An electron is travelling in the positive x direction. A uniform electric field is in the negative y direction. If a uniform magnetic field with the appropriate magnitude and direction also exists in the region, the total force on the electron will be zero. The appropriate direction for the magetic field is:
A) the positive y direction
B) the negative y direction
C) into the page
D) out of the page
E) the negative x direction

Question

An electron is travelling in the positive x direction. A uniform electric field  is in the negative y direction. If a uniform magnetic field with the appropriate magnitude and direction also exists in the region, the total force on the electron will be zero. The appropriate direction for the magetic field is: 
A) the positive y direction 
B) the negative y direction 
C) into the page 
D) out of the page 
E) the negative x direction

Quizplus · Accepted Answer

Since the electric field is in the negative y direction and the electron is travelling in the positive x direction, the force due to the electric field will be in the negative y direction. To cancel out this force, a magnetic field must be present that produces a force in the positive y direction. This can only happen if the magnetic field is directed into the page (i.e. perpendicular to both the electric field and the velocity of the electron). Therefore, the best choice is C) into the page.

An Electron Is Travelling in the Positive X Direction