# Capacitors -- query about plates remaining at same potential

#### logearav

Joined Aug 19, 2011
243
Revered members,
In a tank circuit, When the capacitors discharge, electrical energy begin to collapse and through the current flowing through the inductor , magnetic field is produced and in this process electrical energy is converted into magnetic energy. When the capacitor is fully discharged, the plates of the capacitors remain at the same potential.
I want to know the interpretation of the term same potential. Does it mean both the plates having the same charge, that is either + or - ? And the other possibility is if one plate is having 6C(say) of + charge, the other plate has 6C of negative charge. Thanks in advance, members

#### thatoneguy

Joined Feb 19, 2009
6,359
When a capacitor is discharged, there is 0V potential between the two plates. If you put a voltmeter across the terminals, it would read 0V. No energy is stored in the capacitor.

Keep in mind that a capacitor is discharged when there is the same DC voltage on both terminals (with no AC present). The voltage potential across the terminals only appears when the capacitor is charging or fully charged.

This gets more complicated in an AC circuit, as the capacitor has a changing voltage across it's terminals, the faster the voltage is changing (frequency), the capacitor will act differently. A capacitor appears as an open circuit to low frequencies by storing a charge, thus not letting current pass. While a capacitor will be a low impedance conductor at high frequencies by not staying in a single state of charge.

#### logearav

Joined Aug 19, 2011
243
Keep in mind that a capacitor is discharged when there is the same DC voltage on both terminals (with no AC present). .
Thanks for the reply. I can't understand sir. Do you mean both plates are at 5V(say)?

#### thatoneguy

Joined Feb 19, 2009
6,359
Thanks for the reply. I can't understand sir. Do you mean both plates are at 5V(say)?
Yes. If both terminals are at 5V, with no AC present, the voltage across the capacitor, which represents the stored charge, is 0V.

In your tank circuit, you are dealing with an AC implementation. The core equations for the capacitor and inductor are the same, but the behavior isn't as simple as DC. DC is a special case since frequency (ω) = 0.

See the e-book at the top of the page, Vol. II -AC for full calculations on behavior of capacitors in an AC environment. Capacitors are generally said to "pass AC" and "block DC", in a dynamic circuit, which is true. They are often used in amplifiers to remove a DC bias from an input signal, for example.

It is around this point where the answers aren't quick and short paragraphs, and are much better explained by people who have worked together to write the section of our e-book.

#### logearav

Joined Aug 19, 2011
243
But sir, when we talk about capacitor, one plate have + charge and the other plate has negative charge. So in terms of these charges, what is meant by plates remaining at same potential?

#### thatoneguy

Joined Feb 19, 2009
6,359
But sir, when we talk about capacitor, one plate have + charge and the other plate has negative charge. So in terms of these charges, what is meant by plates remaining at same potential?
Nether plate has a charge, if both plates are at DC 40V, the capacitor is holding the same charge as when both plates are at DC 0V (none). The plate charge is in reference to the other plate, not an external reference.