A proton is accelerated from rest through a potential difference of 1000 V. What is the potential difference through which an alpha particle must be accelerated to gain the same kinetic energy as the accelerated proton?

A. 4000 V
B. 2000 V
C. 500 V
D. 250 V

C

B was the most popular response, presumably as the candidates were thinking ‘twice the charge, twice the potential difference’. A moment’s back checking, however, would show that this would lead to an alpha particle with four times the energy of the proton; therefore the correct response must be C.