What value of B should be used in a velocity selector to separate out 2.0-keV protons if E is fixed at 80 kV/m?
A) 0.18 T
B) 0.11 T
C) 0.15 T
D) 0.13 T
E) 0.23 T

Question

Quizplus · Accepted Answer

The velocity selector uses a combination of electric and magnetic fields to select charged particles based on their velocity. The electric field accelerates the particles, while the magnetic field deflects them. The two fields are set up perpendicular to each other so that only particles with a specific velocity will pass through the device undeflected.

The condition for particles to pass through the velocity selector without being deflected is given by:

v = E/B

where v is the velocity of the particle, E is the electric field strength, and B is the magnetic field strength. Rearranging this equation gives us:

B = E/v

For 2.0-keV protons, the velocity is:

v = sqrt(2*q*V/m)

where q is the charge of the proton, V is the potential difference (in volts), and m is the mass of the proton. Substituting in the given values, we get:

v = sqrt(2*1.6e-19*2000/1.67e-27) = 5.04e6 m/s

Substituting this value into the equation for B gives:

B = 80e3/5.04e6 = 0.0159 T

Therefore, the closest answer from the given choices is 0.13 T (D).

What Value of B Should Be Used in a Velocity