# What's the right way to calculate charge on a capacitor? [closed]

How much charge is on each plate of a 4.00-F capacitor when it is connected to a 12.0-V battery?

I said 2.4 x 10^-5 C because there are two plates of a parallel plate capacitor. But the key said only 4.8 x 10^-5 C

Why?

## closed as off-topic by ACuriousMind♦Dec 15 '16 at 22:57

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The charge on the two plates is opposite, and equal to the key answer. The parameter Q is the charge on one of the plates, not the sum of the absolute value of the two charges. The definition of capacitance is the charge per unit voltage, so that $Q=CV$, so 4F times 12V is 48 units of charge, which for the unit "F" is millionths of a Coulomb C. This gives the book's answer.
The formula $V=\frac{Q}{C}$ gives the amount of charge that is there on one of the plates. The total amount of charge on both the plates taken together is zero! Both of them are oppositely charged.