The radius of a metallic sphere if its capacitance is $1/9\,F$, is

Two metallic charged spheres whose radii are $20\,cm$ and $10\,cm$ respectively, have each $150\,micro - coulomb$ positive charge. The common potential after they are connected by a conducting wire is

A $500 \,\mu F$ capacitor is charged at a steady rate of $100\, \mu C/sec$. The potential difference across the capacitor will be $10\, V$ after an interval of.....$sec$

Answer carefully:

$(a)$ Two large conducting spheres carrying charges $Q _{1}$ and $Q _{2}$ are brought close to each other. Is the magnitude of electrostatic force between them exactly given by $Q _{1} Q _{2} / 4 \pi \varepsilon_{0} r^{2},$ where $r$ is the distance between their centres?

$(b)$ If Coulomb's law involved $1 / r^{3}$ dependence (instead of $1 / r^{2}$ ), would Gauss's law be still true?

$(c)$ $A$ small test charge is released at rest at a point in an electrostatic field configuration. Will it travel along the field line passing through that point?

$(d)$ What is the work done by the field of a nucleus in a complete circular orbit of the electron? What if the orbit is elliptical?

$(e)$ We know that electric field is discontinuous across the surface of a charged conductor. Is electric potential also discontinuous there?

$(f)$ What meaning would you give to the capacitance of a single conductor?

$(g)$ Guess a possible reason why water has a much greater dielectric constant $(=80)$ than say, mica $(=6)$

Assertion : The total charge stored in a capacitor is zero.

Reason : The field just outside the capacitor is $\frac{\sigma }{{{\varepsilon _0}}}$. ( $\sigma $ is the charge density).

The potentials of the two plates of capacitor are $+10\,V$ and $-10\, V$. The charge on one of the plates is $40 \,C$. The capacitance of the capacitor is........$F$