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A transformer is an electrical device which is used for changing the a.c. voltage. It is based on the phenomenon of mutual induction i.e. whenever the amount of magnetic flux linked with a coil changes, an e.m.f. is induced in the neighbouring coil. For an ideal transformer, the resistance of the primary and secondary winding are negligible.
It can be shown that \( \frac{\mathrm{E}_{s}}{\mathrm{E}_{p}}=\frac{\mathrm{I}_{p}}{\mathrm{I}_{s}}=\frac{n_{s}}{n_{p}}=k \) where the symbols have their standard meanings.
For a step up transformer,
For a step down transformer,
\[
n_{\mathrm{s}}n_{p} ; \mathrm{E}_{\mathrm{s}}\mathrm{E}_{p} ; k1 \text {. }
\]
The above relations are on the assumptions that efficiency of transformer is \( 100 \% \)
Infact, efficiency \( \eta=\frac{\text { output power }}{\text { input power }}=\frac{E_{s} I_{s}}{E_{p} I_{p}} \)
The number of turns in primary coil a transformer is 20 and the number of turns in a secondary is 10. If the voltage across the primary is \( 220 \mathrm{ac} \) V. What is the voltage across secondary?
(A) \( 100 \mathrm{~V} \)
(B) \( 120 \mathrm{~V} \)
(C) \( 110 \mathrm{~V} \)
(D) \( 220 \mathrm{~V} \)
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