power line.
1. Transformers are calculated starting from a delivered power of the secondary. The power P=U*I is equal to the voltage multiplied by the current. This power of the secondary need to be delivered by the primary, and because always the voltage of the primary is known it implies a maximum calculated current in the primary coil (it means a maximum diameter of the primary wire). When a short-circuit happens in the secondary, the power delivered by the secondary is higher than the maximum power for which the transformer was computed. It implies the current in the primary will be also higher than the maximum for which it was designed.
If the primary is open the the voltage on the primary coil is zero. It means there will be no current in the primary and therefore no variable magnetic filed. Since there is no variable magnetic field the voltage induced in the secondary will be also zero.
2. As described above a transformer is always designed for a certain power. This power is delivered by the primary coil through the magnetic field to the secondary coil. This way the primary and secondary coils are electrically isolated. This means that the influence of the primary circuit on the secondary circuit is buffered and reduced to a certain maximum current in the secondary. This way a transformer having 1:1 turns ratio although it will have the same secondary voltage as the primary, the current in the secondary will have a certain maximum value which is lower than the maximum value that could be delivered by the voltage source of the primary coil itself. It means a lower chance of electric shock.