Question

A capacitor of capacitance 200 micro farad is charged to a potential of 1000w. The stored energy in wattsecond is

Answer 1

when a charged capacitor is connected with another uncharged capacitor , then charge transfer from charged capacitor to uncharged capacitor untill both attend same potential .

now, A capacitor of capacitance 200 pF charged by 300V battery
Means charge on this capacitor = CV = 200 × 300 pC = 6 × 10⁴ pC

Now, charged capacitor of charge 6 × 10⁴ pC is connected with uncharged capacitor . Let final potential of both capacitors is V
Then, intial charge = final charge
6 × 10⁴ pC = 200 × V + 100 × V
V = 200 V
Hence, final potential is 200V

Now, energy stored intially on capacitor of 200pF = 1/2 CV²
= 1/2 × (200pF)×( 300V )²
= 100 × 10⁻¹² × 90000 J
= 9 × 10⁻⁶ J
Finally energy stored in both capacitors = 1/2(C + C')V²
= 1/2 × (200pF + 100pF) × (200V)²
= 150 × 10⁻¹² × 40000
= 6 × 10⁻⁶ J

Hence, difference in energy stored = intial energy - final energy
= 9 × 10⁻⁶ J - 6 × 10⁻⁶ J = 3 × 10⁻⁶J


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