pv diagram m kyu curve banta hai
Answers
Explanation:
Consider a gas sealed in a container with a tightly fitting yet movable piston as seen below. We can do work on the gas by pressing the piston downward, and we can heat up the gas by placing the container over a flame or submerging it in a bath of boiling water. When we subject the gas to these thermodynamics processes, the pressure and volume of the gas can change.
A convenient way to visualize these changes in the pressure and volume is by using a Pressure Volume diagram or PV diagram for short. Each point on a PV diagram corresponds to a different state of the gas. The pressure is given on the vertical axis and the volume is given on the horizontal axis, as seen below.
Every point on a PV diagram represents a different state for the gas (one for every possible volume and pressure). As a gas goes through a thermodynamics process, the state of the gas will shift around in the PV diagram, tracing out a path as it moves (as shown in the diagram below).
Being able to decode the information shown in a PV diagram allows us to make statements about the change in internal energy \Delta UΔUdelta, U, heat transferred QQQ, and work done WWW on a gas. In the sections below, we'll explain how to decipher the hidden information contained in a PV diagram.
Note: Unless otherwise specified, we will assume that the work WWW refers to the work done on the gas.
How do we determine the sign of the work done from a PV diagram?
Let's say our gas starts out in the state shown in the PV diagram below.
If we press the piston downward, the volume of the gas will decrease, so the state must shift to the left toward smaller volumes (as seen in the diagram below). Since the gas is being compressed we can also say for sure that positive work WWW is being done on the gas.
Similarly, if we let the gas expand, pushing the piston upward, the volume of the gas will increase, so the state must shift to the right toward larger volumes (as seen in the diagram below). Since the gas is expanding we can also say for sure that negative work WWW is being done on the gas.
So if we ever see a state shifting to the left on a PV diagram we can say for sure that the work done on the gas was positive. Similarly, if we ever see a state shifting to the right on a PV diagram we can say for sure that the work done on the gas was negative.
How do we determine the magnitude of the work done from a PV diagram?
The work done during a thermodynamic process is equal to the area under the curve as seen in the diagram below.
The reason why work is equal to the area under the curve is that,
by gas
=−W
on gas
W, start subscript, start text, b, y, space, g, a, s, end text, end subscript, equals, minus, W, start subscript, start text, o, n, space, g, a, s, end text, end subscript.
It doesn't matter what shape the path takes, the area under the curve will still represent the work done. For any curved path we can imagine breaking the area into an infinite amount of infinitesimally thin rectangles.
The area of each rectangle would represent the work done during each infinitesimal step, and the sum of the areas would represent the total work done for the entire process.
[What if there is a closed path for a cyclic process?]
It should be said that we are always going to assume these processes are taking place slowly enough that the entire gas can be at thermodynamic equilibrium at every moment (i.e. the same temperature throughout the gas). If this seems dubious to you, you're right to question it. However, even though basically no real world processes will exactly satisfy this requirement, our ability to model many thermodynamic processes are not fatally jeopardized by this lack of adherence to ideal circumstances.