# Energy stored by a capacitor

The energy (measured in Joules) stored in a capacitor is equal to the work done to charge it. Consider a capacitance C, holding a charge +q on one plate and -q on the other. Moving a small element of charge $\mathrm {d} q$ from one plate to the other against the potential difference V = q/C requires the work $\mathrm {d} W$ :

$\mathrm {d} W={\frac {q}{C}}\,\mathrm {d} q$ where

W is the work measured in joules
q is the charge measured in coulombs
C is the capacitance, measured in farads

We can find the energy stored in a capacitance by integrating this equation. Starting with an uncharged capacitance (q=0) and moving charge from one plate to the other until the plates have charge +Q and -Q requires the work W:

$W_{charging}=\int _{0}^{Q}{\frac {q}{C}}\,\mathrm {d} q={\frac {1}{2}}{\frac {Q^{2}}{C}}={\frac {1}{2}}CV^{2}=W_{stored}$ Combining this with the above equation for the capacitance of a flat-plate capacitor, we get:

$W_{stored}={\frac {1}{2}}CV^{2}={\frac {1}{2}}\epsilon {\frac {A}{d}}V^{2}$ .

where

W is the energy measured in joules
C is the capacitance, measured in farads
V is the voltage measured in volts