CAPACITOR EQUATIONS

Two capacitors in series (of capacitance C1 and C2 , have the same charge Q stored on both, but a different voltage across each. The sum of the two voltages is the p.d. across the two combined, the voltages across the two are equal to Q/C1 and Q/C2 respectively. The combined volatage is equal to Q/C1 + Q/C2 and of course equal to Q/ C, where C is the combined capacitance. So formula for combined capacitane is series is - 1/ C = 1/C1 + 1/C2

Two capacitors in parallel have a common p.d. Vacross them but different charges on each Q1 and Q2 respectively. Q1 = C1 V and Q2 = C2 V, adding to give the total charge on the combined capacitor, Q = (C1 + C2) V and comparing with the definition equation, we see that : C =C1 + C2

PRACTICAL MEASUREMENT
The circuit drawn opposite can be used to test the capacitance of a capacitor. R is a reed switch, which changes back and forward rapidly, G is a sensitive (light beam) galvonmeter, C is the capacitor to be tested. The system works by charging and discharging the capacitor rapidly by using the reed switch, the discharge circuit runs through a galvonmeter, which measures the discharge current which is proportional to the charge stored.