P-V Diagram

P-V Indicator Diagram

  • Only two thermodynamic variables are sufficient to describe a system because third variable can be calculated from equation of state of the system.
  • P-V Indicator Diagram is just a graph between pressure and volume of a system undergoing an operation.
  • When a system undergoes an expansion from state A (P1 V1) to a state B (P2V2) its indicator diagram is shown as follows.
    P-V Indicator Diagram in case of expansion
  • In case of compression system at state A(P1 V1) goes to a state B(P2V2) its P-V diagram is as follows.

    P-V  Diagram in case of compression
  • Intermediate states of system are represented by points on the curve.
  • The pressure volume curve for a fixed temperature is called isotherm.

Solved Example of PV diagram

Question 1
Consider P-V diagram for an ideal gas

Draw the corresponding T-P diagram
$P= \frac {constant}{V}$
$PV= constant$
So it means temperature is constant.So here is the TP diagram
T-P diagram

Question 2
Draw the corresponding P-V diagram for the given T-V diagram
T-V diagram
Since TV diagram is a straight line
T/V =constant
it means Pressure is constant

Question 3
Draw the corresponding P-V diagram for the given T-V diagram

Since T is constant
PV diagram should be isotherm

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