Why Do Capacitors Accumulate Charge?

[Avichal]

After studying more complicated electronics, I find myself asking this question - 'Why capacitor works?'
Why does charge get accumulated on the plates? Isn't that a higher energy state than battery?

 

[mfb]

Why does charge get accumulated on the plates?

Opposite charges attract each other, and the other side has the opposite charge.

Isn't that a higher energy state than battery?

No, the capacitor gets charged until the energy per electron is the same in both.

 

[Avichal]

No, the capacitor gets charged until the energy per electron is the same in both.

Why is it not a higher energy state? When the circuit is closed I understand why electrons flow. But when it is open(which is in this case), I don't understand why electrons should flow.

 

[Drakkith]

Why is it not a higher energy state? When the circuit is closed I understand why electrons flow. But when it is open(which is in this case), I don't understand why electrons should flow.

The circuit is only "open" when the capacitor is fully charged. Until then current will flow into one plate and out of the other until the voltage drop across the plates is equal to the applied voltage.

 

[CWatters]

If you are familiar with static electricity you will know that it's possible for an object like a plastic comb or rod to become charged up (eg it acquires a surplus or shortage of electrons). Much the same thing happens with each plate of a capacitor...

Imagine one plate (lets call it the top plate) behaves like a comb that you have charged up with extra electrons. If that top plate is close to another plate (eg the bottom plate) then the electrons in the top plate will repel electrons in the bottom plate (like charges repel remember). The electrons in the bottom plate will try to move away from the top plate by escaping down the wire.

So now you have electrons on the top plate and a shortage of electrons on the bottom plate... and for awhile you had current flowing. You put electrons into the top plate and different electrons left the bottom plate.

If you have electrons on one plate and a shortage on the other then there will be an electric field/ voltage between the two plates. This voltage will build up until it's greater than the voltage that caused the surplus of electrons on the top plate. At that point no more electrons flow onto the top plate or out of the bottom plate.

 

[Khashishi]

A capacitor is somewhat analogous to a water tower. You can pump water in, then close off the gate. Later, you can open the gate and water will flow out. Now replace water with charge, and water pressure with voltage.