What in the world is a capacitor and what happens??

[pineappletree]

As a current passes through the capacitor, charge begins to accumulate and the voltage of the capacitor begins to increase exponentially. As voltage increases, the capacitance value decreases, and (q/RC) increases...

So what exactly is a capacitor? Seems to me to be nothing more than a slow resistor that needs time to charge up...

What is the point of bringing Current to 0??
Also, does the current ever reach 0 without a capacitor??
I am very very confused

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[lorenzomicron]

Current actually doesnt pass through a capacitor. Actually, the capacitor ends are separated by air.

the capacitance value is set; it doesnt instead/decrease. capacitance C is its parameter.

what happens is that initially, the charge on both ends of a capacitor, i.e. the voltage drop is zero, for instance. thus, there is no interaction between capacitor ends. however, then, a voltage is applied, causing the upper plate to become charged. but, because the voltage in a conductor tries to be zero, the charge will rearrange itself in order to reach this point of zero charge. notice two points: 1) in the long run, the capacitor becomes an open circuit; i.e. after the initial rearrangement of charge, the capacitor becomes an element of infinite resistance 2) thus, the current through a capacitor lasts only for some initial amount of time before going towards zero for a long amount of time.

it's not really a slow resistor, because resistors produce power by having a current go through them. also, resistors do not become open circuits after a long time. without a capacitor, the current will not naturally go to zero in a circuit which has only R and L and voltage sources. however, in reality, when the battery dies, the current will go to zero.

tried to answer some questions. anything still unclear?

 

[pineappletree]

Thank you,

But what does a Discharge mean??
I've no idea whatsoever. All I can imagine is the planet exploding from the star trek movie, which wouldn't make any sense be cause capacitors DON'T LOSE their charge while the circuit is on.
Specifically, the term:
"The capacitor now discharges through the cells"


also, how are
C, I, R, and V
related to the E value for electric field??

 

[pineappletree]

Also, a question about a Berkley review passage

A RC circuit is shown (illustration not important, just a standard one path RC circuit)
it is stated that, when the capacitor is charged, the voltage across the capacitor plates is EQUAL to the voltage across the power source...

WHAT??
How is this possible?
I thought that emf=V(resistor)+V(capacitor)
which means that V(capacitor)=emf-V(resistor)

This seems to COMPLETELY ignore the resistor



Please someone tell me what is going on

 

[pineappletree]

And another question...


Voltage=Resistance??

I didn't know that
So when a capacitor is first turned on, it has zero voltage
hence it has zero Resistance...




Sorry, I have spent the last DAY on this chapter, and I honestly don't understand anything capacitor related...

 

[lorenzomicron]

Thank you,

But what does a Discharge mean??
I've no idea whatsoever. All I can imagine is the planet exploding from the star trek movie, which wouldn't make any sense be cause capacitors DON'T LOSE their charge while the circuit is on.
Specifically, the term:
"The capacitor now discharges through the cells"


also, how are
C, I, R, and V
related to the E value for electric field??

Ok, so imagine if we had a capacitor in a circuit for a long time. in a long time, the plates will be all charged such that there is some constant voltage Vc across the capacitor. at this point, there is no current through the capacitor.
THEN, remove the battery/voltage source. all of a sudden, the capacitor has this voltage/energy that is not being maintained. it then begins to discharge through the circuit, flowing through the wire and resistors in the circuit. this discharge is by means of the voltage of the capacitor causing current to flow through the circuit, with the relationship of i = c dv/dt. after a very short time, most of the energy is discharges (defined as five time constants where a time constant is RC).

lol to the planet exploding. i knew star trek was destroying our minds forever.

well...given the V between two points, a and b, the electric field E will be the negative gradient of voltage at a, taking b as reference. try wikipedia for these specific formulas.

 

[lorenzomicron]

And another question...


Voltage=Resistance??

I didn't know that
So when a capacitor is first turned on, it has zero voltage
hence it has zero Resistance...




Sorry, I have spent the last DAY on this chapter, and I honestly don't understand anything capacitor related...

that's alright, it took me an engineering circuits class before i really understood what these things are.

capacitors don't have real resistance. voltage is not equal to the resistance. when a capcitor is first turned on, it has zero voltage because charge doesnt rearrange itself instantaneously.

i = C dv/dt.

 

[lorenzomicron]

Also, a question about a Berkley review passage

A RC circuit is shown (illustration not important, just a standard one path RC circuit)
it is stated that, when the capacitor is charged, the voltage across the capacitor plates is EQUAL to the voltage across the power source...

WHAT??
How is this possible?
I thought that emf=V(resistor)+V(capacitor)
which means that V(capacitor)=emf-V(resistor)

This seems to COMPLETELY ignore the resistor



Please someone tell me what is going on

well, you're right in so far as you're thinking about the circuit elements. however, recall that capacitors become open circuits when they become charged. thus, because it is an open circuit, no current can flow in the circuit. since there is no current, then resistor has no voltage! because v= i*r, but i =0.
so, in that case, emf = Vc.

 

[lorenzomicron]

remember, capacitors become like infinite resistors when they're charges.

so, then the resistance of the circuit would be R= infinity. thus, v=i*r, means that i = 0. so, then the Vc = Vo.

 

[pineappletree]

THANK YOU sooo much

I'm a biology major, and all of this conceptualizing math is killing me~~~

 

[lorenzomicron]

anytime.

cheers

 

[pineappletree]

ok, so this is what I learned that I was GROSSLY confused on before


C formula is C=q/V
C is a constant, cannot be changed in a given RC circuit
changing q or changing V will change the other, but never change C

this is why resistors in series with a capacitor affect the V of the capacitor and also it's q
But they don't affect the C value

The C value can ONLY be varied via Dielectric constant, materials, area of plates, and distance between plates.


Is this correct?
I was very very confused before because I didn't think that C was a constant value so I kept accounting for C change in my head....

 

[PiBond]

ok, so this is what I learned that I was GROSSLY confused on before


C formula is C=q/V
C is a constant, cannot be changed in a given RC circuit
changing q or changing V will change the other, but never change C

this is why resistors in series with a capacitor affect the V of the capacitor and also it's q
But they don't affect the C value

The C value can ONLY be varied via Dielectric constant, materials, area of plates, and distance between plates.


Is this correct?
I was very very confused before because I didn't think that C was a constant value so I kept accounting for C change in my head....

That's correct