# Dependence of Cut off frequency of RC circuit...

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#### Himanshoo

Joined Apr 3, 2015
260
Thanks man for such an elaborative calculation...Basically I got the calculations...
but here agains comes your OMG moment..
As the voltage amplitude Vin is constant there must be some voltage drop across R1 (assuming that R1 is not infinity)..and some voltage drop across C1....
Now in the second case lets suppose frequency changes, but still the Vin amplitude is same....hence same voltage drop is at R1...BUT..now here comes my devil....as with change with frequency, Xc changes and voltage across C1 will change...now here what KVL says is that Vin+Vr1+VC1=0..now if any one of the three changes the other two would have to change in order to make their sum total zero....but with frequency change VC1 changes which in order to satisfy KVL should change VR1...but then I think why should VR1 change since Vin is constant...

so its just like that....with the change in frequency C1 is telling VR1 to change...but on the other hand Vin is telling VR1 not to change as Vin is constant amplitude wise...Its just a tug of war.....isn't..

#### Papabravo

Joined Feb 24, 2006
14,869
Thanks man for such an elaborative calculation...Basically I got the calculations...
but here agains comes your OMG moment..
As the voltage amplitude Vin is constant there must be some voltage drop across R1 (assuming that R1 is not infinity)..and some voltage drop across C1....
Now in the second case lets suppose frequency changes, but still the Vin amplitude is same....hence same voltage drop is at R1...BUT..now here comes my devil....as with change with frequency, Xc changes and voltage across C1 will change...now here what KVL says is that Vin+Vr1+VC1=0..now if any one of the three changes the other two would have to change in order to make their sum total zero....but with frequency change VC1 changes which in order to satisfy KVL should change VR1...but then I think why should VR1 change since Vin is constant...

so its just like that....with the change in frequency C1 is telling VR1 to change...but on the other hand Vin is telling VR1 not to change as Vin is constant amplitude wise...Its just a tug of war.....isn't..
You don't get to make a set of arbitrary conditions which must all be satisfied. When the reactance of the capacitor changes with frequency it not only changes the voltage drop for the capacitor leg but for the entire RC circuit as well. No violation -- no problem.

#### Jony130

Joined Feb 17, 2009
5,212
But VR1 will and must change. Why ? Because as Xc change the current in the circuit will also changes as the Ohm's law says I = V/R = V/Z = Vin/(√(R^2 + Xc^2). So if current IS changing the VR1 voltage also must change.

Do the same for resistor voltage divider
https://en.wikipedia.org/wiki/Voltage_divider#Resistive_divider

Let R1 = 1K and Vin = 10V now please find VR1 if R2 is 1K and 2K.

#### Papabravo

Joined Feb 24, 2006
14,869
This is starting to feel like feeding a troll.

#### DickCappels

Joined Aug 21, 2008
6,814
Yep.

I think that the lesson has been taught more than once. If questions remain, try opening a new thread.

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