TBR RC circuit question

[carbear02]

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I had a question about an RC passage for TBR. p.197 , passage V, #32 - why does the charge pass exclusively through R1? I read the explanation but I don't quite get it?

 

[SN2ed]

Questions of this nature are better suited to the MCAT Study Q&A forum.

 

[ACSheldor]

I think you can treat it as individual or in parallel and still get the same answer because the resistors have the same value. If you did them together then the total current passing through both would be 4 amps and if that split then you get you get 2 amps going through each resistor. You can treat R1 as a closed circuit too and do V=IR and do it that way since both are identical. I'm not sure where you're confused even though the explanation was sort of confusing.

 

[carbear02]

I think you can treat it as individual or in parallel and still get the same answer because the resistors have the same value. If you did them together then the total current passing through both would be 4 amps and if that split then you get you get 2 amps going through each resistor. You can treat R1 as a closed circuit too and do V=IR and do it that way since both are identical. I'm not sure where you're confused even though the explanation was sort of confusing.

Well, can you elaborate?

I'm confused because it says no current passes through R2 and I don't understand that. I'm not sure why all current goes through R1.

Also, I thought you'd treat it in parallel, so 1/Reff = 1/5+1/5 = 2/5 so Reff = 5/2. I = V/R so 10/(5/2) = 4, not 2.

 

[lorenzomicron]

well, i dont know what the circuit looks like, but if its a resistor in parallel with a series capacitor+resistor, then charge will only pass through the first resistor because the capacitor is an open-circuit for DC voltage.

 

[ACSheldor]

It's 4 total amps for BOTH resistors. When the current gets to that point after switch 2 it splits in half.

 

[carbear02]

It's 4 total amps for BOTH resistors. When the current gets to that point after switch 2 it splits in half.

Hm - yeah I see that now - thanks!

What really threw me off was the answer key says: "A long time after both switches are closed, the current will travel exclusively through resistor R1 because the capacitor passes no current once it is fully charged." Do you understand this?

 

[lorenzomicron]

Hm - yeah I see that now - thanks!

What really threw me off was the answer key says: "A long time after both switches are closed, the current will travel exclusively through resistor R1 because the capacitor passes no current once it is fully charged." Do you understand this?

if its a resistor in parallel with a series capacitor+resistor, then charge will only pass through the first resistor because the capacitor is an open-circuit for DC voltage.

 

[carbear02]

if its a resistor in parallel with a series capacitor+resistor, then charge will only pass through the first resistor because the capacitor is an open-circuit for DC voltage.

Sorry - can you explain this more fully? I don't see this in my TBR or EK book and it's been awhile since I took physics. Thanks!

 

[lorenzomicron]

so, let's say you suddenly put an uncharged capacitor in a circuit. it takes a while to charge, i.e. it takes a while for the charge on its plates to rearrange such that their voltage drop will be zero. when this happens, there is no longer any current in the capacitor. so, the capacitor is like an open circuit, i.e. infinite resistor.
so, in this case, just erase that part of the circuit; no current can flow in that branch.

so, when they say the capacitor is charged or its been in for a long time, it'll mean that the capacitor is an open circuit. ignore its branch, and use normal circuit analysis to find the circuit out.