Question

A $1 \mu F$ capacitor and a $2 \mu F$ capacitor are connected in parallel across a 1200 volts line. The charged capacitors are then disconnected from the line and from each other. These two capacitors are now connected to each other in parallel with terminals of unlike signs together. The charges on the capacitors will now be

• A. $1800 \mu C$each
• B. $400 \mu C$ and
• C. and $400 \mu C$
• D. and

Answer 1

Answer: (B)

$400 \mu C$ and

Answer 2

Initially charge on capacitors can be calculated as follows

Q₁ = 1 × 1200 = 1200 μC and Q₂ = 2 × 1200 = 2400 μC

Finally when battery is disconnected and unlike plates are connected together then common potential $V ^ { \prime } = \frac { Q _ { 2 } - Q _ { 1 } } { C _ { 1 } + C _ { 2 } }$ $= \frac { 2400 - 1200 } { 1 + 2 }$ $= 400 \mathrm {~V}$

Hence, New charge on $C _ { 1 }$ is

And New charge on $C _ { 2 }$ is $2 \times 400 = 800 \mu C$.