- #1
manenbu
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Homework Statement
Taken from Resnick and Halliday:
A Geiger counter has a metal cylinder 2.10 cm in diameter along whose axis is stretched a wire 1.34E-4 cm in diameter. If 855 V is applied between them, find the electric field at the surface of (a) the wire and (b) the cylinder.
Homework Equations
Gauss' Law, potential-field
The Attempt at a Solution
Field inside the counter is by Gauss' law:
[tex]dq = \epsilon_0 E dA = \epsilon_0 E 2 \pi\ r dL[/tex]
rearranging and using [itex]\lambda = \frac{dq}{dL}[/itex]:
[tex]
E = \frac{\lambda}{\epsilon_0 2 \pi r}
[/tex]
now:
[tex]
V = \int E dr = \frac{\lambda}{\epsilon_0 2 \pi}\int\frac{dr}{r}
[/tex]
solving from r1 to r2:
[tex]
V = \frac{\lambda}{\epsilon_0 2 \pi}\ln{\frac{r_2}{r_1}}
[/tex]
solving for [itex]\lambda[/itex] and putting into the expression of E from before:
[tex]E = \frac{V}{r \ln{\frac{r_2}{r_1}}}[/tex]
plugging in the numbers from the beginning of the post, I get for r1 and r1:
4214 V/m
66.06 MV/m
The answers given in the appendix of the book are 2 times my (8kV/m and 132 MV/m).
Where did I miss a factor of 2?