I need some help debugging a calculation command

Question

I am trying to make a calculation but I have a few bugs and I do not understand them/how to fix them. Below is my syntax, the output, and the error messages. I am trying to calculate the point in my plot (done successfully in another window) where the areas are equal. I appreciate any help/advice.

p[v_]:=(8*.95)/(3*v-1)-3/v^2
Plot[p[v],{v,.5,3},PlotRange->{{0,4},
{.5,1}},
AxesLabel->{"v","p"},
PlotLabel->"van der Waals Isotherm for T/T_c=0.95"]
g[v_]:=-(.95)Log[3v-1]+.95/(3v-1)-9/(4v)
ParametricPlot[{p[v],g[v]},{v,.5,2.5},
AxesLabel->{"p","G/NkT_c"},
PlotLabel->"Gibbs free energy"]
pInt[v_]:=(8/3)*.95*Log[3*v-1]+3/v;
AreaDiff[p0_,v1guess_,v2guess_]:=(v1=FindRoot[p[v]==p0,{v,v1guess}][[1,2]];
v2=FindRoot[p[v]==p0,{v,v2guess}][[1,2]];
pInt[v2]-pInt[v1]-p0*(v2-v1));
FindRoot[AreaDiff[p0,.7,2]==0,{p0,.8,.82}]

Welcome to Mathematica.SE! I suggest the following: 1) As you receive help, try to give it too, by answering questions in your area of expertise. 2) Take the tour! 3) When you see good questions and answers, vote them up by clicking the gray triangles, because the credibility of the system is based on the reputation gained by users sharing their knowledge. Also, please remember to accept the answer, if any, that solves your problem, by clicking the checkmark sign! — Michael E2, Apr 04 '22 at 22:06
People here generally like users to post code as Mathematica code instead of just images or TeX, so they can copy-paste it. It makes it convenient for them and more likely you will get someone to help you. You may find the meta Q&A, How to copy code from Mathematica so it looks good on this site, helpful — Michael E2, Apr 04 '22 at 22:06
Define it with p0_?NumericQ. Seehttps://mathematica.stackexchange.com/questions/18393/what-are-the-most-common-pitfalls-awaiting-new-users/26037#26037 — Michael E2, Apr 04 '22 at 22:07
It's best to add the missing code to the question, rather than leave it in the stream of comments. — Michael E2, Apr 04 '22 at 23:03
Edit your question to include copy and pasteable code (InputForm). Include all code necessary to produce results (i.e., include that definition of p`). — Bob Hanlon, Apr 04 '22 at 23:10

score 1 · Answer 1 · answered Apr 05 '22 at 01:37

1

I would use Module[] in the definition of AreaDiff[]. Also, p0_?NumericQ and similar for other variables. See the code below.

p[v_] := (8*.95)/(3*v - 1) - 3/v^2
Plot[p[v], {v, .5, 3}, PlotRange -> {{0, 4}, {.5, 1}}, 
 AxesLabel -> {"v", "p"}, 
 PlotLabel -> "van der Waals Isotherm for T/T_c=0.95"]
g[v_] := -(.95) Log[3 v - 1] + .95/(3 v - 1) - 9/(4 v)
ParametricPlot[{p[v], g[v]}, {v, .5, 2.5}, 
 AxesLabel -> {"p", "G/NkT_c"}, PlotLabel -> "Gibbs free energy"]
pInt[v_] := (8/3)*.95*Log[3*v - 1] + 3/v;
AreaDiff[p0_?NumericQ, v1guess_?NumericQ, v2guess_?NumericQ] := 
  Module[{v1, v2}, (v1 = FindRoot[p[v] == p0, {v, v1guess}][[1, 2]];
    v2 = FindRoot[p[v] == p0, {v, v2guess}][[1, 2]];
    pInt[v2] - pInt[v1] - p0*(v2 - v1))];
FindRoot[AreaDiff[p0, .7, 2] == 0, {p0, .8, .82}]

answered Apr 05 '22 at 01:37

mszynisz

824
5
9

Hey, @mszynisz thanks for your reply. I fixed the syntax and I still get an error message. COuld it be an issue with my software? – Dwight Shelford Apr 05 '22 at 01:51
What's the error message? Remember to clear all the variables before running my code. – mszynisz Apr 05 '22 at 01:56
adding image to original question – Dwight Shelford Apr 05 '22 at 01:59
Close Mathematica completely, open a clean notebook, and run the code? – mszynisz Apr 05 '22 at 02:03
3

@DwightShelford, mszynisz -- probably forgot ClearAll[AreaDiff]; before defining the function AreaDiff. I always start every definition of every function with this. Saves headaches. – Michael E2 Apr 05 '22 at 04:12

I need some help debugging a calculation command

1 Answers1