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

Explanation / Answer

For the positive particle, apply v = E/B

4000 = 500/B

B = .125 T

For the negative particle

The Electric Force would pull it to the toward the -y and the Magnetic Field would pull it toward the +y. The velocity remains unchanged so it still goes in a straight line

For the last part

Since the velocity is greater, the B field is stronger (because F = qvB) than the E field so the negative charge will travel in a circular path in the X-Y plane

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