Rc Circuit question

[AA|FCB|DOC]

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I just had some confusion after doing a Berkley Review passage. It is question #1 from passage #1 of section 9 (circuits). I was always under the impression that the resistor in the RC circuit affects the charging time AND the discharging time. However, on this problem explanation it states that the resistor R1 will affect the charging time of the capacitor, but not the discharging time. Can anyone make any sense of this? Thanks in advance.

 

[milski]

What page is that? There are two passage I-s for chapter nine in my edition, and neither is about RC-circuits. One is about designing a resistor and the other is about changing current in a circuit with 3 resistors.

 

[AA|FCB|DOC]

hey sorry for the late reply milski, for me that is page 189. It is a passage about some sort of electrophoresis.

 

[milski]

It seems that we have different editions - page 189 for me is the beginning of the circuits chapter - well before the passages start.

Without any other information - in general you are correct, if the capacitor is charged and discharged through the resistor, the resistance will have an effect on both times. With that said, there are ways to have different paths for the current for the charging and discharging portion. I don't think diodes or anything similar is on the MCAT, so I'm not really sure what's going there, sorry.

 

[AA|FCB|DOC]

thanks for your help either way on all the questions I have been asking.

 

[BerkReviewTeach]

Can anyone make any sense of this? Thanks in advance.

You have a different version of the book than mine as well (the oldest I have is 2010), so I don't know the exact question but I think I remember the passage. If it's the question I think it is, the route to fill the capacitor passes through R₁. But this is not the same as the route taken to discharge the capacitor. A battery sends current through the circuit to fill the capacitor, and that current passes through R₁, so the time to fill the capacitor depends on R₁. But when the capacitor discharges, the current passes through the cells that are situated in the cell suspension between the anode and catchode plates, so that flow of charge does not pass through R₁. The discharge time depends on the conductivity of the cell suspension.

 

[ihatebluescrubs]

I actually also have a similar question based on this exact problem.

To rephrase the problem: we are given a simple circuit: battery --> resistor --> capacitor

The question choices then ask if increasing the resistance will affect the capacitance. The answer explanation states that "changing the resistance affects only how long it takes the capacitor to charge up in the first place".

What I don't understand is how changing the resistance doesn't also change a capacitor's ability to store charge. If we increase the resistance of the resistor, doesn't this mean that there is less charge thats going to the capacitor and thus decreasing the capacitor's ability to store charge?

 

[milski]

I actually also have a similar question based on this exact problem.

To rephrase the problem: we are given a simple circuit: battery --> resistor --> capacitor

The question choices then ask if increasing the resistance will affect the capacitance. The answer explanation states that "changing the resistance affects only how long it takes the capacitor to charge up in the first place".

What I don't understand is how changing the resistance doesn't also change a capacitor's ability to store charge. If we increase the resistance of the resistor, doesn't this mean that there is less charge thats going to the capacitor and thus decreasing the capacitor's ability to store charge?

The ability to store charge is an internal property of the capacitor - it does not depend on how the charge gets there, it depends only on the geometry and the materials of the capacitor. By increasing the resistance you make it harder for the charges to get to the capacitor, but eventually the same amount of them will make it.

Note that when the capacitor is fully charged, there is no current going through the circuit, thus the voltage across the resistor is zero and the voltage across the capacitor is the same as the voltage of the battery.

One way to think about it is filling a bucked with water. If you close the faucet a bit, it will take longer time to fill the bucket but at the end you'll put the same amount of water in the bucket. And yet another note: the capacitance is not exactly the same thing as the volume of the bucket, but for what we are discussing here, that does not matter so much.