Question

An electric heater operating at $220V$ boils $5l$ of water in $5min$. If it is used on $110V$, it will boil the same amount of water in:

• A. $10min$
• B. $20min$
• C. $15min$
• D. $25min$

Answer 1

The correct option is B $20min$

Given

220 V heater boiling 5-litre water in 5 mint.

Let 110 V heater will boil 5-litre water in t mint.

The amount of heat supplied by the heater to water $H=i^{2}Rt=\frac{V^2}{R}t$.

$\because$ Same amount of water has to build with both heater so amount of heat supply will be same.

$Q_1=Q_2$

$\frac{V_1^2{t}_1}{R_1}=\frac{V_2^2{t}_2}{R_2}$

$\frac{(220{)}^2}{R}\times 5=\frac{(110{)}^2}{R}\times t_2$

$\Rightarrow t_2=4\times 5=20min$