An electric field of 3.0 kV/m is perpendicular to a magnetic field of 0.20 T. An electron moving in a direction perpendicular to both and is not deflected if it has a velocity of
A) 6 km/s
B) 9 km/s
C) 12 km/s
D) 15 km/s
E) 6.7 m/s

Question

An electric field of 3.0 kV/m is perpendicular to a magnetic field of 0.20 T. An electron moving in a direction perpendicular to both  and  is not deflected if it has a velocity of
A) 6 km/s 
B) 9 km/s 
C) 12 km/s 
D) 15 km/s 
E) 6.7 m/s

Quizplus · Accepted Answer

The force on an electron moving through an electric field E and magnetic field B is given by the Lorentz force law: F = q(E + v x B). For the electron to not be deflected, the force must be zero, which means E = -v x B. Taking the magnitude of both sides and solving for v, we get:

|E| = |vB|
v = |E|/|B| = 3.0 kV/m / 0.20 T = 15 km/s

Therefore, the electron must have a velocity of 15 km/s to not be deflected. Answer choice D is the correct answer.

An Electric Field of 3