electricity - Why do charges enter the capacitor in the first place?

I have been studying Capacitors for the past year now and the one thing I don't understand is how a charge is stored on the capacitor. Essentially, a circuit with a capacitor is an incomplete circuit right?

So why do the electrons start to gather up on one of the plates. Its like they are tricked into thinking its a complete circuit and get trapped once they realise it isn't! What is this sorcery!

What is happening to make the electrons do that and if it simply is electrons moving and then turning back or getting stuck, would a switch have the same effect? (Assuming the switch is open).

Hopefully someone can finally fill the major gap in my knowledge because everything else I read doesn't explain this.

Answer

Imagine taking a disc of some conductor and putting it in an electric field (with the plane of the disc at right angles to the field lines). The free electrons in the conductor will move in response to the field, so the face at the positive side gets a net negative charge and the other face gets a net positive charge. You get a transient current through the body of the disk when you turn on the electric field and a transient current in the other direction when you turn it off.

Now split the disk through the middle, i.e. turn it into the two plates of a capacitor, and connect a wire between the two outside faces, then switch on the external electric field again. Once more the disk gets polarised, but this time you get a transient current flowing from one side of the (now split) disk to the other through the wire. You end up with the same charge separation as before, but the electrons flowed through the piece of wire instead of through the body of the disk.

Finally, connect a battery in the middle of the piece of wire. This produces an electric field between the two sides of our split disk (i.e. capacitor), and just like the externally applied field it will polarise the disk and electrons will flow through the wire from one face of the disk to the other.

That's why when you connect a battery to a capacitor you get a transient current as the capacitor polarises.