RC circuit problems

  • #1

Homework Statement



A 2.00μF capacitor with an initial charge of 5.10μC is discharged through a 1.3 kΩ resistor.

a) calculate the current through the resistor 9.00ms after the resistor is connected across the capacitor

b) what charge remains on the capacitor after 8.00ms

c) what is the maximum current in the resistor?

Homework Equations



Discharging capacitor
I = (Io-Vo)e^-t/RC
q = Qe^-t/RC

C= Q/(Vf-Vi)

I = V/R

The Attempt at a Solution



C= Q/(Vf-Vi)
V= 5.1/2
V=2.55V

I=V/R
I=2.55/1300
I=1.9mA

Unable to find Io and have no idea where to go from there.
 

Answers and Replies

  • #2
SammyS
Staff Emeritus
Science Advisor
Homework Helper
Gold Member
11,398
1,045
What is the voltage across the capacitor with its initial charge?
 
  • #3
It should be:

V=C/Q

V=2/5.1 = 392mV
 
  • #4
245
0
If you want to find I0, you should know V0. But what is V0?
Do you know the capacitor relation C=Q/V. Can you use this to find V0?
So knowing V0 and R you can find I0.

And can you check your first equation? how the term -V0 exist there?
 
  • #5
So,

Io = Vo/R
Io = 301mA
I=(Io)e^-t/RC

(Io)e^-t(650000000)
(Io)e^-1.38461538 × 10-11
1 * Io
I = 301mA?
 
  • #6
gneill
Mentor
20,934
2,877
It should be:

V=C/Q

V=2/5.1 = 392mV

Check your units. Always check your units!

Capacitance has units [Coul]/[Volt]. Charge has units [Coul]. So the basic expression is

C = Q/V giving you V = Q/C .
 

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