# PV Diagrams (Curve v. Straight)

Why are some lines on a PV digram curved and some lines are straight?

Be more specific. Give an example of a PV diagram you don't understand.

Assuming that the no gas enters or leaves, there's a third relevant variable: temperature. If you heat something, it will either expand, increase in pressure, or a little of both.
edit: There are only these 3 variables if nothing enters or leaves the system if the gas is assumed to be ideal (the molecules neither take up space nor attract each other).

Last edited:
That still doesn't explain why the state at (Po, To) is curved when it goes to state (0.5Po, 2Vo). If it's directly proportional, shouldn't it be a decreasing straight line slope?

Pressure and temperature are not directly proportional. PV = constant if the temperature is constant.

So how do you know if one of the lines need to be straight or need to be curved?

You can just make a back-of-the-envelope graph. For example if the temperature is constant (which would be expected if everything happens slowly and there's poor insulation) you can assume PV = 1 (although any constant would do) and then make a rough graph with P being 1/4, 1/2, 1, 2, and 4 (V would be the inverse).

Redbelly98
Staff Emeritus
Homework Helper
So how do you know if one of the lines need to be straight or need to be curved?
That's a pretty general question, so here is a general answer.

If the line is curved, then a problem statement will either:
(1) tell you that the process is isothermal
(2) tell you that the process is adiabatic
(3) present you with a diagram that shows a curved line
or (4) provide some other information that allows you to conclude that the line is curved.

If the line is straight, then a problem statement will either:
(1) tell you that the process is at constant pressure (for a horizontal line)
(2) tell you that the process is at constant volume (for a vertical line)
(3) present you with a diagram that shows a straight line (could be horizontal, vertical, or slanted)
or (4) provide some other information that allows you to conclude that the line is straight

That's a pretty general question, so here is a general answer.

If the line is curved, then a problem statement will either:
(1) tell you that the process is isothermal
(2) tell you that the process is adiabatic
(3) present you with a diagram that shows a curved line
or (4) provide some other information that allows you to conclude that the line is curved.

If the line is straight, then a problem statement will either:
(1) tell you that the process is at constant pressure (for a horizontal line)
(2) tell you that the process is at constant volume (for a vertical line)
(3) present you with a diagram that shows a straight line (could be horizontal, vertical, or slanted)
or (4) provide some other information that allows you to conclude that the line is straight

That is so much clearer. Thank you!!!!