Science:Math Exam Resources/Courses/MATH 180/December 2017/Question 06

MATH 180 December 2017

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Question 06

The Buckingham potential $$V(r)={\frac {A}{e^{Br}}}-{\frac {C}{r^{6}}},\quad r>0,$$ where ${\textstyle A}$, ${\textstyle B}$ and ${\textstyle C}$ are positive constants, is a refinement of the Lennard-Jones potential that describes the potential energy ${\textstyle V(r)}$ of a diatomic molecule where the atoms are a distance ${\textstyle r}$ apart. Find and justify all vertical and horizontal asymptotes of ${\textstyle V(r)}$.

Make sure you understand the problem fully: What is the question asking you to do? Are there specific conditions or constraints that you should take note of? How will you know if your answer is correct from your work only? Can you rephrase the question in your own words in a way that makes sense to you?

If you are stuck, check the hints below. Read the first one and consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it! If after a while you are still stuck, go for the next hint.

Hint 1
Near the vertical asymptotes, the function tends to (plus or minus) infinity.

Hint 2
The horizontal asymptotes are approached by the function for very positive or negative values of ${\textstyle r}$.

Checking a solution serves two purposes: helping you if, after having used all the hints, you still are stuck on the problem; or if you have solved the problem and would like to check your work. If you are stuck on a problem: Read the solution slowly and as soon as you feel you could finish the problem on your own, hide it and work on the problem. Come back later to the solution if you are stuck or if you want to check your work. If you want to check your work: Don't only focus on the answer, problems are mostly marked for the work you do, make sure you understand all the steps that were required to complete the problem and see if you made mistakes or forgot some aspects. Your goal is to check that your mental process was correct, not only the result.

Solution

Found a typo? Is this solution unclear? Let us know here. Please rate my easiness! It's quick and helps everyone guide their studies. Observe that if $V(r)={\dfrac {A}{e^{Br}}}-{\dfrac {C}{r^{6}}}$ approaches positive/negative infinity, then either ${\dfrac {A}{e^{Br}}}$ or $-{\dfrac {C}{r^{6}}}$ approaches positive/negative infinity, as $r\to a$, for some $a$. In other words, if $r=a$ is a vertical asymptote of $V$, then $r=a$ is a vertical asymptote of ${\dfrac {A}{e^{Br}}}$ or of $-{\dfrac {C}{r^{6}}}$. Recall that the exponential function can never be zero, so ${\dfrac {A}{e^{Br}}}$ is finite for any $r$. On the other hand, the rational function $-{\dfrac {C}{r^{6}}}$ is not defined at $r=0$, which makes $r=0$ a candidate for a vertical asymptote of $-{\dfrac {C}{r^{6}}}$ and hence of $V$. In fact, $$\lim \limits _{r\to 0^{+}}V(r)=\lim \limits _{r\to 0^{+}}\left({\dfrac {A}{e^{Br}}}-{\dfrac {C}{r^{6}}}\right)=A-\lim \limits _{r\to 0^{+}}{\dfrac {C}{r^{6}}}=-\infty .$$ So ${\textstyle r=0}$ is the only vertical asymptote. For horizontal asymptotes, since ${\textstyle V(r)}$ is only defined for ${\textstyle r>0}$, we find that $$\lim \limits _{r\to +\infty }V(r)=\lim \limits _{r\to +\infty }\left({\dfrac {A}{e^{Br}}}-{\dfrac {C}{r^{6}}}\right)=0-0=0.$$ So the only horizontal asymptote is ${\textstyle V=0}$. Answer: the vertical asymptote is ${\textstyle {\color {blue}r=0}}$ and the horizontal asymptote is ${\textstyle {\color {blue}V=0}}$.