# Potential energy separation curve

#### cyt91

Here's my understanding of the Force /Separation graph, I'm open to
criticism/ridicule if I've got it wrong:

If the two atoms are pushed together, then there is a repulsive force between
them of a positive magnitude. If they are separated by a distance r, then the
two forces (repulsive/attractive) are equal but opposite and the atoms are in a
state of equilibrium. At a separation greater than r, the force is that of
attraction, and if the separation is greater than 2r, then the attraction
tends to zero.

Potential Energy/Separation:

If the two atoms are separated by an infinite distance, then there is no
attraction between them, and there is no Potential Energy. But as they come
closer together, potential energyhttp://www.physicsforums.com/library.php?do=view_item&itemid=269 decrease until at a distance of r, potential
energy is at a minimum. If the separation is reduced further, the line of
Potential Energy passes through zero.

Why is the potential energy zero? What actually happens to the 2 atoms when the potential energy becomes zero?

I'm confused, can anyone clear this up for me please?

Thanks.

#### werehk

PHF Hall of Fame
The potential energy is defined to be 0J at infinity. By definition, potential energy of an atom is the energy required to move one atom from infinity to that point. So when you move one atom from same point of infinity to same point of infinity, the displacement is 0m and the work done is then 0J.
In fact, it is just a matter of definition and this definition is generally accepted.

#### cyt91

The potential energy is defined to be 0J at infinity. By definition, potential energy of an atom is the energy required to move one atom from infinity to that point. So when you move one atom from same point of infinity to same point of infinity, the displacement is 0m and the work done is then 0J.
In fact, it is just a matter of definition and this definition is generally accepted.
Ok....but,referring to the U-r graph, when the potential is zero (at the x-intercept) isn't the 2 atoms close together? If they are close together, they are not separated by an infinite distance. Another point to note is that the potential energy of the system of 2 atoms becomes positive when separation,r between the atoms is further decreased past the x-intercept.

#### werehk

PHF Hall of Fame
For the U-r graph with potential energy = 0J , it doesn't necessarily imply that they are separted at infinity.But it is just that the energy required to move another atom close to another atom from infinity to that point is 0J. The positive poential energy means that work has to be done to overcome the repulsion forces between the electron clouds of two atoms in order to move that atom so close to each other.

Note one very importanat thing which is a force always exists in a direction opposite to the drop in potential energy. Just like gravitational pull is always pointing towards the centre of the Earth

#### cyt91

Thanks.

Note one very importanat thing which is a force always exists in a direction opposite to the drop in potential energy. Just like gravitational pull is always pointing towards the centre of the Earth
Shouldn't it be "a force always exists in a direction opposite to the direction of increasing potential energy" since the gravitational potential energy of a mass increases as we move the mass further from the centre of the earth while gravitational pull acts towards the centre of the earth?

#### werehk

PHF Hall of Fame
You are right. Sorry for the typing mistake.