 # A proton is kept at rest. `
Question:

A proton is kept at rest. A positively charged particle is released from rest at a distance d in its field. Consider two experiments; one in

which the charged particle is also a proton and in another, a positron. In the same time t, the work done on the two moving charged

particles is

(a) same as the same force law is involved in the two experiments

(b) less for the case of a positron, as the positron moves away more rapidly and the force on it weakens

(c) more for the case of a positron, as the positron moves away a larger distance

(d) same as the work done by charged particle on the stationary proton

Solution:

(c) Force between two protons is equal to the force between proton and a

positron because their charges are same. As the mass of positron is much lesser than proton, (1/1840 times) it moves away through much larger

distance compared to proton.

Change in their momentum will be same. So, velocity of lighter particle will be greater than that of a heavier particle. So, positron is moved through

a larger distance.

As work done = force x distance. As forces are same in case of proton and positron but distance moved by positron is larger, hence, work done

will be more.