# What is the Formula -RC series circuit impedance & current

#### ericgibbs

Joined Jan 29, 2010
12,272
hi,
Remember original question...
What is the Formula -RC series circuit impedance & current.

E

#### MrAl

Joined Jun 17, 2014
8,067
Hi there,

I am not sure what you guys are calculating, but an RC series circuit requires either using complex impedance or adding the impedances by adding their squares and then taking the square root to get the total impedance, then dividing that into the voltage to get the total current and because the current is the same in each element in a series circuit the current will be the same in both R and C.

Now in this problem the difference will be small, on the order of 0.01 percent, because the capacitive reactance is much higher than the resistance, but in another problem it could be much much higher even hundreds of percent so it is best to learn the more complete solution.

For example, if the resistor value was the same value as the reactance of the cap (which is around 15k at 1000Hz) then the current would be around 4e-4 rather than around 6e-4 meaning the result would be off by around 50 percent (although the real calculation would yield around 30 percent difference).
So i would advise looking at the more complete solution it isnt that much more difficult if you want to use the 'squared/square root' solution for the impedance.

Just to note, if the resistance was closer to 150k then the result would be off by 10 times. That's because the resistance would then dominate the response for some frequencies.

#### ericgibbs

Joined Jan 29, 2010
12,272
I am not sure what you guys are calculating,
hi Al,
This TS does not even know the Ohms Law equation, so I kept as simple as possible.
Hopefully overtime he will gain more knowledge and realise that circuits can be more complex.
E

#### MrAl

Joined Jun 17, 2014
8,067
I am not sure what you guys are calculating,
hi Al,
This TS does not even know the Ohms Law equation, so I kept as simple as possible.
Hopefully overtime he will gain more knowledge and realise that circuits can be more complex.
E
Oh i see, then i guess Ohm's Law is a good place to start.