# Drift velocity and charging a capacitor.

1. Jun 25, 2012

### jhammin

I'm having trouble understanding where the charging time in a capacitor actually comes from. Is it possible to derive the $\tau$ of a capacitive circuit from the drift velocity of the electrons? Are charges literally moving from the conductors onto the metal plates of the capacitor?

2. Jun 25, 2012

### drummin

Yes, the drift velocity is quite slow but there are a lot of electrons in the "electron gas" of the metal conductor.

3. Jun 25, 2012

### jhammin

The charging time of capacitor depends on its energy absorption, which can be used directly to get the drift velocity. Looks like I answered my own question.

I was caught up in the notion that electrons never actually enter or leave a wire. I'm not sure how i reached that idea because obviously the dispersion region in a diode depends on how many excess charges are available to be shared across the junction.

4. Jun 25, 2012

### jhammin

Ya, I was thrown off by the shear number of electrons. I was also thinking too much about the individual Coulomb forces between each electron.