Question

A charge $+q$ moves parallel to a very long straight wire carrying a current $I$ as shown. The force on the charge is

• A. along $oy$
• B. opposite to $ox$
• C. along $ox$
• D. opposite to $oy$

Answer 1

The correct option is B opposite to $ox$


We know that magnetic field $\overrightarrow{B}$ due to infinitely long straight wire into the plane at point $\text{P}i.e.(⨂)$.

So, the magnetic force on the charge particle $+q$ will be

$\overrightarrow{F}=q(\overrightarrow{v}\times \overrightarrow{B})$

Using right-hand thumb rule, $\overrightarrow{F}$ on $+q$ will be along negative $x-$ axis i,e., opposite to $ox$.

Therefore, option $\left(a\right)$ is correct.

Why this question ? To understand how the direction is decided for magnetic force using right-hand thumb rule or vector equations.