1.2 kg of liquid water initially at 15°C is to be heated to 95°C in a teapot equipped with a 1200-W electric heating element inside. The teapot is 0.5 kg and has an average specific heat of 0.7?kJ/kg•K. Taking the specific heat of water to be 4.18 kJ/kg•K and disregarding any heat loss from the teapot, determine how long it will take for the water to be heated.

Question

What will be an ideal response?

Answer 1

Liquid water is to be heated in an electric teapot. The heating time is to be determined.

Assumptions 1 Heat loss from the teapot is negligible. 2 Constant properties can be used for both the teapot and the water.

Properties The average specific heats are given to be 0.7 kJ/kg•K for the teapot and 4.18 kJ/kg•K for water.

Analysis We take the teapot and the water in it as the system, which is a closed system (fixed mass). The energy balance in this case can be expressed as



Discussion In reality, it will take more than 6 minutes to accomplish this heating process since some heat loss is inevitable during heating. Also, the specific heat units kJ/kg • °C and kJ/kg • K are equivalent, and can be interchanged.