On 22 February 2012 19:24, Matthew Rocklin <[email protected]> wrote: > This is true in most unit systems. Consider this image describing the > possibilities to describe a Joule > > http://upload.wikimedia.org/wikipedia/en/math/6/5/7/65761e9c7ec650ec33b3f3af5f7124fd.png I did not meant that. I meant that in electrodynamics even the base units can not be defined in a natural unique manner. I will try to find an old problem set with nice examples that I had, but it won't be anytime soon. > > Probably your internal representation would consist only of the most > natural/basic units (these have a name that I've forgotten) like meters, kg, > seconds, coulombs, etc.... When printing though there is the choice of how > you display these. Perhaps there is some way to find the simplest > representation of the units in this case (least complex expression?) My > guess is that this is likely to be annoying/messy. > > > On Wed, Feb 22, 2012 at 6:03 PM, [email protected] > <[email protected]> wrote: >> >> I have a question about the automatic addition of units to expressions >> written in natural units (i.e. dimensionless). If I recollect >> correctly there are too many degrees of freedom in the definition of >> the units for fields in electrodynamics (and probably in other >> theories too) therefore it is impossible to find a unique set of units >> for the expression. Can you comment on this? Maybe I am wrong or maybe >> I have just misunderstood what you are proposing. >> >> Stefan >> >> On 22 February 2012 16:27, Sergiu Ivanov <[email protected]> >> wrote: >> > On Mon, Feb 20, 2012 at 11:21 PM, Harold E. <[email protected]> >> > wrote: >> >> On 20 fév, 15:19, Aaron Meurer <[email protected]> wrote: >> >>> >> >>> If you're not going to use units until the very end, they're not very >> >>> useful. Granted, there is a certain level of physical correctness >> >>> that is lost without them, but I think the whole point of having a >> >>> strong units system is that it can do dimensional analysis for you. >> >>> This means that you use them in every point in the calculation, so >> >>> that at the end, you know exactly the units of the end result. >> >> >> >> I'm not sure to understand exactly what you mean, but I answer anyway, >> >> so say me if I'm wrong. >> >> My main point was that during computation, you can write either 0.7*c >> >> or just 0.7 when you have a velocity, and, in the first case, having >> >> method to see if we have to add c factors. It means as you say that >> >> the system can do dimensional analysis. But also maybe it's not very >> >> useful, and so we can encourage user to be consistent when using units >> >> if they want detailed results. >> >> I did not yet have thought to this point in deep, but I think it's >> >> possible to get something usable. >> > >> > I'm inclined to strongly agree with Aaron. Dimensional analysis is >> > often a relatively cheap way to check the physical meaning of the >> > computation, as far as I can remember. I think that this feature >> > would not only be very useful, but it also should be made imperative, >> > i.e., the user should be forced to use units in all cases where this >> > is appropriate. >> > >> > On the other hand, being strict with specifying units will simplify >> > your life as the developer of this module: you will always know that >> > constants are dimensionless, whatever the context. >> > >> > Sergiu >> > >> > -- >> > You received this message because you are subscribed to the Google >> > Groups "sympy" group. >> > To post to this group, send email to [email protected]. >> > To unsubscribe from this group, send email to >> > [email protected]. >> > For more options, visit this group at >> > http://groups.google.com/group/sympy?hl=en. >> > >> >> -- >> You received this message because you are subscribed to the Google Groups >> "sympy" group. >> To post to this group, send email to [email protected]. >> To unsubscribe from this group, send email to >> [email protected]. >> For more options, visit this group at >> http://groups.google.com/group/sympy?hl=en. >> > > -- > You received this message because you are subscribed to the Google Groups > "sympy" group. > To post to this group, send email to [email protected]. > To unsubscribe from this group, send email to > [email protected]. > For more options, visit this group at > http://groups.google.com/group/sympy?hl=en.
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