Question

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

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

Answer 1

The correct option is C

$90\%$

Step1: Given data and assumptions.

Output power, $P_{out}=100W$

Primary voltage, $V_p=220V$

Primary current, $I_p=0.5A$

Secondary voltage, $V_s=110V$

Step1: Find the efficiency of a transformer.

Formula used:

$\eta =\frac{P_{out}}{P_{in}}\times 100$

Where, $\eta$ is the efficiency, $P_{out}$ is the output power, $P_{in}$ is the input power.

Now,

$\eta =\frac{P_{out}}{V_p{I}_p}\times 100$

$\Rightarrow$$\eta =\frac{100}{220\times 0.5}\times 100$

$\Rightarrow$$\eta =90\%$

Hence, option C is correct.