Question

Two capacitors of 3 µF and 6 µF are connected in series and a potential difference of 900 V is applied across the combination. They are then disconnected and reconnected in parallel. The potential difference across the combination is ______.

Options

• Zero

• 100 V

• 200 V

• 400 V   

Answer 1

Two capacitors of 3 µF and 6 µF are connected in series and a potential difference of 900 V is applied across the combination. They are then disconnected and reconnected in parallel. The potential difference across the combination is 400 V.

When connected in series,

∴ C α `1/V`

∴ `"C"_1/"C"_2="V"_2/"V"_1⇒ 3/6="V"_2/"V"_1⇒ "V"_1=2"V"_2 `

Also `  "V"_1 + "V"_2 = 900 "V"`

∴ `2"V"_2+"V"_2=900"V"`

`"V"_2=300 "V" and "V"_1=600"V"`

After connecting in parallel, common potential

`"V"=("C"_1"V"_1+"C"_2"V"_2)/("C"_1+"C"_2)`

=`((3xx10^-6xx600)+(6xx10^-6xx300))/((3xx10^-6)+(6xx10^-6))`

=`((1800xx10^-6)+(1800xx10^-6))/(9xx10^-6)`

`=3600/9`

=400 V