Electromagnetism - Lorentz Force

A proton moves in the +z direction after being accelerated from rest through a potential difference V. The proton then passes through a region with a uniform electric field E in the +x direction and a uniform magnetic field B in the +y direction, but the proton’s trajectory is not affected. If the experiment were repeated using a potential difference of 2V, the proton would then be

A. Deflected in the +x-direction
B. Deflected in the -x-direction
C. Deflected in the +y-direction
D. Deflected in the -y-direction
E. Undeflected

(GR0177 #58)

Solution:

Lorentz Force: 

Since it is not deflected, 

Potential energy transforms into kinetic energy, qV = ½ mv²

Thus, velocity is  

For 2V, velocity is  

  

Therefore,

Particle is deflected in −x direction

Answer: B