Water is boiled in a pan covered with a poorly fitting lid at a specified location. Heat is supplied to the pan by a 2-kW resistance heater. The amount of water in the pan is observed to decrease by 1.19 kg in 30 minutes. If it is estimated that 75 percent of electricity consumed by the heater is transferred to the water as heat, determine the local atmospheric pressure in that location

Respuesta :

Answer:

[tex]P_{atm} = 87.5\,kPa[/tex]

Explanation:

The heat required to boil the water in the pan is:

[tex]Q = \eta_{e}\cdot \dot W_{e} \cdot \Delta t[/tex]

[tex]Q = 0.75 \cdot (2\,kW)\cdot (30\, min)\cdot (\frac{60\,sec}{1\, min} )[/tex]

[tex]Q = 2700\,kJ[/tex]

Since the pan is accompained with a poorly fitting lid, the heating process is isobaric and change on specific enthalpy is obtained by following expression:

[tex]\Delta h = \frac{Q}{\Delta m}[/tex]

[tex]\Delta h = \frac{2700\,kJ}{1.19\,kg}[/tex]

[tex]\Delta h = 2268.908\,\frac{kJ}{kg}[/tex]

Then, the local atmospheric pressure can be estimated by looking for the saturation pressure related to the change on specific enthalpy at property tables for saturated water:

[tex]P_{atm} = 87.5\,kPa[/tex]

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