Why won't it solve?

Forum|Forum|7 years ago
February 4, 2019
2 replies
8805 views

I had to revert back to using freeware after hitting this snag with Prime 4.0... I don't understand what the difficulty is at step 5. Troubleshooting further down the page was successful after using new variables, so it seems to be a problem with my variables...

I even tried clearing the variables like my reference "Essential PTC Mathcad Prime 3.0" textbook suggests. Any help would be greatly appreciated in getting me back going...

1_~KCL-(conservation-of-electric-charge-or-mass).zip

Electrical_Engineering

Best answer by Werner_E

The problem is caused by Prime's infamous "auto labelling". Especially in combination with symbolic calculations this "feature" is usually more pain than help.

The I0 on the left hand side is labelled as "variable" but the I0 after "solve," is labelled "-", meaning "automatic". For the symbolics you are demanding to solve for a variable which does not even occur in the system of equations.

You have to change the label of one of the I0 to match the other, e.g. you may change the label of the second I0 from "-" to "variable". After doing so you get the solution you are looking for.

BTW, I wonder why you are using symbolics while you are just after numeric results. You may consider using Primes numerical methods like "root" or a "solve block". They are more rebust wrt labelling and can deal much better with units.

W

Werner_E

Answer

25-Diamond I

The problem is caused by Prime's infamous "auto labelling". Especially in combination with symbolic calculations this "feature" is usually more pain than help.

The I0 on the left hand side is labelled as "variable" but the I0 after "solve," is labelled "-", meaning "automatic". For the symbolics you are demanding to solve for a variable which does not even occur in the system of equations.

You have to change the label of one of the I0 to match the other, e.g. you may change the label of the second I0 from "-" to "variable". After doing so you get the solution you are looking for.

BTW, I wonder why you are using symbolics while you are just after numeric results. You may consider using Primes numerical methods like "root" or a "solve block". They are more rebust wrt labelling and can deal much better with units.

1_~KCL-(conservation-of-electric-charge-or-mass)-2.zip

L

LucMeekes

23-Emerald IV

One way to prevent such annoying situations is by changing the appearance of each of the 7 possible 'labels' such that they are readily distinguishable.

By default 'automatic' and 'variable' use exactly the same font, colour, and size. The same is true for 'system' and 'keyword', and their difference to 'function' isn't particularly obvious.

You might consider changing the colour of 'automatic' to orange, so that it sticks out.

While you're at it: Note that 'unit' is blue and italic, it should be upright (roman) style according to international standards...

Success!

Luc

W

Werner_E

25-Diamond I

@LucMeekes wrote:

You might consider changing the colour of 'automatic' to orange, so that it sticks out.

As far as I know you can't change the color of "automatic' but if you have a different appearance of all six possibilities you can easily see if the automatic had chosen the wrong choice.