Units and solving of system of linear equation - one problem or bug

[ValeryOchkov]

One task (see pictures and load files Mathcad Prime 2.0):

All is OK!

But it is a linear system and it is better use the lsolve function:

Mathcad Prime can not works here with units and I must do so:

It it a bug in new possibility of Mathcad Prime (different dimensions in array) or I do something wrong? Thanks!

[jkucan]

In a mixed unit matrix, you need to assign the units to 0's explicitly.

Define:

and

--Jakov

[ValeryOchkov]

Thanks, Jakov!

But in the Solve block 0 and 0*Unit is equal - it was my...

Better use not V, ohm and A but mV, mohm and mA

And second:

[Werner_E]

Not sure what the problem should be? v*M^(-1) can't work - using units or not.

[ValeryOchkov]

Yes!

0 and 0V is not equal.

M^-1*v and v*M^-1 is not equal too

[Werner_E]

M^-1*v and v*M^-1 is not equal too

It can't be!!

You cannot multiply a n*1 vector with n*n matrix!

But you can multiply a n*n with a n*1 or a 1*n with a n*n as shown.

And yes, I agree about the distinction between 0 and 0V. It would be convenient not having to specify units in that case.

[ValeryOchkov]

OK!

Can you do so in Mathcad? Sorry in Mathcad Mad House - http://communities.ptc.com/groups/mathcad-paradoxes

[Werner_E]

Valery Ochkov wrote:

OK!

Can you do so in Mathcad?

Sure, but wouldn't the following be more appropriate for this thread?

Feel free to change the values in the assignments. Seems it is what you are looking for 😉

[ValeryOchkov · Accepted solution]

I was a students nd I was wondering why there can not be to change by place factors.

Sometime I ask this of mathematicians now too. They do not tell me I'm a fool - they tell me that I have an engineering approach to mathematics

Why!?

See alphabet!

[jkucan]

PTC Mathcad is smart enough (sometimes) to infer that a 0 should have particular unit. PTC Mathcad 15.0 can always do that thanks to Static Unit Checking. However, static unit checking prevents mixed units in a matrix. This is why PTC Mathcad Prime 2.0 uses Dynamic Unit Checking. The compromise: sometimes (and I recommend always) one must specify units with 0's.

--Jakov

[Werner_E]

The compromise: sometimes (and I recommend always) one must specify units with 0's.

--Jakov

I guess thats bearable, considering we can have mixed units in matrices as a reward. And sometimes it also has advantages of being forced into a certain consistency, too.

Thanks for the clarification.

[ValeryOchkov]

Jakov Kucan wrote:

...mixed units in a matrix.
--Jakov

I think more correct to say not mixed units in a matrix but mixed dimensions.

Mixed units are (may be) m, mm, km or W, kW, MW etc

Mixed dimensions are V, A, km, W etc

[Werner_E]

Mixed dimensions are V, A, km, W etc

No, these are still units. Dimensions are Voltage, Current, Length, Power, etc.

I won't argue with a native speaker, but I think we could say the Prime2 allows mixed units in a matrix, even units of different dimensions.

[ValeryOchkov]

Werner Exinger wrote:

Mixed dimensions are V, A, km, W etc

No, these are still units. Dimensions are Voltage, Current, Length, Power, etc.

I won't argue with a native speaker, but I think we could say the Prime2 allows mixed units in a matrix, even units of different dimensions.

Mixed dimensions are 1V,1A, 1m, 1W etc

Sorry, no - mixed dimensions in a matrix!

[Werner_E]

voltage --> dimension

V, mV,.. --> unit

1 V --> physical quantity

[ValeryOchkov]

1. V=1V

2. Was ist das 0V?

kg is an unit

but kg water/steam and kg gas (in gas-steam power plant for example) - one dimension but two physical quantity.

[Werner_E]

1. V=1V

In my opinion thats not the same. Its only the same in Mathcad, because MC handles units like variables.

The same applies for 0V and 0. While you may "simplify" 0V to 0 you loose information in doing so. On the other hand thats the reason Mathcad can add the correct unit with static unit checking. While it may be cumbersome it it sure good habit to add the unit even if the absolute measure is zero.

A physical quantity as I understand it consists of a dimensionsless number, the absolute measure and a unit.

A dimension as understood by Mathcad is sort of a catagory (length, pressure, current, ...). So you cannot put a dimension (in that sense) in a matrix but just a quantity of that dimensions, which would consist of absolutre measure and unit.

But whatever you may want to call it, the good thing is that we CAN put different units rsp. different phys.quantities in a matrix.

The bad thing however is that we are "moderated" again 😞

[ValeryOchkov]

but kg water/steam and kg gas (in gas-steam power plant for example) - one dimension but two physical quantity.

see one article about same calculation http://twt.mpei.ac.ru/ochkov/WSPHB/Web-function-Power-Eng.pdf