Page 75 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

a) From equation 2.13,

Q12 = mCv(T2 – T1)

= 3.4 x 0.72(420 – 290)

= 318.24 kJ

b) From equation 2.14,

PV = mRT

Hence for state 1,
P1V1 = mRT1

mRT 4 . 3 kg x . 0 287 kJ/kgK x 290 K
2
P 1 = 1 = = 307 6 . kN/m
V 1 . 0 92 m 3

c) For state 2,

P2V2 = mRT2

mRT 4 . 3 kg x . 0 287 kJ/kgK x 42 K 0
2
P = 2 = = 445 5 . kN/m
2
V 2 . 0 92 m 3

2.6 Specific Heat Capacity at Constant Pressure (Cp)

If 1 kg of a gas is supplied with an amount of heat energy sufficient to raise the
temperature of the gas by 1 degree whilst the pressure of the gas remains constant, then

the amount of heat energy supplied is known as the specific heat capacity at constant

pressure, and is denoted by Cp. The unit of Cp is J/kg K or kJ/kg K.

For a reversible non-flow process at constant pressure, we have

dQ = mCpdT (2.22)

For a perfect gas the values of Cp are constant for any one gas at all pressures and
temperatures. Equation (2.22) can then be expanded as follows:

Heat flow in a reversible constant pressure process Q = mCp(T2 – T1) (2.23)
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