# Capacitors (charging after long time)

• jisbon
In summary, the conversation discussed the flow of current through a resistor after a long time, the voltage drop across a capacitor being equal to the EMF of the cell, and finding the energy stored in a capacitor after a switch has been closed for a long time. The final answer was solved to be ##1.25x10^{-4}##.
jisbon
Homework Statement
Find energy stored in capacitor after switch has been closed for long time.
Relevant Equations
-

Hi there,
So as far as I know, after long time, current doesn't flow through the resistor That means the voltage drop across the capacitor is equal to the EMF of the cell = 9V
So will the charge be ##U = 0.5CV^2## = ##0.5(10)(81)##?
Answer appears to be wrong, not sure what I am doing wrong here.

jisbon said:
Homework Statement: Find energy stored in capacitor after switch has been closed for long time.

Hi there,
So as far as I know, after long time, current doesn't flow through the resistor .
through what resistor? I think after a long time there won't be any current through the capacitor, and therefore no current through the 3 ohm resistor.
That means the voltage drop across the capacitor is equal to the EMF of the cell = 9V
I can really see no reason for that, no matter which resistor you meant.

If there's no current through the capacitor, you can remove it from the circuit, solve the remaining circuit to find the voltage across the capacitor.

Delta2
willem2 said:
If there's no current through the capacitor, you can remove it from the circuit, solve the remaining circuit to find the voltage across the capacitor.
Won't the voltage across the capacitor be 9v?

jisbon said:
Won't the voltage across the capacitor be 9v?
Isn't the 15Ω resistor connected too? Think about its effect.

Ah
sophiecentaur said:
Isn't the 15Ω resistor connected too? Think about its effect.
Ah..Solved it :) ##1.25x10^{-4}##
Thanks!

sophiecentaur

## What is a capacitor?

A capacitor is an electronic component that stores electrical energy in the form of an electric field. It consists of two conductive plates separated by an insulating material called a dielectric.

## How does a capacitor charge after a long time?

When a capacitor is connected to a power source, it charges by accumulating electrons on one plate and removing them from the other plate. After a long time, the capacitor will reach its maximum charge and the flow of electrons will stop.

## Why does a capacitor take a long time to charge?

A capacitor takes a longer time to charge compared to other electronic components because it has a limited amount of space to store electrons. As more electrons are added, the repulsive force between them increases, making it harder for additional electrons to be added.

## What happens when a capacitor is overcharged?

When a capacitor is overcharged, it can lead to damage or failure of the component. This is because the dielectric material can break down due to the high amount of energy stored, causing a short circuit.

## Can a capacitor discharge after a long time?

Yes, a capacitor can discharge after a long time if it is connected to a circuit with a lower voltage than its stored charge. This can happen if the power source is disconnected or if the capacitor is connected to a load that has a lower voltage than the capacitor's stored charge.

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