# Homework Help: Dissociation energy of two particles-variables only equation-mastering physics

1. Oct 12, 2011

### Same-same

1. The problem statement, all variables and given/known data

The potential energy of two atoms in a diatomic molecule is approximated by U(r)= ar$^{-12}$ - βr$^{-6}$, where is the spacing between atoms and α and β are positive constants.
From earlier parts of the equation, it has been that determined that force between two atoms as a function of r is
F(r)= 12ar$^{-13}$ - 6βr$^{-7}$.
and that the equilibrium distance between them r$_{min}$ is
$\sqrt[6]{2α/β}$

Part a: Is the equilibrium stable? (My note: a stable equilibrium is a local minimum in a potential energy function. An unstable is a local maximum)

Part b:
Asssuming that the molecules are at said equlimbrium distance apart, find the energy required to bring the molecules an infiinite distance apart., and express this equation in terms of α and β

2. Relevant equations
W=∫ F dr= ΔU

3. The attempt at a solution
To part a:
Since potential energy is negatively related to displacement, and there are only two scenarios where Force could be 0 (at this distance or at infinity), I believe this equilibrium is unstable.

to part b:
∫$^{∞}_{equilibrium}$ (12ar$^{-13}$ - 6βr$^{-7}$) dx
which appears to be negative infinity, which I'm fairly certain is wrong.

Part b is what's really giving me problems. I'm not sure exactly what I'm looking for; I've honestly never dealt with dissociation before. I considered integrating the potential energy function from 0 to infinity, but that would give me the wrong units. Perhaps I'm looking for an energy equal to the maximum potential energy possible with the function?

Any help would be greatly appreciated

2. Oct 12, 2011

### DukeLuke

Part a: You might want to look up what a local maximum or minimum of a function is, and how to tell one from the other (hint: telling a maximum from a minimum has to do with the sign of the 2nd derivative at the max/min).

Part b: I don't think you are evaluating your limits correctly. Infinity to a negative power puts it in the denominator. If you divide a number by infinity what do you get?

3. Oct 12, 2011

### Same-same

"Part b: I don't think you are evaluating your limits correctly. Infinity to a negative power puts it in the denominator. If you divide a number by infinity what do you get? "

My thought process (which may very well be wrong) was as follows "At infinity the function should be zero, but the area under the curve to get there appears to be infinite."

4. Oct 12, 2011

### DukeLuke

You are right about the function approaching zero as it heads out to +- infinity, but that doesn't mean the area under the curve is infinite (unless you integrate over zero in the case of your function). Maybe if you plot it this will be easier to see.