Corrigendum: ‘Measurement of the ratio h / e with a photomultiplier tube and a set of LEDs’ (2017 Eur. J. Phys. 38 025208)

Should the concept of stopping potential be removed from teaching for A-level?

“…of the emitted electrons is both widespread, and intuitively attractive, it is wrong, as was first emphasised in this Journal half a century ago [3]. Other critical papers in various journals devoted to Physics education followed [4][5][6][7][8][9][10], but the error took a long time to be removed from advanced texts, and still occasionally crops up at University level [9], so it is unsurprising to find the same erroneous analysis in A-level texts. Nevertheless, errors need to be corrected.…”

“…Figure 2 above, reproduced 5 from [9], shows the variation of energies for the photoelectrons of maximum k.e. when they reach the collector, as the externally applied potential V is varied.…”

“…This energy reduction comes about because, although no external retarding potential is being applied, the higher work function of the collector means that the real potential just outside the collector surface is above that at the emitter surface. 9 Thus electrons which were emitted with a k.e. close to zero will not be able to reach the collector.…”

“…The experiment is still making the wrong measurement, and the results are only 'rescued' by the accidental coincidence. This near-coincidence is quite possible in a photocell which has been manufactured by evaporating a volatile metal with a low work function, such as K or Cs, on to the photocathode [9]. Here there is a considerable probability that the volatile metal will also be deposited on the photocell anode (or the first dynode of a photomultiplier), and experience in several teaching laboratories indicates that this does happen in practise.…”

“…For the purposes of the argument, this electron energy is referred to EF, rather than the usual reference of the vacuum outside the emitter surface 8. This was true for Millikan's own experiment 9. This is the origin of the contact potential difference 10.…”