What causes a positively charged particle to gain speed when it is accelerated through a potential difference?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The particle accelerates because it loses potential energy as it moves from high to low potential
A charge \( q \) at a point of potential \( V \) has electric potential energy \( qV \). When the charge moves freely, the electric force does work on it. The loss in potential energy appears as kinetic energy, and the sum of the two stays constant.
For a positive charge moving from potential \( V_1 \) to a lower potential \( V_2 \), the change in potential energy is:
\( \Delta U=q\,(V_2-V_1)\lt0 \)
The gain in kinetic energy is:
\( \begin{aligned} \Delta K &= -\Delta U \\ &= q\,(V_1-V_2) \\ &\gt 0 \end{aligned} \)
Option (1) The particle accelerates because it loses potential energy as it moves from high to low potential — Correct. A positive charge moves on its own from high to low potential. Its potential energy \( qV \) falls, and the lost energy becomes kinetic energy. So its speed increases.
Option (2) The particle accelerates because it loses potential energy as it moves from low to high potential — Incorrect. Moving from low to high potential raises the potential energy \( qV \) of a positive charge, so it would slow down. Work has to be done against the field to move it this way.
Option (3) The particle accelerates because it gains potential energy — Incorrect. A gain in potential energy comes at the cost of kinetic energy. So the particle would slow down.
Option (4) The particle accelerates because it moves towards negative charges — Incorrect. The gain in speed comes from the fall in potential along its path. Negative charges need not be present. A positive charge also speeds up while moving away from another positive charge.
NCERT (2026-27):
Introduction – “… If the external force is removed on reaching P, the electric force will take the charge away from Q – the stored energy (potential energy) at P is used to provide kinetic energy to the charge q in such a way that the sum of the kinetic and potential energies is conserved.”