What is the Charge on a Capacitor in an RC Circuit?

[musiliu]

Homework Statement

An RC circuit is made of a 2.2 kOhm resistor and a 0.001 microF capacitor.
The circuit is connected to a 10V source of emf for 5 x 10^-5 s. What is the
resulting charge on the capacitor after that?

Homework Equations

Q(t) = -ECe^(-t/RC) + EC , where E is the emf and C is the capacitance

The Attempt at a Solution

it seems like a simple plug-in problem, so I used E = 10V, C = 1 x 10^-9 F, R = 2200 Ohms, and t = 5 x 10^-5 s.

edit: oops.. i did the calculation wrong.. now i get:

Q = 2.27 x 10^-11 C

is this answer correct now and did i do the problem correctly?

 

[Delphi51]

The method looks okay but I get a much larger answer, on the order of 10^-8. Did you get EC = 1 x 10^-8 ? RC = 2.2 x 10^-6?
The 5 x 10^-5 seconds is at least two time constants, so the first term should be just about zero.

 

[musiliu]

ok.., i recalculated and now i get 1 x 10^-8 C... is this correct? is it exactly 1?

 

[Delphi51]

No, it isn't exactly 1. That would only be one digit accuracy. Likely you are expected to have 3 digit accuracy, meaning intermediate steps must keep at least 4 digits.

 

[musiliu]

ok..., i used a graphing calculator now and i get

9.99 * 10^-9 C... is this correct?

 

[Delphi51]

Oops, you're right and I dropped a power of ten. Correct answer!