A quick and simple question/clarification on circuits

[Pablo1122]

Homework Statement

http://i.imgur.com/zXJpa9J.png

Homework Equations

It = vT/rT
P = I^2R

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.

Thanks in advance.

 

[TSny]

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

 

[Pablo1122]

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

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?

 

[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?

 

[Pablo1122]

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.

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?

 

[TSny]

Yes, I believe so.

 

[Pablo1122]

Yes, I believe so.

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.

 

[TSny]

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

 

[Pablo1122]

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

Perfect, thank you very much.

 

[Pablo1122]

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

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?

 

[TSny]

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.

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.

The resistor would still have a resistance of 50 ohms even if there's no current going through it right?

Yes, that's right.

 

[Pablo1122]

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.

Perfect, thanks once again.