# A quick and simple question/clarification on circuits

1. Feb 27, 2017

### Pablo1122

1. The problem statement, all variables and given/known data
http://i.imgur.com/zXJpa9J.png

2. Relevant equations
It = vT/rT
P = I^2R

3. The attempt at a solution

http://i.imgur.com/zXJpa9J.png
This is the question at hand. I've solved them all and I have a question for part c and d. Since the capicators are fully charged, the circuit becomes:

http://i.imgur.com/jGjQRLJ.png

Now, assume the question on my exam was i5 on r5 for c) and r5 for d). r5 does not exist on this circuit so will the answer for c and d both be zero or just d? It seems like they both would be zero since there is no resistor but part c is throwing me off as it's saying i4 = itotal.

Last edited by a moderator: Feb 27, 2017
2. Feb 27, 2017

### TSny

When the capacitors are fully charged, can there be any current in R5?

3. Feb 27, 2017

### Pablo1122

No, since that resistor would be gone. So both c and d with be zero right? I was just confused because it says i4 = it. And in this case, if we wrote it as i5=it then we'd get a current. What would I write to say that the current is zero? Simply that since there is no longer charge going through resistor 5, the current on the resistor is zero?

4. Feb 27, 2017

### TSny

The fact that R5 is gone in the simplified circuit is not the reason why i5 = 0. Rather, it's the other way around. The fact that there is no current in R5 is the reason why R5 doesn't appear in the simpified circuit.

To give a reason why i5 = 0, suppose i5 ≠ 0. What would that imply concerning the charges on the capacitors?

Last edited: Feb 27, 2017
5. Feb 27, 2017

### Pablo1122

Alright, so current flows until capacitors are full. Therefore, since the capacitors are full there is no more current flowing through resistor 5.

Would that be the correct answer and would that be sufficient to write on an exam as the reasoning for it?

6. Feb 27, 2017

### TSny

Yes, I believe so.

7. Feb 27, 2017

### Pablo1122

Alright, so i5 = 0A.

Then for d)"Compute the power delivered to resistor r5 in the steady state"

I would just write p = I^2R = 0^2(0) = 0 right? Or just state it's 0 due to the fact that the current is equal to zero? Should I write it as 0^2(50) or 0^2(0)?

Thank you very much for your help.

8. Feb 27, 2017

### TSny

Yes, the power dissipated in R5 is zero because i5 = 0.

9. Feb 27, 2017

### Pablo1122

Perfect, thank you very much.

10. Feb 28, 2017

### Pablo1122

Would it make sense if I showed that by writing

p = I^2R = 0^2(50) = 0 or should I just write the power dissipated in R5 is zero because i5 = 0. The resistor would still have a resistance of 50 ohms even if there's no current going through it right?

11. Feb 28, 2017

### TSny

Either statement seems OK to me, but writing out p = I^2R = 0^2(50) = 0 shows more explicitly why the power for R5 is zero. So, I would recommend using the more explicit argument on a test.
Yes, that's right.

12. Feb 28, 2017

### Pablo1122

Perfect, thanks once again.