A coil takes 15 minutes to boil a certain amount of water. Another coil takes 20 minutes for the same process. Time taken to boil the same amount of water when both coils are connected in series is

Step-by-step solution

From Joule's law,             H = V 2 t RJ   cal Where V is potential difference, t is the time and R is the resistance.             R = R 1 + R 2 Let t R = HJ V 2 =   constant = k ⇒ R 1 = kt 1   and   R 2 = kt 2 ∴   kt = kt 1 + kt 2 t = 15 + 20            t = 35 minutes