Question

An ideal gas of adiabatic exponent (Y=7/5) is expanding at constant pressure. The ratio of
dQ : dU: DW is (Symbols have their usual meanings)
(1) 7:5:1
(2) 7:2:5
(3) 2:1:1
(4) 7:5:2​

Answer 1

Answer:

7:5:2

Explanation:

For an ideal gas going through constant pressure process,

dQ= Cp ∆T

dU= Cv ∆T

From First law of thermodynamics,

dQ=dU+dW

=> dW=dQ-dU

dW= (Cp-Cv) ∆T

Now,

dQ:dU:dW= Cp ∆T : Cv ∆T : Cp-Cv

= Cp : Cv : Cp-Cv

= Cp/Cv : 1 : Cp/Cv - 1

= Y : 1 : (Y - 1) { Cp - Cv = Y}

= 7/5 : 1 : 2/5

= 7 : 5 : 2 (Ans)

Answer 2

Explanation:

dQ = dU + dW

In isobaric process

dQ = nCpdT

dU = nCVdT

dW = n(Cp – CV)dT

dQ : dU : dW = CP : CV : (CP – CV)

= 7/2R : 5/2R : R

= 7:5:2