# capacitance equations

Discussion in 'Homework Help' started by squat, Feb 11, 2013.

1. ### squat Thread Starter New Member

Feb 11, 2013
1
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Hi guys, need some help finding how to do a few math probs in a caps circuit. I have three caps in series 10mf,5mf,4mf The supply voltage is a 12v DC supply. I need to know how to find

the total capacitance
the total charge
the voltage across each cap
the energy stored in each cap

then i have three caps in parrell 200nf,300nf,600nf its a dc supply 15v,I have to find.

the charge in each cap
the total capacitance
the total charge
the energy stored in each cap

I would really appreciate the help

2. ### DiodeMan New Member

Feb 3, 2013
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I'm going to assume that you have some electrical background when answering this question, and not answer it directly because that is no way to learn. The way which you phrased it has me unsure whether you have experience or not though.

if you have dealt with resistors in parallel before, and used the "product-over-sum" rule, capacitors in series are treated this way. However, capacitors in parallel are treated similar to resistors in series.

The total charge can be found by taking the total capacitance and multiplying this by the supply voltage. The equation is Q = CV

Charge acts similar to current, as in series it is the same for all for all the capacitors, and in parallel it is dissimilar.

The voltage across each capacitor is found using the above formula in series, and in parallel using the fact that parallel branches have equal voltages.

To find the energy stored in a capacitor, the equation is W = 1/2 * C * V^2.

If you have had teachings in electricity before, then these are concepts that you should be familiar with. If you are doing this for any other reason, I am sorry I could not have been more help, and wish you the best of luck.

3. ### WBahn Moderator

Mar 31, 2012
23,381
7,095
Not a whole lot to add to the previous response.

In each case you need to identify which quantities have to be equal to the total quantity for each of the caps in the circuit and which quantities have to sum to the total for the circuit. The keep in mind that the total capacitance is the total charge divided by the total voltage.

As a check, the total energy calculated for the total capacitance has to equal the sum of the energies for the individual caps.