1. May 15, 2016

i_hate_math

1. The problem statement, all variables and given/known data
The electric potential energy v(r) of a charged particle located between two uniformly charged concentric spheres with radii r1 and r2 satisfies the second order differential equation

rv′′+2v′=0, r1≤r≤r2
where r is the distance of the charged particle from the common centre of the spheres.

(a) Determine the general solution of the differential equation, by trying a solution of the form v(r)=rm, m∈R.

(b) Using your answer to part (a), find the electric potential energy of a charged particle between two concentric spheres with radii r1 = 2 cm and r2 = 20 cm, kept at potentials v1 = 220 Volts and v2 = 130 Volts respectively.

2. Relevant equations
rv′′+2v′=0

3. The attempt at a solution
I tried to use v(r)=rm as a solution:
v'(r)=mrm
v''(r)=m2rm
and sub back in:
m2rm+1+2mrm=0
this yields:
m(mr+2)=0, which leads me no where near the solution

2. May 15, 2016

vela

Staff Emeritus
You should rethink what v'(r) and v''(r) are equal to.

3. May 15, 2016

i_hate_math

Darn it I took the exponential. I'll rework my solution

4. May 15, 2016

blerghh

i found the general solution to be v(r)=A+Be^(-r). then, what's next?

5. May 15, 2016

i_hate_math

Apply these conditions:
and you get A=200 and B=120

6. May 15, 2016

blerghh

is there any other way to simplify the exponential before applying the conditions?

7. May 15, 2016

blerghh

nawhh i just got the answer! thank you!

8. May 15, 2016

i_hate_math

wait howd u get the exponential? the solution i got was v=A/r+B

9. May 15, 2016

blerghh

yeah, i was wrong at first. no exponential. you are right :)

10. May 15, 2016

crazy too

isn't the general solution supposed to be v(r)=A+B/r? because both r^0 and r^-1 satisfy the DE

11. May 15, 2016

crazy too

oops you guys updated... yeah, and after getting v(r)=A+B/r, how do i sub in the conditions... the conditions doesn't make sense to me

12. May 15, 2016

blerghh

you will eventually get A=120 and B=200. just sub those conditions into v(r)=A+B/r separately and solve A and B simultaneously

13. May 15, 2016

crazy too

ohhhhh okay, i got the answer! even tho it doesn't make much sense... thank you for your help!