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 $=4200{\text{JKg}}^{\text{-1}}{\text{C}}^{\text{-1}}$ )