Question

A proton moving horizontally downward enters in a magnetic feild pointing towards north. In which direction proton will deflect ?

• A. East
• B. West
• C. North
• D. south

Answer 1

Answer: (A)

Apply Fleming’s left hand rule to find the direction of Lorentz force on proton.
Proton will represent the direction of current. So, the direction of current is vertically downward.
According to Fleming’s left hand rule, if middle finger represents the direction of current and forefinger repressents the direction of magnetic field then thumb will represent the direction of Lorentz force acting on the proton which deflects the proton in east direction.
Alternative : Force on proton in magnetic field

$\vec{F}=q\left(\vec{V}\times \vec{B}\right)$
Here, $\vec{V}=v\hat{i},\vec{B}=B\hat{j}$
$\therefore \vec{F}=q\left(v\hat{i}\times B\hat{j}\right)$
$-qvB\hat{k}\left(\because \hat{i}\times \hat{j}-\hat{k}\right)$
Hence, force will act on the proton in $Z$-direction, i.e., towardseast.