# B Negative potential energy vs Positive potential energy

#### Kaushik

Summary
Negative potential energy vs Positive potential energy
What is the physical difference between negative potential energy and positive potential energy?

PS- When I was reading about surface tension I came through the following line - ' The molecules at bulk inside the fluid has lesser negative potential energy than those on the surface.' In this context, what does lesser negative potential energy mean?

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#### ZapperZ

Staff Emeritus
2018 Award
Summary: Negative potential energy vs Positive potential energy

What is the physical difference between negative potential energy and positive potential energy?
One is attractive, and has the possibility of having a bound state, the other is repulsive and probably has no bound state.

Zz.

#### Kaushik

One is attractive, and has the possibility of having a bound state, the other is repulsive and probably has no bound state.

Zz.
So the one with positive potential energy is not stable?

#### ZapperZ

Staff Emeritus
2018 Award
So the one with positive potential energy is not stable?
Define "stable".

Zz.

#### Kaushik

Define "stable".

Zz.
Stable means that the particle is confined to a space. So when it is less stable, it is confined within a larger area.
PS - this is my understanding

#### ZapperZ

Staff Emeritus
2018 Award
Stable means that the particle is confined to a space. So when it is less stable, it is confined within a larger area.
PS - this is my understanding
That's not my definition of "stable". Why not just use the term "bound state" the way I stated it? An unbound state is not "unstable".

"confined within a larger area" is still a bound state! "large" is relative.

Zz.

#### Kaushik

That's not my definition of "stable". Why not just use the term "bound state" the way I stated it? An unbound state is not "unstable".

"confined within a larger area" is still a bound state! "large" is relative.

Zz.
I don't know about bound state.

#### ZapperZ

Staff Emeritus
2018 Award
I don't know about bound state.
I also don't know what you don't know.

Zz.

#### ZapperZ

Staff Emeritus
2018 Award
What is BOUND STATE?
See, this is another example where you try to go one step forward, but we end up having to take 2 steps back. It happens when you are trying to ask something beyond what you can understand, or if you are asking something without proper context. In this case, you notice that you never included any kind of context of what you mean by "positive" and "negative" potential energy.

So there is a HUGE missing info here, especially with regard to whether this is just classical physics idea of negative and positive potential energy, or if this were to included quantum mechanics.

Because I don't know what you don't know, and you don't seem to want to let us know what you know and don't know, I will have to prod you. I hate to now have to explain what a "bound state" is because I can already anticipate that I will have to then explain my explanation, which is what I'm doing now (1 step forward, 2 steps back).

So instead, I will ask you to go back to your original question. Why don't you sketch out the profile of what you THINK is a positive and negative potential energy, i.e. sketch out a graph of PE versus distance from the source.

Zz.

#### Dale

Mentor
@Kaushik If a proton and an electron are in the ground state then they are expected to be close together, or bound, and importantly the expected distance does not change over time. As you add internal energy to the system the electron is expected to be further away from the proton, but that expected distance does not change over time so it is still bound. If you add enough energy then the system becomes unbound and the expected distance increases arbitrarily over time. The hydrogen atom is now ionized.

#### Kaushik

See, this is another example where you try to go one step forward, but we end up having to take 2 steps back. It happens when you are trying to ask something beyond what you can understand, or if you are asking something without proper context. In this case, you notice that you never included any kind of context of what you mean by "positive" and "negative" potential energy.

So there is a HUGE missing info here, especially with regard to whether this is just classical physics idea of negative and positive potential energy, or if this were to included quantum mechanics.

Because I don't know what you don't know, and you don't seem to want to let us know what you know and don't know, I will have to prod you. I hate to now have to explain what a "bound state" is because I can already anticipate that I will have to then explain my explanation, which is what I'm doing now (1 step forward, 2 steps back).

So instead, I will ask you to go back to your original question. Why don't you sketch out the profile of what you THINK is a positive and negative potential energy, i.e. sketch out a graph of PE versus distance from the source.

Zz.
When I was reading about surface tension I came through the following line - ' The molecules at bulk inside the fluid has lesser negative potential energy than those on the surface.' In this context, what does lesser negative potential energy mean?

My interpretation -
The molecules inside the fluid are surrounded by other molecules. Those molecules attract the one at the center. So the net force is 0. It is 'stable' i.e., it does not move. Hence, it is considered to have less energy. Where as the molecules at the center experience a net downward force. This means that the molecules are 'less stable'. Due to this extra energy, they try to minimize their surface (using the extra energy) to get minimum energy possible. If we are to increase the surface area then we must do work on it.

#### Dale

Mentor
what does lesser negative potential energy mean?
Less negative means greater. So the potential energy of the molecules in the bulk is greater than those on the surface.

#### Kaushik

Less negative means greater. So the potential energy of the molecules in the bulk is greater than those on the surface.
So we must do some work to displace the molecule? The energy required to move the molecules in bulk is greater than those on the surface. Isn't?

#### Dale

Mentor
So we must do some work to displace the molecule? The energy required to move the molecules in bulk is greater than those on the surface. Isn't?
It would require energy to move the molecule from the surface to the bulk. It would not necessarily require energy to move surface molecules around the surface or bulk molecules within the bulk.

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