Disconnect/Maintain Battery Connections

[SaintJude]

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Here are 2 slight different short scenarios.

1. A parallel-plate capacitor is charged to a voltage V by a battery. The battery is then disconnected.

Describe the implications, if any, disconnecting the battery has on the charge on the plates.

2. A parallel-plate capacitor is charged to a voltage V by a battery. The batter remains connected to the capacitor.

What will this mean for the voltage of the capacitor in relation to the battery?

 

[411309]

1. None because charge is conserved, if the battery is disconnected charge is still fixed and remains constant.

As for number two im confused on what they are asking specifically. Im too tired. Ill let one of the SDN geniuses answer question two, lol.

 

[chiddler]

Here are 2 slight different short scenarios.

1. A parallel-plate capacitor is charged to a voltage V by a battery. The battery is then disconnected.

Describe the implications, if any, disconnecting the battery has on the charge on the plates.

2. A parallel-plate capacitor is charged to a voltage V by a battery. The batter remains connected to the capacitor.

What will this mean for the voltage of the capacitor in relation to the battery?

1. When the battery is disconnected, the capacitor discharges and therefore the charge on the plates decrease as current flows through the circuit.

2. Pretty sure the voltage does not change. What makes me think this is true is that if voltage decreases, then the capacitor will discharge since current goes from high potential to low potential.

Oh "in relation to the battery"...wouldn't this depend? For example, if it's in series with another identical capacitor, then half the voltage goes to both capacitors and therefore their capacitance is halved. So half voltage goes to the capacitor in this case.

 

[SaintJude]

re: 1. Colesmalls is right. Once the battery is disconnected, the charge will stay constant. This was a revelation for me as well. I think it's the case b/c capacitor are capable of storing charge, i mean I guess that's what they do for a living according to the definition. Anyway, it was crucial in a problem that asked me to predict how the voltage would be affected after an insulator was placed in the capacitor. B/c I then had to use

Q = CV with the assumption that Q remained constant. B/c only then, can one make a prediction that if C increased, V decreases.

re: 2. Yeah, voltage doesn't change and equals that of the battery. When I saw the picture below that finally made sense to me.

The voltage is that of the battery. The question stem says " A parallel-plate capacitor" and I think that requires you to assume it's a single capacitor. But you are right for multiple plate capacitors & thanks for reminding me about that.

 

[chiddler]

huh. then a question: when a capacitor discharges, why doesn't q decrease?

 

[SaintJude]

huh. then a question: when a capacitor discharges, why doesn't q decrease?

Probably better colesmalls answers this..

But charge does decrease. How it happens, I don't know.

 

[chiddler]

huh. then a question: when a capacitor discharges, why doesn't q decrease?

bump.

"Once the battery is disconnected, the charge [on the capacitor] will stay constant"

 

[411309]

bump.

"Once the battery is disconnected, the charge [on the capacitor] will stay constant"

even if you disconnect a capacitor from a voltage source it still keeps it's charge (although a small amount slowly decays away) because it gets stored in the electric field between the two plates. It sort of acts like an imperfect insulator.

 

[chiddler]

even if you disconnect a capacitor from a voltage source it still keeps it's charge (although a small amount slowly decays away) because it gets stored in the electric field between the two plates. It sort of acts like an imperfect insulator.

i don't understand.

i thought the whole point of a capacitor is to store charge, then release when voltage source is disconnected.

what acts like an imperfect insulator? do you mean an electrical or thermal insulator?

 

[411309]

i don't understand.

i thought the whole point of a capacitor is to store charge, then release when voltage source is disconnected.

what acts like an imperfect insulator? do you mean an electrical or thermal insulator?

Well charge is stored on a capacitor because it has two conducting plates with a dialetric in the middle thats nonconducting and this causes charge to accumulate on the plates and allows it store it.

to answer your first question, theres a good little summary that explains discharging capacitors. I recommend looking here http://www.techitoutuk.com/knowledge/electronics/components/capacitors/capac.html

 

[chiddler]

Well charge is stored on a capacitor because it has two conducting plates with a dialetric in the middle thats nonconducting and this causes charge to accumulate on the plates and allows it store it.

to answer your first question, theres a good little summary that explains discharging capacitors. I recommend looking here http://www.techitoutuk.com/knowledge/electronics/components/capacitors/capac.html

thanks it appears i need to do more reading

 

[mcloaf]

You guys are both right, it just depends on whether or not the capacitor is in a closed or open circuit once the battery is removed. If closed, it will discharge. If open, it will store the charge until it can discharge through a closed circuit.

 

[chiddler]

You guys are both right, it just depends on whether or not the capacitor is in a closed or open circuit once the battery is removed. If closed, it will discharge. If open, it will store the charge until it can discharge through a closed circuit.

thank you. i was pretty upset about this because it seemed that i was missing a very fundamental concept.

 

[drechie]

so, can someone re-cap this discussion succinctly