Question

A negatively charged particle is moving through a magnetic field as pictured below.
What is the direction of the force on the particle due to the magnetic field at the instant in the picture?

• A. North
• B. West
• C. East
• D. Up
• E. Down

Answer 1

The correct option is E Down

From figure, $\overrightarrow{v}=-v$ $\hat{y}$ $\overrightarrow{B}=B$ $\hat{x}$

Magnetic force on the particle $\overrightarrow{F}=-q(\overrightarrow{v}\times \overrightarrow{B})$

$\therefore$ $\overrightarrow{F}=-q\left[\right(-v)\hat{y}\times B\hat{x}]=qvB(\hat{y}\times \hat{x})=-qvB$ $\hat{z}$

Thus magnetic force on the particle acts downwards (i.e into the page).