# Q & A: Capacitor homework

Q:
1) a constant current of 1mA flows for 100s onto one plate of an uncharged capacitor connected to a battery in a cicuit. At the end of this time the p.d across the capacitor is 10V. a) what current flows off the other plate of the capactor? b) what is the charge on one plate after 100s? c) How any columbs are needed on one plate to give a p.d of 1v? d)what is the capacitance of the capacitor? e)If the fully charged plates were allowed to touch what would be the total charge on the plates?
- rita
british
A:
We're not going to do your homework for you, but can give you some hints.

a) If the capacitor remains uncharged, then as much charge must flow off as flows on. If the charge flows off of the second plate, it will have a charge that is opposite in sign to the charge on the first plate. Current times time is charge.

b) Current times time is charge.

c) Capacitance is the relationship between charge and potential difference. The potential difference times C = the charge on one plate, where C is the capacitance. Use the setup of the problem to calculate C, then use this again to calculate the charge. It'll be in Coulombs if your units are amperes and seconds. C will be in Farads.

d) You will have calculated C in part c)

e) The same as the sum of the charges on the two plates before they touched -- you cannot create or destroy net electric charge. See the answer to part a).

Tom

(published on 10/22/2007)