An electric heater of resistance  10Ω connected to 220V power supply is immersed in water of mass 1kg. How long the heater has to be switched on to increase its temperature from 300C to 600C. (Specific heat of water =4200JKg-1C-1 )

# An electric heater of resistance  $10\mathrm{\Omega }$ connected to 220V power supply is immersed in water of mass 1kg. How long the heater has to be switched on to increase its temperature from 300C to 600C. (Specific heat of water $=4200{\text{JKg}}^{\text{-1}}{\text{C}}^{\text{-1}}$ )

1. A

20s

2. B

26s

3. C

16s

4. D

42s

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$\begin{array}{l}\mathrm{Q}=\mathrm{ms}△\mathrm{T}={\mathrm{I}}^{2}\mathrm{Rt}=\frac{{\mathrm{v}}^{2}\mathrm{t}}{\mathrm{R}}⇒\mathrm{t}=\frac{\mathrm{ms}△\mathrm{TR}}{{\mathrm{v}}^{2}}\\ ⇒\mathrm{t}=\frac{1×4200×30×10}{220×220}=26.03\text{s}\end{array}$