# resistor circuit

Discussion in 'Homework Help' started by sara_jayne, May 12, 2008.

1. ### sara_jayne Thread Starter New Member

May 12, 2008
3
0
hi. im just finishing my first year in gcse electronics and have been given the question regarding the attachment.

in the following circuit, determine the current in the 4 ohm resistor. i only have basic knowledge in electronic. im only 15 . but i do undertand ohms law. any help would be apreciated. but please put it in basic terms for me . many thanks . sara

ps one last question . i understand that when a resistor and capactor are connected in series then the time constant is r x c in ohms and farads but what is the formula for thetime constant if the resistor and capcitor are in paralell.

View attachment q3.pdf

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
An easy way to it is to find the Thevenin equivalent circuit at points A and C and then find the current through the 40 ohm resistor. Others ways is to use Nodal voltage analysis, Mesh current analysis or superposition theorem. The best for this case is superposition.

3. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Your question seems to relate to a problem different from the one you have attached. Was that intentional on your part?

hgmjr

4. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
Opps.

To Sarah, I think you'll find we'll be glad to help however we can. Everyone opps now and again. In my case it's again, and again, and again...

5. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Bill is correct. We are error prone beings by design.

If you can clarify the discrepancy between your question and the posted circuit we should be able to assist you.

I would encourage you to show any effort you have made to solve the circuit. Our members better appreciate a request for assistance when it accompanied by a genuine attempt on the poster's part to solve it themselves.

hgmjr

6. ### sara_jayne Thread Starter New Member

May 12, 2008
3
0
thanks for the reply's sorry about the way i wrote the thread. Theres spose to be 2 completly different question. one is that i would like to know how to find out the time constant for a rc parrell circuit and the question regarding the attachment is

"in the following circuit, determine the current in the 40 ohm resistor". by all means ill show my workings and answers. i just need somewhere to start. plus im still none the wiser with mik3 reply. thanks sara

7. ### Digi Dave Member

Apr 25, 2008
26
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the time constant is the same RxC parallel or series

8. ### sara_jayne Thread Starter New Member

May 12, 2008
3
0
many thanks for the reply digi dave. thats heled very much.

9. ### Caveman Active Member

Apr 15, 2008
471
0
Remember the voltage around any loop is equal to zero. So this is true:
V(AB) + V(BC) + V(CD) + V(DA) = 0

Just in case you don't understand my notation, V(AB) is the voltage of node A minus the voltage of node B, so in your circuit that is the voltage across the 10 ohm resistor on the left.

You have two loops in your circuit, so write that equation for both in terms of the two currents, I1 and I2. I'll show you how the first one is done, then let you go the rest of the way on your own.
V(AB) = 10*I1
V(BC) = -10
V(CD) = 20*I2
V(DA) = -20

Now substitute these in your equation, write the equation for the other loop. You will have two equations with two unknowns, so you should be able to solve using substitution.

Got it?