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PHYSICS DOUBTS (Electrostatics)

Q 1: The electric strength of air is 2*10^7 N/C. The maximum charge that a metallic sphere of diameter 6mm can hold is?
a. 3nC b. 20nC c. 1.5nC d. 2nC
SOURCE: Arihant Physics for JEE Main
MY DOUBT: The obvious formula here would be Q=4 pi e0 r^2 E . However, instead of using E = 2*10^7 N/C an additional step is printed: "As maximum value of E = 10% dielectric strength, hence E = 10%(2*10^7 ) = 2*10^6 N/C"
Where did that step come from? Is there any such law? If yes, does it always hold true?

Q 2: A 4microF capacitor is charged to 400V. If its plates are joined through a resistance of 4kOhm, then heat produced in the resistance will be?
a. 0.16J b. 0.32J c.0.64J d. 1.28J
SOURCE: MH-CET 2000
MY DOUBT: How do you solve this? Since V=IR does not hold true, I guess it would be right to use the formula for discharging of a capacitor (?)

Q 3: A solid conducting sphere having a charge Q is surrounded by an uncharged concentric conducting hollow spherical shell. Let the potential difference between the surface of the solid sphere and that of the outer surface of the hollow shell be V. If the shell is now given a charge of -3Q, the new potential difference between the same two surfaces is?
a. V b. 2V c. 4V d. -2V