Which one of the operations of $$\mathrm{n}-\mathrm{p}-\mathrm{n}$$ transistor differs from that of $$p-n-p$$ transistor?

A hollow metal pipe is held vertically and bar magnet is dropped through it with its length along the axis of the pipe. The acceleration of the falling magnet is ( $$\mathrm{g}=$$ acceleration due to gravity)

A metal sphere of mass '$$m$$' and density '$$\sigma_1$$' falls with terminal velocity through a container containing liquid. The density of liquid is '$$\sigma_2$$'. The viscous force acting on the sphere is

Two uniform wires of same material are vibrating under the same tension. If the first overtone of first wire is equal to the $$2^{\text {nd }}$$ overtone of $$2^{\text {nd }}$$ wire and radius of the first wire is twice the radius of the $$2^{\text {nd }}$$ wire then the ratio of length of first wire to $$2^{\text {nd }}$$ wire is