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Problems in capacitors Udit Gupta Ex-VMC Physics Faculty MNNIT Allahabad

Subjective Problem -1 A parallel plate capacitor has plates of area 400cm2 and separation between the plates 2.00mm. What potenticl difference will be developed if a charge of 1.00nC is given to the capacitor? If the plate separation is now increased to 4.00mm, what will be the new potential difference? 2

Approach Level 1-Easy Basic definitions -) Parallel plate capacitor

Solution: On increasing the separation from 2mm to 4mm, the capacitance is decreased by a factor of 2 If the charge remains same, the potential difference will increase by a factor of 2. Thus the new potential difference will be 5.65 x 2 11.3V The capacitance of a parallel plate capacitor is given as: EgA C= C = 8.85 10-12 x too x 10-4 C = 0.177 10-9F 2 10-3 The potential dittererics between the plates is: 5.65V = = C 0.177

Subjective Problem - 2 An isolated sphere has a capacitance of 25pF Calculate its radius. b) How much charge should be placed on it to raise its potential to 104V? 5

Approach Level 1-Easy Basic definitions Spherical Capacitor

Solution (a): The capacitance of an isolated sphere is: 25 10-12- 9 109 R 22.5cm Solution (b): Q CV 25 x 10-12 x 10 0.25uC

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