Why does the PE of particles during steam condensation decrease?

[shirozack]

Homework Statement

Why does the PE of particles during steam condensation decrease?

Relevant Equations

na

The answer says it decreases but shouldn't it increase? according to the kinetic model, KE accounts for particle vibrations. PE accounts for the attractive forces between particles.

Since condensation means gas -> liquid, it means the particles become closer and hence the attractive forces between them become stronger? so PE should increase right?

OR

Since steam is losing thermal energy to the surroundings, it must mean that the resulting particles have lesser energy. Since temperature is constant, KE is constant, so PE decreases? i suppose this accounts for a bigger drop than the 1st part?

thanks.

 

[haruspex]

the attractive forces between them become stronger? so PE should increase right?

As a meteorite falls to Earth, does the gravitational attraction get stronger or weaker? Does its GPE increase or decrease?

 

[shirozack]

it decreases ?

but the KE model says solid have the greatest potential energy because the particles are close together, as opposed to gases where they are spread apart so their PE is negligible. Since it is going from gas -> liquid, shouldn't the PE increase?

 

[haruspex]

but the KE model says solid have the greatest potential energy because the particles are close together,

That does not follow, as I showed with the meteorite analogy. The force is proportional to the rate of loss of PE as the distance decreases, not to the value of the PE.
Consider two particles that attract each other. At a given separation they have a certain PE. If you pull them apart you have to do work, so you are increasing the PE, but the force of attraction may decrease (as with an inverse square law) or increase (if they are the ends of an elastic string).

 

[shirozack]

oh so in a solid, the forces between particles are very strong, but the PE of the particles are weak.

in a gas, the forces between the particles are weak, but the PE is strong?

 

[nasu]

The value of PE is not meaningful anyway. Only the change in PE matters. If we take the PE in the gas to be zero, the PE in solid has a negative value. So, you need to provide energy to the solid to separate the atoms and bring them from negative PE to zero in the gas state.

 

[haruspex]

oh so in a solid, the forces between particles are very strong, but the PE of the particles are weak.

in a gas, the forces between the particles are weak, but the PE is strong?

I would not describe an energy level as weak or strong. One level can be lower or higher than another.

 

[shirozack]

this is confusing. my textbook says in solids, each particle stores a large amount of PE due to strong intermolecular forces. in gases, due to negligible intermolecular forces, the PE is negligible.

so is this wrong?

the textbooks says PE is the attractive forces between each particle.
when we say PE in solid or gas, we are talking about this attractive force between particles right?

 

[haruspex]

this is confusing. my textbook says in solids, each particle stores a large amount of PE due to strong intermolecular forces. in gases, due to negligible intermolecular forces, the PE is negligible.

so is this wrong?

It is certainly misleading. What they have in mind is that so long as the gas remains a gas there is little variation in the PE because the forces, at those separations, are weak. But on transition to a solid, by freezing, significant PE would be released, making the remaining PE lower.

the textbooks says PE is the attractive forces between each particle.

That's really bad, if that is exactly what it says. PE is an energy, not a force. It relates to the attractive forces in that if the attractive force at separation $x$ is$F(x)$ then to increase the separation by $\Delta x$ requires $F\cdot\Delta x$ of PE to be added.

 

[nasu]

this is confusing. my textbook says in solids, each particle stores a large amount of PE due to strong intermolecular forces. in gases, due to negligible intermolecular forces, the PE is negligible.

so is this wrong?

the textbooks says PE is the attractive forces between each particle.
when we say PE in solid or gas, we are talking about this attractive force between particles right?

"Amount of PE" is meaningless until you specify the reference level for PE. The only thing that is physically meaningful is that you need to provide energy for the system to go from solid to gas. So the PE inceases during this transformation. You can make the absolute value to be zero either in the gas state or in the solid state or anywhere in between.

 

[jbriggs444]

the textbooks says PE is the attractive forces between each particle.

If we have an attractive force then the force will tend to bring the particles together. This reduces potential energy as the particles get closer to one another.

Like a ball rolling down a hill, a stretched spring compressing or a positive and negative charge approaching one another. In each such case, potential energy decreases. Meanwhile kinetic or thermal energy increases to compensate. Energy is conserved, you know.


If you can arrange to get a set of particles that are attracted to one another to expand, you can increase potential energy. This obviously requires an energy input of some sort. You may have to turn on the burner to get the water in the kettle to boil away. The vapor will have more potential energy than the liquid. If the vapor is allowed to cool, we get its potential energy back as thermal energy released due to the state change.

 

[shirozack]

ok i am starting to understand.
"when a liquid is vaporising to gas, the temp remains constant. KE is constant, but PE of particles is increasing." We have to put in energy to separate the particles, resulting in more PE.

but then i saw another question that confused me again.

"when a liquid is heated, the temperature rises and KE of particles rise. PE of particles however remain the same."

why does the PE of particles remain constant?
I understand why KE rises because according to the kinetic model of matter, temp is the measure of the average of the kinetic energy of particles. but what about PE? why does it not increase also?

 

[haruspex]

ok i am starting to understand.
"when a liquid is vaporising to gas, the temp remains constant. KE is constant, but PE of particles is increasing." We have to put in energy to separate the particles, resulting in more PE.

but then i saw another question that confused me again.

"when a liquid is heated, the temperature rises and KE of particles rise. PE of particles however remain the same."

why does the PE of particles remain constant?
I understand why KE rises because according to the kinetic model of matter, temp is the measure of the average of the kinetic energy of particles. but what about PE? why does it not increase also?

You are right that the PE will also change, but in most circumstances the PE change will be insignificant compared to the KE change. Exceptions to that are likely to be when close to freezing point. Note how water expands as it cools below 4°C. That, I would guess, is because the crystal structure of ice has a lower PE than the denser liquid form at 4°C.

 

[shirozack]

if this was a high school exam, what should i put as the answer?

the official exam answer given is "there is no change in PE when liquid is heated. only KE increases as temp rise."
my textbook says "KE & PE increases when liquid is heated and temp rises."

 

[haruspex]

if this was a high school exam, what should i put as the answer?

the official exam answer given is "there is no change in PE when liquid is heated. only KE increases as temp rise."
my textbook says "KE & PE increases when liquid is heated and temp rises."

Can you contact the exam author?

For future exams, if it is multiple choice, go with the textbook and, if necessary and possible, challenge the exam official answer later. If an essay, use the textbook answer but qualify it by noting that the PE increase is generally much less.