Some equations relating to capacitors:

When a capacitor is discharged to produce an electric current, the decrease in the charge stored in the capacitor is exponential - it has a constant half-life. The equation for this discharge is:

x = x0 e-(t / CR)

t is the time during which the capacitor discharges, C is the capacitance of the capacitor, and R is the resistance of the circuit connected to it. x can stand for the charge in the capacitor, Q, and it can also stand for the current and voltage of the resulting electric flow, I and V. Therefore as a capacitor discharges, charge in the capacitor and current and voltage in the circuit all decrease exponentially.

C × R, capacitance times resistance, is known as the time constant of a capacitor and is represented by τ, the letter tau. It is equal to the time, in seconds, taken by the values of Q, I and V (represented by x in the above equation) to decrease by a factor of e, the exponential function.

If capacitors are placed in parallel, the total capacitance is the sum of the individual capacitances: Ctotal = C1 + C2 + C3 etc. If they are placed in series, the total capacitance decreases according to the equation: 1/Ctotal = 1/C1 + 1/C2 + 1/C3 etc. Note that is the reverse of the case for resistors, which become less effective in parallel and more effective in series.

See also capacitor time constant