A) 250V
B) 300V
C) 400V
D) 600V
Correct Answer: C
Solution :
Key Idea: The common potential on the capacitor is the ratio of net charge to the net capacitance. \[\text{Common}\,\text{potential (V) = }\frac{\text{Net}\,\text{charge}}{\text{Net capacitance}}\] \[=\frac{{{q}_{1}}+{{q}_{2}}}{{{C}_{1}}+{{C}_{2}}}\] \[=\frac{{{C}_{1}}{{V}_{1}}+{{C}_{2}}{{V}_{2}}}{{{C}_{1}}+{{C}_{2}}}\] Here, \[{{C}_{1}}=20\times {{10}^{-6}}F,\] \[{{V}_{1}}=500\,V\] \[{{C}_{2}}=10\times {{10}^{-6}}F,\] \[{{V}_{2}}=200V\] \[\therefore \] \[V=\frac{20\times {{10}^{-6}}\times 500+10\times {{10}^{-6}}\times 200}{30\times {{10}^{-6}}}\] \[=400V\] Note: When a capacitor is charged by battery, then 50% of energy is liberated or wastage) as heat always.
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