A velocity selector having uniform perpendicular electric and magnetic fields is

Question

A velocity selector having uniform perpendicular electric and magnetic fields is shown in the figure (Figure 1) . The electric field is provided by a 150 V dc battery connected across two large parallel metal plates that are 4.50 cm apart.

a) What must be the magnitude of the magnetic field so that charges having a velocity of 3.15 km/s perpendicular to the fields will pass through undeflected?

b) Show how the magnetic field should point in the region between the plates. (pick from one of the choices below)

In order for F B to be upward, to oppose F E, B  must be into the page.

In order for F B to be downward, to oppose F E, B  must be into the page.

In order for F B to be upward, to oppose F E, B  must be out of the page.

In order for F B to be downward, to oppose F E, B  must be out of the page.

Explanation / Answer

a) E = V/d = 150V / 0.045 m = 3333.33 V / m

to remain charge undeflected,

magnetic force = electric force

q v B = qE

B = E / v = 3333.33 / (3.15 x 10^3) = 1.06 T

b) v X B have the opposite diretion as E.

for more figure will be needed.

and figure is missing from the question.

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