A velocity selector uses a combination of electric and magnetic fields to filter

Question

A velocity selector uses a combination of electric and magnetic fields to filter out charged particles with velocities other than the one desired. It does this by balancing the force on a moving charge by the magnetic field with the force on a charge by the electric field. In this case, a proton moves undeflected at a speed of 6.67 x 10^5 m/s through an electric field measuring 2.00x10^5 N/C, as oriented in the figure. Find the magnetic field strength and direction that keeps this proton traveling in a straight line.

Explanation / Answer

magnetic and electric force will have same magnitude and opposite direction.

then Fnet = 0 and particle will move undeflected.

Fe = Fm

qE = q(v x B )

E = vB

2 x 10^5 = 6.67 x 10^5 x B

B = 0.3 T

for direction,

E -> along j

v -> along i

and E = v X B

so direction will be -k

such that i x -k = j

so into the page (direction)

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