Question

As a negatively charged bowling ball rolls eastward toward some bowling pins, in what direction does it experience a force, due to the earth's magnetic field (Assume the earth's field is pointing straight north)?

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

Answer 1

The correct option is E Down

According to the figure, $\overrightarrow{v}=v\hat{y}$ $\overrightarrow{B}=-B\hat{x}$

Charge of the ball $Q=-q$

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

Thus the ball experience a magnetic force in downward direction.