3
\$\begingroup\$

Encyclopedia of Electronic Components - Vol1, pg:98 by Charles Platt

Electrons from the power source will migrate on­ to the plate attached to the negative side of the source, and will tend to repel electrons from the other plate. This may be thought of as creating electron holes in the other plate or as attracting positive charges,

Isn't this dead wrong! Electrons don't migrate from the source (battery, power supply). Electrons aren't little rolling balls that leave the battery on a journey to the dielectric where they polarize atoms!

Instead, the battery/PSU provides a Electric field (that travels at c=speed of light approximately) that polarizes the dielectric across a region of space within it; electrons are displaced from their favorite haunts around their home atoms and you won't find them there anymore (or you might - the probability distribution is altered).

This does not create a hole, or ion! A hole is the promotion of a valence band electron into the conduction band where it's free to move about in the material lattice.

What is important is that a capacitor stores an electric field by aligning it's (atoms, molecules - charge carriers) in one way.

Maybe electrons in the wire will move into the battery and cause a chemical reaction that is used to maintain the Electric field.

So that para is wrong -right?

One thing i don't understand though is why, when the cap is disconnected, it still retains the orientation (in the absence of the external E-field).

\$\endgroup\$
  • \$\begingroup\$ What atoms are getting aligned in a capacitor consisting of two plates separated by a vacuum? \$\endgroup\$ – Spehro Pefhany May 30 '14 at 4:32
1
\$\begingroup\$

Do electrons migrate from the source to the dielectric? No. They flow from one plate to the other plate.

Imagine a capacitor disconnected from everything, with no energy stored in it initially. This means that each of the two electrodes has a net charge of 0. For example, if each electrode/plate has million protons, then it must also have a million electrons.

Now let’s connect a battery to the cap. What happens now is that the battery pumps a percentage of the million electronics from plate1 (connected to the plus side of the battery) to plate2 of the capacitor. This flow of charge (current) goes through the battery (since the 2 plates are electrically isolated by a dielectric), and into the other plate, until the cap reaches the battery voltage. In this sense, the electrons migrate from the source to the plate.

(Imagine the battery as a water pump, and the water molecules are the electrons particles. Yes, you can imagine these as "little rolling balls that leave the battery".)

Now once you disconnect the battery, the electrons that have already been pumped onto plate2 remain there since the two plates are electrically isolated.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy