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Help me please

by judefrance
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judefrance
#1
May23-04, 03:38 AM
P: 4
Can you help me to solve this:

(dē e(r)/drē)+(1/r)*(d e(r)/dr)=0

There is no initials conditions, please use general form
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arildno
#2
May23-04, 03:44 AM
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Hint:
Convince yourself of the following equality:
[tex]\frac{d^{2}e}{dr^{2}}+\frac{1}{r}\frac{de}{dr}=\frac{1}{r}\frac{d}{dr}( r\frac{de}{dr})[/tex]
judefrance
#3
May23-04, 03:57 AM
P: 4
And, if i want to find the general form of e(r)?

arildno
#4
May23-04, 04:10 AM
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P: 12,016
Help me please

You get:
[tex]\frac{d}{dr}(r\frac{de}{dr})=0[/tex]
This differential equation can be directly integrated to find the general solution.
judefrance
#5
May23-04, 04:50 AM
P: 4
If I understand, you've made:
1/r*d/dr*(r*de/dr)=0
so:
d/dr*(r*de/dr)=0

If i integrate, i find:
e(r)=A*ln(r)+B

Wrong or not ?
judefrance
#6
May23-04, 05:56 AM
P: 4
THANKS!!!!! you save me!!!
TALewis
#7
May23-04, 07:50 AM
P: 199
I would also do it this way:

Rewriting:

[tex]e''+\frac{1}{r}e'=0[/tex]

I would then multiply through by r^2:

[tex]r^2e''+re'=0[/tex]

I would recognize this as a d.e. of the Euler-Cauchy form:

[tex]x^2y''+axy' + by=0[/tex]

In the case of the given equation, a=1 and b=0. The characteristic equation for the Euler-Cauchy is:

[tex]m^2+(a-1)m+b=0[/tex]

In our case:

[tex]
\begin{align*}
m^2&=0\\
m&=0
\end{align*}
[/tex]

For the case of a real double root in the characteristic equation, the general solution for the Euler-Caucy is given as:

[tex]y=(A + B\ln x)x^m[/tex]

So in our case:

[tex]
\begin{align*}
e(r)&=(A + B\ln r)x^0\\
e(r)&=A + B\ln r
\end{align*}
[/tex]

I guess this solution depends on having the Euler-Cauchy form available to you in your course, which may not be the case.
HallsofIvy
#8
May23-04, 04:32 PM
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Thanks
PF Gold
P: 39,310
Another way to do this problem is let u= e' so that u'= e" and the equation reduces to the separable first order equation u'+ (1/r)u= 0. Then du/u= -dr/r and so
ln(u)= -ln(r)+ C1 or u= e'= C1/r. Integrating again, e= C1ln|r|+ C2.


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