Describe the concept used for the selection of velocity of a charged particle. Explain the principle of the device with the help of a diagram where the same concept is used. What is resonating condition for the said device

A velocity selector is a region in which there is a uniform electric field and a uniform magnetic field. The fields are perpendicular to one another, and perpendicular to the initial velocity of the charged particles that are passing through the region.

The force exerted on a charged particle by the electric field is given by:

F = qE

The magnitude of the force exerted by the magnetic field is F = qvB, as long as the velocity is perpendicular to the field.

The idea is that, if the two forces are equal and opposite, the net force is zero, and the particle passes through the region without changing direction. With the magnetic force being speed dependent, however, any charges traveling faster or slower than the ones that go straight through will be deflected one way or another out of the beam. You can investigate these concepts with this simulation.

That device is the CYCLOTRON 

Cyclotron

A cyclotron is a particle accelerator that uses electrical power to accelerate charged particles in a spiral path. The collision of particles produces certain radioactive isotopes for medicine, industry and scientific research.

Working of cyclotron
 

Cyclotrons contain two hollow D-shaped electrodes, called dees, in a vacuum chamber between the poles of a powerful electromagnet. This vacuum chamber is flat and sits in a narrow gap between poles of an electromagnet, which creates a perpendicular magnetic field. A high-frequency alternating voltage is applied across the electrode to accelerate the charged particles fed into the center of the chamber.

As particles gain more energy from the accelerating voltage, they spiral outwards until they reach the outer edge of the chamber. The size of the vacuum chamber determines the spiral path of each particle and the amount of energy attained from the cyclotron. The direction of the spiral can also be reversed by connecting the dee to an AC voltage source that reverses direction at regular intervals.

The most common use for a cyclotron is for positron emission tomography scans.





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