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can someone solve question 4 C(!!)
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Physics (9702)/9702_w09_qp_42.pdf
The capacitance of the sphere, as calculated in the previous part of the same question, is 7 * 10^-11 Farads.
When the capacitor discharges, the energy stored in the sphere decrease to 25 % of the original, so the final energy is 1/4th of the initial energy:
0.5 * C * V(final)^2 = 0.5 * C * V(initial)^2 * 0.25
V(initial) is equal to 1.2 * 10^6 Volts, as is shown in the question.
C = capacitance = 7.0 * 10^-11 Farads
Therefore,
V(final)^2 = V(initial)^2 * 0.25
V(final) = 6.0 * 10^5 Volts
Hope this helped!
Good Luck for all your exams!