$A$ step-down transformer transforms $220 \text{ V}$ to $11 \text{ V}$. If the currents in the primary and secondary coils are $5 \text{ A}$ and $90 \text{ A}$ respectively,the efficiency of the transformer is: (in $\%$)

$A$ step-up transformer operates on a $230\, V$ line and supplies a load of $2\, A$. The ratio of the primary and secondary windings is $1 : 25$. The current in the primary is.... (in $, A$)

$A$ generator at a utility company produces $100 \, A$ of current at $4000 \, V$. The voltage is stepped up to $240,000 \, V$ by a transformer before it is sent on a high voltage transmission line. The current in the transmission line is ....... $A$.

Write the principle of a transformer.

Write the working procedure of a transformer.

In an ideal step-up transformer,if the input voltage and input power are $V_1$ and $P_1$ respectively,and the output voltage and output power are $V_2$ and $P_2$ respectively,then