# Same Polarity Charges Accumulating on Both sides of a Capacitor Simultaneously??

#### Metin Ozsavran

Jul 30, 2016
1
Dear Guys,

Here is a really fundamental and elementary question for you.

A- When you charge a capacitor with a DC voltage source, you see opposite charges accumulate on either side, and measure as such relative to your ground.

B- When you apply an ordinary AC sinusoidal signal with equal positive and negative peaks to the same capacitor, both scopes and simulators show simultaneously positive or negative voltages on both sides, with a slight voltage drop related to its reactive impedance Xc. Even when you take down frequency as low as 0.1Hz or 10 seconds each cycle, or change capacitor sizes between picos and micros same thing is observed.

Question:
=======
Shouldn't there be a 180 degree (pi radians, half a cycle) voltage phase difference across a capacitor when applied AC. When one side of the capacitor is at positive peak of the wave, shouldn't the other side be at negative peak?

No book, or web source I digged gave me a satisfactory explanation as to the Physics of what is going on here. Even if its related to charge discharge times, still both sides should not show same polarity simultaneously, and no difference is visible with same source with different size caps. Measurement direction is the same in both DC and AC applications too.

Can a few people take a stab at this glaring gap in my humble little electronics knowledge chain please? It is one of those painful ones that stops most progress in my small head.

#### davenn

Moderator
Sep 5, 2009
14,011
A- When you charge a capacitor with a DC voltage source, you see opposite charges accumulate on either side, and measure as such relative to your ground.

no, measured relative to the 2 plates

B- When you apply an ordinary AC sinusoidal signal with equal positive and negative peaks to the same capacitor, both scopes and simulators show simultaneously positive or negative voltages on both sides

that surprises me

When one side of the capacitor is at positive peak of the wave, shouldn't the other side be at negative peak?

that is what I would expect

lets see what others say

Dave

#### Ratch

Mar 10, 2013
1,094
Dear Guys,

Here is a really fundamental and elementary question for you.

A- When you charge a capacitor with a DC voltage source, you see opposite charges accumulate on either side, and measure as such relative to your ground.

B- When you apply an ordinary AC sinusoidal signal with equal positive and negative peaks to the same capacitor, both scopes and simulators show simultaneously positive or negative voltages on both sides, with a slight voltage drop related to its reactive impedance Xc. Even when you take down frequency as low as 0.1Hz or 10 seconds each cycle, or change capacitor sizes between picos and micros same thing is observed.

Question:
=======
Shouldn't there be a 180 degree (pi radians, half a cycle) voltage phase difference across a capacitor when applied AC. When one side of the capacitor is at positive peak of the wave, shouldn't the other side be at negative peak?

No book, or web source I digged gave me a satisfactory explanation as to the Physics of what is going on here. Even if its related to charge discharge times, still both sides should not show same polarity simultaneously, and no difference is visible with same source with different size caps. Measurement direction is the same in both DC and AC applications too.

Can a few people take a stab at this glaring gap in my humble little electronics knowledge chain please? It is one of those painful ones that stops most progress in my small head.

I just love to talk about capacitors. But, let's get a few facts straight first. We have to be precise about this. What do you mean when you say you are "charging" a capacitor? What are you charging it with?

Ratch