# Homework Help: Calculating ratio of voltage help

1. Aug 19, 2011

### joe007

1. The problem statement, all variables and given/known data

At the given 10 Hz frequency calculate the magnitude of the voltage across a:

(1)4.7kohm resistor
(2)100nF capacitor

2. Relevant equations

Vr/v=R/z=R/sqrtR^2+(1/wC)

Vc/v=(1/wC)/z=(1/wC)//sqrtR^2+(1/wC)

3. The attempt at a solution
(1)4.7/sqrt(4.7^2+2*PI*4.7^2) ------i am not sure if this correct

and for part 2

(2)1/(2*PI*100*10^-9)/sqrt()

not sure of what value to use for the R

2. Aug 19, 2011

### MisterX

There should be more information. What information was given about the power source? What circuit was described?

3. Aug 19, 2011

### joe007

oh it had 10V power source

4. Aug 19, 2011

### joe007

i think i have to calculate the voltage across each component as they are in series but not sure how to use impedance on this question

5. Aug 20, 2011

### Staff: Mentor

It appears that you are using as the formula for the total impedance:

z=sqrtR^2+(1/wC)

You have not derived this correctly.

You are aware that the total impedance is the sum of the impedance of R + the impedance of C. But because these individual impedances are at right angles to each other, you must add them like vectors at right angles to each other.

Complete this: the impedance of a capacitor of 100nF at 10Hz =

6. Aug 20, 2011

### joe007

ok z=1/2PI*10*100*10^-9 then

7. Aug 20, 2011

### Staff: Mentor

Consider using parentheses to clarify the mathematical operator precedence in your expressions. The X2 and X2 icons in the post composition header let's you create superscripts and subscripts, too. Thus:

Zc = 1/(2*pi*10*100*10-9)

8. Aug 20, 2011

### Staff: Mentor

Now, basing your calculations on a right-angled triangle, add the two series impedances.