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A
amcmillan1-Visitor
1-Visitor
November 6, 2013
Question

Coupled differential equations issue

  • November 6, 2013
  • 2 replies
  • 5116 views

I Am having an issue with writing down coupled differential equations for radioactive decay! What is know is

dN/dt=-λN

so N=N0exp-λt

where λ=ln(2)/τ where τ is the half life of each element

I calculated all of the λ values and was then asked to write down the couples differential equations for the decay/creation of each element

dNn/dt=λn-1*Nn-1 -λnNn

NNot sure how to do this on MathCad!

They will be getting solved via rkfixed function but when I input the equations in the form above I get "a name is required here" error

There is an image below with my question sheet! Any help would be great

2 replies

W
Werner_E25-Diamond I
25-Diamond I
November 6, 2013

Maybe you would like to show what you have done so far by posting your Mahcad sheet.

A
amcmillan1-VisitorAuthor
1-Visitor
November 6, 2013

HHow do I do that?

W
Werner_E25-Diamond I
25-Diamond I
November 6, 2013

Alison McMillan wrote:

HHow do I do that?

The same way as you attached your pic.

But in a reply you have to switch to "advanced editor" first. You do so by clicking "Use advanced editor" at the upper right.

Now you have the option to attach files at the end.

There is no need to zip the files yourself as this will be done automatically.

    W
    Werner_E25-Diamond I
    25-Diamond I
    November 6, 2013

    Mathcad offers a more natural way to write and solve differential equations, namely via a solve block and the function odesolve(). You can select the numeric algorithm used by right clicking the word Odesolve. Normally there should be no reason to change the default "Adams/BDF"- Interestingly enough even chosing "Fixed" gives a satisfacory result this time.

    Again also this function unfortunately does not support units in Mathcad 15 and below. One of the few advantages of Prime is that you are allowed to use units with odesolve, too.

    decay3.png

    EDIT: Again, see Alans post. It should NOT be surprising that "Fixed" works here. The reason is that I had set the number of points to 10^4 in this sheet (without being aware of 😉

      A
      amcmillan1-VisitorAuthor
      1-Visitor
      November 6, 2013

      Thanks so much guys, you have been great help:)

      I think I understand where I was going wrong! MathCad is extremely tedious at time especially for a beginner like me!

      Thanks again much appreciated

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