Current of RC Circuit After Switch is Opened

[Math-U-Up]

Homework Statement

See attached image for circuit diagram and givens.
The switch has been closed for a very long time.
a) What is the charge on the capacitor?
b) The switch is opened at t=0 s. What current initially flows?

Homework Equations

V = IR
Q=CV
I=I₀e^-t/(RC)

The Attempt at a Solution

Since the switch has been open for a long time, no current flows through the 10 Ohm resistor. Therefore,
I = 100V/(60 Ohm + 40 Ohm) = 1 Amp, V = 1Amp*40 Ohm, and Q = 2uF * 40V = 80uC.

So my question is: Is the current that initially flows after the switch opens at t=0 s just due to the battery? So it would just be 1Amp? I'm having trouble understanding how to find the current at part b.

 

[BvU]

Hello MUU,

First step is to establish where the current flows. Through an open switch ? Or elsewhere ?

 

[collinsmark]

Hello @Math-U-Up. Welcome to PF!

Homework Statement

See attached image for circuit diagram and givens.
The switch has been closed for a very long time.
a) What is the charge on the capacitor?
b) The switch is opened at t=0 s. What current initially flows?

Homework Equations

V = IR
Q=CV
I=I₀e^-t/(RC)

The Attempt at a Solution

Since the switch has been open for a long time, no current flows through the 10 Ohm resistor. Therefore,
I = 100V/(60 Ohm + 40 Ohm) = 1 Amp, V = 1Amp*40 Ohm, and Q = 2uF * 40V = 80uC.

So my question is: Is the current that initially flows after the switch opens at t=0 s just due to the battery? So it would just be 1Amp? I'm having trouble understanding how to find the current at part b.

I think you might have misread the problem statement. According to the wording in the problem statement, initially, the switch has been closed for a very long time. That means that current has been flowing through the battery for a very long time.

At time $t = 0$, the switch is opened, meaning that after $t = 0$ no current flows through the battery, and any current flowing in the circuit is the result of whatever residual charge is left in the capacitor.

At least that's the way that I interpret the problem.

(A "closed" switch means that the switch can conduct current. An "open" switch does not conduct current.)

 

[BvU]

The idea in PF is that the thread poster discovers that after a small nudge from a helper ...