# Question #18ca7

May 4, 2017

Same potential across each in parallel so:

$Q = C V \implies {Q}_{1} / 5 = {Q}_{2} / 7 = \implies {Q}_{1} = \frac{5}{7} {Q}_{2}$

In addition we have conservation of charge.

Total charge before they are joined, ${Q}_{i}$ was:

$\sum Q i = \frac{5 \cdot 450 + 7 \cdot 750}{{10}^{6}} = \frac{3}{400} C$

${Q}_{i} = {Q}_{1} + {Q}_{2}$

$\implies {Q}_{2} = \frac{7}{1600} C , {Q}_{1} = \frac{1}{320} C$

Voltage:

$V = {Q}_{2} / {C}_{2} = \frac{\frac{7}{1600}}{7 \cdot {10}^{- 6}} = 625 V$