Electric and Magnetic Fields

In brief explain the working of velocity selector.

What is a Helmholtz coil?

Write a short note on Helmholtz coil.

Which of the following conditions is used to select charged particles of a particular velocity out of a beam of charges moving with different speeds?

A beam of alpha particles passes undeflected  through a velocity selector having electric and magnetic fields of $120\mathrm{kV}/\mathrm{m}$ and $60 \mathrm{mT}$,

respectively. The velocity of the beam is :

The following statement is false for Helmholtz coils:

Which device you will use to produce uniform magnetic field?

Helmholtz coils are used

A velocity selector is used to select alpha particles of energy $200 \mathrm{KeV}$. From a beam containing particles of several energies. The electric field strength is $900 \mathrm{kV} {\mathrm{m}}^{-1}$. What must be the magnetic field strength in $\mathrm{mT}$?