-4
\$\begingroup\$

A capacitor consisting of two concentric spheres of radius R1 and R2 = 2.50·R1 has a capacitance of C = 6.00 picoFarads and is charged to a potential difference of 74.0 Volts.

  1. Calculate the energy stored in the capacitor.

  2. Calculate the charge on this capacitor, when the electrical energy stored is 19.9·10-8 Joules.

  3. If the radius of the outer sphere is increased by a factor of 3.00 while keeping the charge on the plates constant, by what factor does the stored energy change?

I've already answered #1 and #2 Questions, but I don't know how to calculate the third one. Help please!

\$\endgroup\$
  • 1
    \$\begingroup\$ What are the formulas you found for #1? This is part of the solution for #3. \$\endgroup\$ – jippie Feb 26 '13 at 8:02
  • 2
    \$\begingroup\$ This sounds homeworkesque \$\endgroup\$ – Grady Player Feb 26 '13 at 8:09
  • \$\begingroup\$ If this is homework, you better mention this in your question. \$\endgroup\$ – Johan.A Feb 26 '13 at 8:48
  • \$\begingroup\$ There is nothing wrong with asking a homework question, generally users are expected to show some work on their homework, but this user seems to have calculated 1 and 2. \$\endgroup\$ – Kortuk Feb 26 '13 at 16:28
1
\$\begingroup\$

What you missing for part C is the formulae for the capacitance of two concentric spheres: $$ C= \frac{4\pi\epsilon}{1/a - 1/b}$$ with $$ b>a $$ From that you should be able to determine the ratio of C wrt the change in R

\$\endgroup\$
0
\$\begingroup\$

Energy stored in a capacitor is: \$U = \frac{1}{2}\cdot\frac{Q^2}{C}\$

You know the amount of charge.

What effect would tripling the radius of the outer "plate" of the capacitor have on the structure's capacitance?

\$\endgroup\$
  • \$\begingroup\$ @Kortuk - If he has calculated #1 he can calculate #3, it's a bit lazy... and the fact it's obviously "please do my homework for me" makes it doubly so. \$\endgroup\$ – John U Feb 26 '13 at 13:27
  • \$\begingroup\$ My point, which I am worrying I am not making clear, is the user knows how to solve the first 2 parts but not the third, and this just reads as restating it. However, I remember when I have had to restate a question before to have a student understand it, so I will remove the -1. \$\endgroup\$ – Kortuk Feb 27 '13 at 1:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.