$A$ transformer is employed to

In an ideal step-up transformer, the number of turns in the primary coil and secondary coil are $100$ and $200$ respectively. If the output current is found to be $5 \text{ A}$, then the input current will be . . . . . . . (in $\text{ A}$)

The output power in a step-up transformer used in practice is . . . . . .

$A$ transformer is used to light a $100 \,W$ and $110 \,V$ lamp from a $220 \,V$ mains. If the main current is $0.5 \,A$, the efficiency of the transformer is approximately (in $\%$)

The alternating voltage induced in the secondary coil of a transformer is mainly due to

What is a transformer? Write its principle and describe its construction.