# RC circuit question

1. Feb 8, 2006

### Shay10825

Hello I need some help with the following problems:

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3. In the figure shown, if I= .5 A and R= 12 ohms, determine E.

http://img263.imageshack.us/img263/8938/phy237kc.png [Broken]

I did R_eq = 1/[(1/24)+(1/24)] + 12 = 24 ohms
V=IR
V = (.5)(12) = 6

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4. In an RC circuit, how many time constants must elapse if an initially uncharged capacitor is to reach 80% of its final potential difference?

I had no clue how to do this one. I there a formula or something for this? I looked in my book and I did not find anything that seemed to help.

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6. If I= .4 A in the circuit segment shown, what is the potential difference V_ab?

http://img263.imageshack.us/img263/8363/phy269iw.png [Broken]

I did:

V_ab = 20(I) + 8(I_2) + 10(I)
V_ab = 10(I) + 12(I - I_2) + 10(I)

I plugged .4 in for I and solved for I_2. I found I_2 to be .24. Then I plugged I and I_2 back into one of the equations and got V_ab to be 9.92
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Any help would be greatly appreciated

Thanks

Last edited by a moderator: May 2, 2017
2. Feb 8, 2006

### Staff: Mentor

3. I don't understand the circuit fragment. And what do you mean by R=12 Ohms? All the resistors in the figure are labeled already. What is supposed to be to the right in the figure? Or is that not supposed to matter maybe?

4. Do you know what an RC time constant is? If not, solve for the Vo/Vi transfer function of a simple Resistor-Capacitor circuit (draw the resistor horizontal, with the input voltage at the left side, and the capacitor connected to the right side of the resistor and going to ground). If you put a step voltage into the left end of the resistor, what does the voltage do at the connection between the resistor and cap? Since you will have a differential equation for Vo/Vi, the solution will involve an exponential function with time in it. The R-C time constant has something to do with the term in the exponential....

6. I got the same answer as you. I combined the 8 and 12 Ohm resistors, and then added them to the two 10 Ohm resistors. If they are saying that 0.4A flows from point a to b, then I agree that you would measure 9.92V between those two points. Do they maybe mean that there is 0.4A flowing through some other segment, not all the way from a to b?