question_answer

A proton moving vertically downward enters a magnetic field pointing towards north. In which direction proton will deflect?

A)  East           

B)  West

C)  North         

D)  South

Correct Answer: A

Solution :

 Key Idea : Apply Flemings 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 Flemings left hand rule, if middle finger represents the direction of current and fore-finger represents 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. (Vertically downward) \[\vec{F}=q(\vec{V}\times \vec{B})\] Here, \[\vec{F}=q(\vec{V}\times \vec{B})\] \[\therefore \] \[\vec{F}=q(v\,\hat{i}\times B\hat{j})\] \[=qvB\hat{k}\,\,\,(\because \,\,\,\hat{i}\times \hat{j}=\hat{k})\] Hence, force will act on the proton in Z-direction, i.e., towards east.