A solid metallic sphere has a charge $+3Q$. Concentric with this sphere is a conducting spherical shell having charge $-Q$. The radius of the sphere is $a$ and that of the spherical shell is $b(b>a)$. What is the electric field at a distance $R(a<R<b)$ from the centre?

If $g_E$ and $g_M$ are the accelerations due to gravity on the surfaces of the Earth and the Moon respectively and if Millikan's oil drop experiment could be performed on the two surfaces, one will find the ratio $\frac{\text{electronic }\text{charge }\text{on }\text{the }\text{moon }}{\text{electronic }\text{charge }\text{on }\text{the }\text{earth }}$  to be

In the following figure, three paths of $\alpha$ particle crossing a nucleus $N$ are shown. The correct path is 

(a) 

(b) 

(c) 

A conducting sphere of radius $10 \mathrm{cm}$ is charged to $10 \mu \mathrm{C}$. Another uncharged sphere of radius $20 \mathrm{cm}$ is allowed to touch it for some time. After that, if the spheres are separated, then surface density of charges on the spheres will be in the ratio of

Two spherical conductors $A_1$ and $A_2$ of radii $r_1$ and $r_2$ are placed concentrically in air. The two are connected by a copper wire as shown in figure. Then the equivalent capacitance of the system is