A velocity selector is a device that is used in association with a mass spectrom

Question

A velocity selector is a device that is used in association with a mass spectrometer. The velocity selector contains an electric field and a magnetic field at right angles to each other. Suppose the electric field points in the +y direction and the magnetic field points in the +z direction as shown in the diagram. The electric field strength is 500 N/C. A positively charged particle is then shot into the region, traveling in the +x direction with velocity of 4000 m/s.

1. If the particle is to pass through the velocity selector along a straight path, what must be the magnitude of the magnetic field?

2. If the particle were negatively charged, what trajectory would it follow?

3. Suppose the positively-charged particle is travelling at a higher speed while E and B remain unchanged. What trajectory would it follow?

Explanation / Answer

1) Particle is moving in straight line with constant speed.
So electric and magnetic fields are perpendicular to each other as well as velocity of particle is perpendicular to both.
In this case, electric force(qE) on the electron should balance the magnetic force(qvB) on it.
qvB=qE

B = E / v = 0.125 T

2) It would still follow the same trajectory, as the forces still cancel out each other.

3) If v is higher, magnetic force will increase, so there will be a net force in -y direction, so, the particle will tend to move towards -y direction in circular path

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