Question

A magnetic field is produced by bar magnet as shown in figure and a beam of electron is passing through magnetic field. How will the magnetic field affect the electron beam?

• A. The beam will rotate around the two magnets.
• B. The beam will bend up away from the table.
• C. The beam will bend down into the table
• D. The beam will be attracted to the north pole of a magnet.
• E. The beam will be attracted to the south pole of a magnet.

Answer 1

The correct option is B The beam will bend up away from the table.

Given : $\overrightarrow{B}=B\hat{y}$

Velocity of electron beam $\overrightarrow{v}=-v\hat{x}$

Magnetic force on electron $\overrightarrow{F}=-q(\overrightarrow{v}\times \overrightarrow{B})=q(\overrightarrow{B}\times \overrightarrow{v})$

$\therefore$ $\overrightarrow{F}=q[B\hat{y}\times (-v\hat{x}\left)\right]=qvB$ $\hat{z}$

Thus the electron beam will bend up away from the table.