Bohr and Biot purchased two electric kettles A and B of the same size and thickness and…

Bohr and Biot purchased two electric kettles A and B of the same size and thickness and same volume of 0.4 litre. They studied the specification of kettles as under
Kettle A : specific heat capacity;1680 J/kg–k, mass = 200 gm cost = Rs. 400
Kettle B : specific heat capacity = 2450 J/kg–K mass = 400 gm cost = Rs 400 . When kettle A is switched on with constant potential source, the tea begins to boil in 6 minutes. When kettle B is switched on with same source separately, tea begins to boil in 8 minutes. They made discussion on specification and efficiency of kettles and prepared a list of questions to draw a conclusion. Some of these are as under (take specific heat of tea liquid = 4200 J/kg–k, and density = 1000 kg/m³)
 
Efficiency of kettle B is