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2 A mass of 15 kg of air in a piston–cylinder device is heated from 25 to 95°C by passing current through a resistance heater inside the cylinder. The pressure inside the cylinder is held constant at 300 kPa during the process, and a heat loss of 60 kJ occurs. Determine the electric energy supplied, in kWh

Respuesta :

Answer:0.2338KWh

Explanation:

Given

mass of air[tex]\left ( m\right )[/tex]=15kg

Initial Temperature[tex]\left ( T_i\right )=25^{\circ}C[/tex]

Final Temperature[tex]\left ( T_f\right )=95^{\circ}C[/tex]

pressure is constant 300 KPa

heat loss =60 KJ

Enthalpy at [tex]25^{\circ}C\ is\ h_1=298.2KJ/kg[/tex]

Enthalpy at [tex]95^{\circ}C\ is\ h_2=350.49KJ/kg[/tex]

[tex]h_2-h_1=52.29 KJ/kg[/tex]

and for 15 kg air

[tex]\Delta h=784.35 KJ[/tex]

Thererfore net heat input=[tex]\Delta h+60[/tex]

                                          =784.35+60=844.35KJ

                                          =[tex]844.35\times 0.000277=0.2338KWh[/tex]

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