If the beams of electrons and protons are moving parallel to each other in the same direction, then, they will
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → attract each other due to stronger electrostatic force compared to magnetic force.
The electric force between the beams is attractive (opposite charges). The magnetic force is repulsive: the electron beam is a current opposite to the proton beam, and antiparallel currents repel. The two compare as
\( \displaystyle \frac{F_{\mathrm{magnetic}}}{F_{\mathrm{electric}}} = \frac{v^{2}}{c^{2}} \)
Since v is much less than c, the electric attraction wins and the beams attract each other.
Why the other options are incorrect
Option (1) keep moving as parallel beams. — Incorrect. The net force between the beams is not zero.
Option (2) repel each other due to stronger magnetic force compared to electrostatic force. — Incorrect. The magnetic force is repulsive but weaker than the electric attraction.
Option (3) attract each other due to stronger magnetic force compared to electrostatic force. — Incorrect. The magnetic force here is repulsive, and it is the weaker force.