# what is the voltage across C1?

Discussion in 'Electronics Homework Help' started by charmcaster.engg, Jan 20, 2015.

1. ### charmcaster.engg

79
1
Mar 20, 2014
If C1, a 4.7 F capacitor, and C2, a 3.3 F capacitor are in series with 18 Vdcapplied, what is the voltage across C1?
Frequency is not mentioned.

2. ### davennModerator

13,880
1,964
Sep 5, 2009

because DC voltage doesn't have a frequency

3. ### charmcaster.engg

79
1
Mar 20, 2014
Then how to find Xc?

4. ### davennModerator

13,880
1,964
Sep 5, 2009
There is NO Xc ( capacitive reactance) ..... again ... Its DC voltage

did you google voltage across capacitors in series ?

5. ### charmcaster.engg

79
1
Mar 20, 2014
V=Vc1+Vc2 and Vc1=Q/C1, Vc2=Q/C2. I find out total capacitance using C1*C1/C1+C2=1.94uF. Then Q=C*V=34.92. n then Vc1=Q/C1=7.42.
Am I right?

6. ### Laplace

1,252
184
Apr 4, 2010
Correct, but there is no need to calculate the charge Q. Just recognize that the charge is equal in both capacitors. C=Q/V, Q1=Q2, C1∙V1=C2∙V2. The voltage law gives V1+V2=18.

So just solve these two equations for V1: C1∙V1=C2∙V2 & V1+V2=18

V2=(C1/C2)V1, (1+C1/C2)V1=18, V1=18/(1+4.7/3.3) = 7.425

7. ### charmcaster.engg

79
1
Mar 20, 2014
Thanks!  