# Capacitance Theory

Discussion in 'Homework Help' started by ajpik, Mar 13, 2014.

1. ### ajpik Thread Starter New Member

Feb 13, 2014
15
0
I posted a page from homework with solution. I'm going to have a quiz tomorrow morning on a similiar topic and just do NOT understand the theory behind their equation. If anyone could give it a look, I'd appreciate it.

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2. ### blah2222 Well-Known Member

May 3, 2010
565
33
One way to look at this is by thinking of what is happening to the charge leading up to steady-state. Since all the capacitors are said to be initially fully discharged, they have zero potential difference across their plates before the supplies are turned on. The problem just boils down to conservation of charge.

When the supplies turn on a number of things happen (disregarding the circuits arbitrary +/- signs):

Capacitor C1 accumulates 'positive' charge on its left plate, causing 'positive' charge to leave its right plate, moving into the Vx node.

Capacitor C2 accumulates 'positive' charge on its left plate, causing 'positive' charge to leave its right plate, moving into the Vx node as well.

Capacitor C4 accumulates 'positive' charge on its right plate, causing 'positive' charge to leave its left plate, moving into the Vx node also.

Following the conservation of charge, all the charge moving into node Vx must be equal to the charge leaving, which in this case deposits onto the top plate of capacitor C3, followed by charge exiting its bottom plate to ground.

The equal for Qeq is just another way of writing that the charge held on C3 is the sum of C1, C2, and C4.

Hope that helped.

ajpik likes this.
3. ### ajpik Thread Starter New Member

Feb 13, 2014
15
0
haha it did help a lot but the teacher quized us on something completely different :-(