A mass spectrometer, constructed as shown in the diagram above, is to be used fo

Question

A mass spectrometer, constructed as shown in the diagram above, is to be used for determining the mass of singly ionized positively charged ions. There is a uniform magnetic field B = 0.20 tesla perpendicular to the page in the shaded region of the diagram. A potential difference V = 1,500 volts is applied across the parallel plates L and K, which are separated by a distance d = 0.012 meter and which act as a velocity selector. In which direction, relative to the coordinate system shown above on the right, should the magnetic field point in order for positive ions to move along the path shown by the dashed line the diagram above? Calculate the magnitude of the electric field between the plates. Calculate the speed of a particle that can pass between the parallel plates without being

Explanation / Answer

a) Force in magnetic field B i sgiven by, F = q v X B

since q = positive, v X B should point in positive x direction

v is in - j direction and F should be i derection

So B should be - k direction, that is into plane of paper

b) V = 1500 V

d = 0.012 m

E = V/d = 1500/0.012 = 125000 N/C

c) Force due to electric field = qE (to left)

force due to magnetic field = qvB (to right)

To avoid deflection, qE = qvB

=> v = E/B = 125000/0.2 = 625000 m/s

So velocity to avoid deflection = 625000 m/s

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