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SECTION 26.2 • Calculating Capacitance 801 Example 26.2 The Cylindrical Capacitor A solid cylindrical conductor of radius a and charge Q is Solution It is difficult to apply physical arguments to this where E is the electric field in the region between the cylinders. In Chapter 24, we showed using Gauss’s law that e - /r (Eq. 24.7). The same result applies here because, accord- V b $ V a # $ " b a
E+d
s Figure 26.6 (Example 26.2) (a) A cylindrical capacitor consists of a solid cylindrical conductor of radius a and length ! surrounded by a coaxial cylindrical shell of radius b. (b) End view. The electric field lines are radial. The dashed line represents the end of the cylindrical gaussian surface of radius r and length !. b a ! (a) (b) Gaussian surface –Q a Q b r not contribute to the electric field inside it. Using this re- E is along r, we find that Substituting this result into Equation 26.1 and using the fact (26.4) where !V is the magnitude of the potential difference #V a $ V b # # 2k e - ln(b/a), a positive quantity. As predicted, the capaci- tance is proportional to the length of the cylinders. As we (26.5) An example of this type of geometric arrangement is a coax- What If? Suppose b " 2.00a for the cylindrical capacitor. We would like to increase the capacitance, and we can do Answer According to Equation 26.4, C is proportional to !, # 0.721
! k e C # ! 2k e
ln(b/a) # ! 2k e
ln(2.00) # ! 2k e
(0.693) C ! # 1 2k e
ln(b/a) ! 2k e ln(b/a) C # Q ! V # Q (2k e
Q
/!)ln(b/a) # V b $ V a # $ " b a E r dr # $2k e
-
" b a
dr r # $ 2k e
- ln
$ b a % Cylindrical and Spherical Capacitors From the definition of capacitance, we can, in principle, find the capacitance of any |