# B When does potential energy change?

#### Angela Liang

Why doesn't the potential energy of a substance change when the pressure changes? Or in what case will the potential energy of a substance change?

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#### drvrm

Why doesn't the potential energy of a substance change when the pressure changes? Or in what case will the potential energy of a substance change?
pl.elaborate about your substance.....is it a body or a system of particle or a liquid.?
if the external pressure has deformed it and the work done in deformation can be accounted for as capacity of the body to do mechanical work then the potential energy does change by applying pressure.
a common example is a spring of length l and one changes its length to say l-dl !

#### Angela Liang

pl.elaborate about your substance.....is it a body or a system of particle or a liquid.?
if the external pressure has deformed it and the work done in deformation can be accounted for as capacity of the body to do mechanical work then the potential energy does change by applying pressure.
a common example is a spring of length l and one changes its length to say l-dl !
How about gas in an enclosed system?

#### drvrm

How about gas in an enclosed system?
for gas as substance the relation with pressure changes is well anaysed using p, v, t relations and the energy of the system is related to temperature and work -energy theorems deal with it.....gas molecules are in incessant motion and a state of potential energy is not well defined.

#### Angela Liang

for gas as substance the relation with pressure changes is well anaysed using p, v, t relations and the energy of the system is related to temperature and work -energy theorems deal with it.....gas molecules are in incessant motion and a state of potential energy is not well defined.
Thanks. How to describe it using kinetic model of matter?

#### drvrm

Thanks. How to describe it using kinetic model of matter?
Kinetic theory is based on the postulates,/ assumptions like gases are composed of a large number of particles that behave like hard, spherical objects in a state of constant, random motion.
These particles move in a straight line until they collide with another particle or the walls of the container. the collisions are perfectly elastic
the interaction between them is during collision

The average kinetic energy of the gas particles depends only on the absolute temperature of the system.
you can see the details in any text book of physics.

How about gas in an enclosed system?
If you've been told that PE doesn't change you've been dealing with a theoretical gas called an ideal gas where any changes of PE are considered to be negligibly small. No gas is perfectly ideal but they approach ideal behaviour as the pressure approaches zero, in other words for very largeparticle separations. There are PE changes with real gases where the particles are more closely packed and to find out why this is so search for the interatomic / intermolecular force curve.

#### Khashishi

Potential energy of what object are you talking about?

#### Angela Liang

There is a fixed mass of gas. By increasing the volume, the internal energy doesn't change. So does the temperature change? Cos in my understanding, internal energy= internal KE + internal PE. Does increasing the volume increase the PE of the particles? If so, the internal KE should have decreased as the total internal energy remains unchanged?

#### drvrm

There is a fixed mass of gas. By increasing the volume, the internal energy doesn't change. So does the temperature change? Cos in my understanding, internal energy= internal KE + internal PE. Does increasing the volume increase the PE of the particles? If so, the internal KE should have decreased as the total internal energy remains unchanged?
when one is handling a gas in a container -it has three parameters pressure, volume and temperature and if you are changing two of them the third may be kept constant .
suppose the volume is being increased keeping temperature constant then the behaviour is pressure decreases .if however the volume is being increased keeping pressure constant then the temperature falls /decreases meaning thereby that the internal energy decreases .the role of PE in in internal energy of a gas is minimal as the energy is most of the time kinetic in nature -it has minimal time interacting with each other .
there are ideal gas laws governing the variation of P ,V and T.

#### robphy

Homework Helper
Gold Member
Generally, the "potential energy of a system" can be thought of as the "energy of configuration" [ a function of the configuration variables ].
When the configuration changes (e.g. a displacement, a volume, [and in the case of a capacitor, an amount of charge displaced from equilibrium]), that energy generally changes.

#### Angela Liang

when one is handling a gas in a container -it has three parameters pressure, volume and temperature and if you are changing two of them the third may be kept constant .
suppose the volume is being increased keeping temperature constant then the behaviour is pressure decreases .if however the volume is being increased keeping pressure constant then the temperature falls /decreases meaning thereby that the internal energy decreases .the role of PE in in internal energy of a gas is minimal as the energy is most of the time kinetic in nature -it has minimal time interacting with each other .
there are ideal gas laws governing the variation of P ,V and T.
Thank you very much:)

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