Q96 / 160 · JIPMER 2009 · Physics

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

  • A
    $30 \%$
  • B
    $50 \%$
  • Correct answer
    C
    $90 \%$
  • D
    $10 \%$

Solution

The efficiency of transformer $=\frac{\text { Energy obtained from the secondary coil }}{\text { Energy given to the primary coil }}$ or $\begin{array}{lrl} \text { or } & \eta & =\frac{\text { Output power }}{\text { Input power }} \\ \text { or } & \eta & =\frac{V_s I_s}{V_p I_p} \\ \text { Given, } & V_s I_s & =100 \mathrm{~W}, V_p=220 \mathrm{~V}, I_p=0.5 \mathrm{~A} \\ \text { Hence, } & \eta & =\frac{100}{220 \times 0.5}=0.90=90 \% \end{array}$

Hence, the correct answer is option: 'C'.

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