Question

A proton moves in the positive $z$-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 positive $x$-direction and a uniform magnetic field $B$ in the positive $y$-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 positive $x$-direction
• B. deflected in negative $x$-direction
• C. deflected in positive $y$-direction
• D. deflected in negative $y$-direction

Answer 1

Answer: (B)

The electrostatic force on proton is along positive $x$-axis and the magnetic force is along negative $x$-axis. Initially net force on charge $Q$ is zero. Since the velocity of proton is increased in repeat experiment, the magnetic force on proton shall increase and the proton would then be deflected in negative $x$-direction.