Question

A uniform magnetic field is directed vertically upwards. In which direction in this field should an alpha particle be projected so that it is deflected southward ? Name and state the rule you have used to find the direction in this case .

Answer 1

Since alpha particle is positively charge so the current will be in direction in which alpha particle will be projected .
mathematically:

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

$\left(\widehat{-j}\right)=q(\overrightarrow{V}\times \hat{k})$

$\left(\widehat{-j}\right)=q(\hat{i}\times \hat{k})$

$\therefore \hat{V}=\hat{i},eastwardorrightside$

Also, by applying flaming left hand rule .

Forefinger will point in upward direction.

And thumb should point in direction in which deflection will takes place ie.south ward so in this case middle finger will point in right direction ie. East

hence alpha particle should be projected in east or right side