# RC circuit question regarding current flow with capacitors

Discussion in 'General Electronics Chat' started by Archaeus, Sep 10, 2016.

1. ### Archaeus Thread Starter Member

Sep 10, 2016
34
0
Hello,

I am a new hobbyist and I have a question which has conflicting answers online. I'm currently learning about rc timing circuits- more specifically- using a resistor/capacitor network to switch on a transistor after a certain time period. Charging and placing the capacitor in parallel with the base of the transistor is clear, but I am having trouble understanding the branch current calculations. When the switch is closed, I know caps act as a short immediately until charge and voltage build. What I don't understand is why I am taught by some that current takes every path available based on ohms law, while a reputable lecturer says that the initial 'short' from a capacitor will divert ALL current (even from a branch in parallel) until voltage builds. Can anyone help clear up how current behaves in this situation? Thank you.

2. ### DGElder Member

Apr 3, 2016
351
87
"When the switch is closed, I know caps act as a short immediately until charge and voltage build. What I don't understand is why I am taught by some that current takes every path available based on ohms law, while a reputable lecturer says that the initial 'short' from a capacitor will divert ALL current (even from a branch in parallel) until voltage builds. Can anyone help clear up how current behaves in this situation? Thank you."

They are both correct. The voltage in a capacitor can not change instantly, so if it is at zero volts initially then immediately after switching in a power source it will still be zero volts. That means a parallel resistor, for example, will see zero volts across it immediately after switch-on and the current through the resistor will be determined by ohms law: I = V/R or I = 0 volts /R ohms = 0 amps. So, for an instant, all the current is going through the capacitor, I = C dV/dt.

Last edited: Sep 10, 2016
3. ### Archaeus Thread Starter Member

Sep 10, 2016
34
0
Thank you very much for this reply. Appreciated.

Apr 3, 2016
351
87