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From: PD on 23 Jul 2010 14:52
On Jul 23, 1:24 pm, va...(a)icmf.inf.cu wrote:
> On 23 jul, 08:40, PD <thedraperfam...(a)gmail.com> wrote:
>
>
>
> > On Jul 23, 7:25 am, va...(a)icmf.inf.cu wrote:
>
> > > On 22 jul, 13:58, PD <thedraperfam...(a)gmail.com> wrote:
>
> > > > On Jul 19, 4:56 pm, va...(a)icmf.inf.cu wrote:
>
> > > > > On 19 jul, 13:59, PD <thedraperfam...(a)gmail.com> wrote:> On Jul 16, 2:16 pm, va...(a)icmf.inf.cu wrote:
>
> > > > > > > I suppose we are all in agreement about the increase of U(r) with an
> > > > > > > increase of r. Has U(r) a finite maximal limit value when r tends to
> > > > > > > infinite? In case of positive answer, which is that maximal value?
>
> > > > > > You may have heard in freshman physics that the zero point of
> > > > > > potential is physically arbitary. In all interactions, the only thing
> > > > > > that is important is the *change* in potential energy between initial
> > > > > > and final states, and that number is independent of the overall scale.
>
> > > > > Well, you don’t answer my question, but what you say is compatible
> > > > > with an infinite value for U(r) when r tends to infinite (any
> > > > > arbitrary additive constant doesn’t change the infinite).
>
> > > > Read what I wrote. U(r)=0 is quite finite.
>
> > > I don’t understand your comment. When you write U(r)=0 you are only
> > > assigning to U(r) the finite value 0 at some arbitrary r.
>
> > Yes, and for example, it is often customary to set U(r)=0 when
> > r=infinite.
>
> > > What
> > > relation do you establish between this and my question (not answered
> > > by you yet) about if exist or not a finite maximal limit value for
> > > U(r) when r tends to infinite?
>
> > Now do you see?
>
> > Have you NEVER studied out of a freshman physics text?
>
> I continue without seeing an answer from you to my original questions
> opening this thread. To say that it is often customary to set U(r)=0
> when r=infinite is not an answer at all.
> You say in a previous post
> that to put U(r)=0 when r=infinite is only a choice among others. If
> we set per example U(1)=0, that says nothing about the value of U(r)
> when r=infinite.

And that is very easy to calculate, once you've made the choice to set
U(1 m)=0. The answer is a finite number, 2.3E-28 J.
Of course, you get a different answer for U(r) when r=infinite, if you
chose instead U(5)= 0.

> Is so difficult to you to answer my original questions? If following
> you, this can be found in a freshman physics text, what excuse has you
> to not put your answers here? I showed you already the way to know my
> own ones.

Well, there's a couple reasons for that.
First of all, this is not a good forum to have reproduced for you the
contents of a freshman physics text. A newsgroup is not a place to get
a quick and easy education on physics. It also does a disservice to
the authors of freshman physics texts, who normally expect royalties
from the hard work they've put into putting all that stuff into a book
for sale, and to physics teachers, who normally expect salaries for
teaching physics to students. Third, there is a minimum expectation
for folks who are intersted in physics that they do some homework on
their own, using resources that they spend effort to acquire.
Declining to expend that effort and asking to have everything spoonfed
to you is a sure way to lose the interest of someone who could help
you.

In response to questions of the sort you asked originally, you'll find
that most physicists will attempt to answer by *reminding* you of how
to calculate it yourself. And if it turns out that you didn't know how
to calculate it in the first place, then you may discover that they
will suggest you teach yourself from first-course materials before
coming to the newsgroup asking questions about basics.

>
>
>
>
>
>
>
> > > My last comment is related with the
> > > fact that if any finite value is selected for the arbitrary r, this is
> > > compatible with an infinite value for U(r) when r tends to infinite
> > > (and not compatible if the arbitrary r is infinite).
>
> > > > > Anyway, I
> > > > > consider more interesting the other alternative with a finite maximal
> > > > > limit value. I just address it in an answer to dlzc (David) in this
> > > > > same thread. I consider adequate to refer you to it, instead of
> > > > > repeating here my analysis.
>
> > > > > > It is frequently customary to put U(r=infinity) = 0, so that all
> > > > > > values of U(r) for finite r are negative. But this by no means
> > > > > > required and many problems are more convenient to solve with a
> > > > > > completely different choice.
>
> > > > > > PD
>
> > > > > RVHG (Rafael Valls Hidalgo-Gato)
>
> > > RVHG (Rafael Valls Hidalgo-Gato
>
> RVHG (Rafael Valls Hidalgo-Gato)- Hide quoted text -
>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -

From: valls on 29 Jul 2010 06:32
On 23 jul, 13:52, PD <thedraperfam...(a)gmail.com> wrote:
> On Jul 23, 1:24 pm, va...(a)icmf.inf.cu wrote:
>
>
>
>
>
> > On 23 jul, 08:40, PD <thedraperfam...(a)gmail.com> wrote:
>
> > > On Jul 23, 7:25 am, va...(a)icmf.inf.cu wrote:
>
> > > > On 22 jul, 13:58, PD <thedraperfam...(a)gmail.com> wrote:
>
> > > > > On Jul 19, 4:56 pm, va...(a)icmf.inf.cu wrote:
>
> > > > > > On 19 jul, 13:59, PD <thedraperfam...(a)gmail.com> wrote:> On Jul 16, 2:16 pm, va...(a)icmf.inf.cu wrote:
>
> > > > > > > > I suppose we are all in agreement about the increase of U(r) with an
> > > > > > > > increase of r. Has U(r) a finite maximal limit value when r tends to
> > > > > > > > infinite? In case of positive answer, which is that maximal value?
>
> > > > > > > You may have heard in freshman physics that the zero point of
> > > > > > > potential is physically arbitary. In all interactions, the only thing
> > > > > > > that is important is the *change* in potential energy between initial
> > > > > > > and final states, and that number is independent of the overall scale.
>
> > > > > > Well, you don’t answer my question, but what you say is compatible
> > > > > > with an infinite value for U(r) when r tends to infinite (any
> > > > > > arbitrary additive constant doesn’t change the infinite).
>
> > > > > Read what I wrote. U(r)=0 is quite finite.
>
> > > > I don’t understand your comment. When you write U(r)=0 you are only
> > > > assigning to U(r) the finite value 0 at some arbitrary r.
>
> > > Yes, and for example, it is often customary to set U(r)=0 when
> > > r=infinite.
>
> > > > What
> > > > relation do you establish between this and my question (not answered
> > > > by you yet) about if exist or not a finite maximal limit value for
> > > > U(r) when r tends to infinite?
>
> > > Now do you see?
>
> > > Have you NEVER studied out of a freshman physics text?
>
> > I continue without seeing an answer from you to my original questions
> > opening this thread. To say that it is often customary to set U(r)=0
> > when r=infinite is not an answer at all.
> > You say in a previous post
> > that to put U(r)=0 when r=infinite is only a choice among others. If
> > we set per example U(1)=0, that says nothing about the value of U(r)
> > when r=infinite.
>
> And that is very easy to calculate, once you've made the choice to set
> U(1 m)=0. The answer is a finite number, 2.3E-28 J.
> Of course, you get a different answer for U(r) when r=infinite, if you
> chose instead U(5)= 0.
>
Well, I see that you continue without answering my first question,
relative to the existence or not of a maximal finite limit for the
potential energy when r tends to infinite. I take note of your reasons
for not to put here a freshman physics text formula (I will try to not
interfere with that decision of your part). Anyway, from your
quantitative assertions I derived already what formula are you
managing.
Let us suppose that we decided to put U(0)=0. What happens with the
potential energy value when r tends to infinite? This is my first
question (not answered for you yet) for the particular case stated.
I am not sure now if you followed or not the path I gave you already
to see my own answers, because you make no comment about them.

RVHG (Rafael Valls Hidalgo-Gato)
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