Question

A proton, an alpha particle and an electron are projected perpendicularly into uniform transverse electric and magnetic fields. It is observed that proton travels undeflected. Then,
a) deflection for alpha particle > deflection for proton
b) alpha particle travels in clockwise, and electron in anti-clockwise directions.
c) both alpha particle and electron travel without deflection
d) alpha particle gets deflected but not electron.

• A. Only a is correct
• B. Only b, c are wrong
• C. Only d is correct
• D. a, b, d are wrong

Answer 1

Answer: (D)

a, b, d are wrong

The proton moves undeflected it means force due to magnetic field = force due to electric field
$\Rightarrow q(\overrightarrow{V}\times \overrightarrow{B})=-q\overrightarrow{E}$
So, $(\overrightarrow{V}\times \overrightarrow{B})=\overrightarrow{E}$
So, neither alpha-particle will be deflected nor electron will be deflected because net force on both particles will be zero.