NDA (I) 2018 General Ability Test · Question 108 of 150

Correct — C, Rubidium. A photoelectric cell works only if visible light carries enough energy to knock electrons out of the metal surface, and that threshold — the work function — is lowest for the heavy alkali metals. Rubidium's first ionisation energy is only about 403 kilojoules per mole, among the lowest of any element, because its single outer electron is far from the nucleus and well screened. Ordinary light is therefore enough to eject it, and a current flows. Rubidium is listed among the standard uses of the metal as a photocell component, alongside caesium, which is the other alkali metal photocathodes are commonly made from. None of the other three metals in the list will emit electrons under visible light at all.

• (a)Tungsten — Tungsten is the filament metal of an incandescent lamp, chosen for its melting point of about 3,422 °C — the highest of any metal. That makes it ideal for glowing white-hot and emitting electrons when heated, which is thermionic emission, but its work function is far too high for visible light to do the job. The word 'filaments' in the stem is what pulls candidates towards this option.
• (b)Copper — Copper is chosen for wiring because it conducts electricity well, but conductivity and photoemission are different properties. Copper's work function is high enough that visible light cannot dislodge its electrons; ultraviolet would be needed.
• (d)Aluminium — Aluminium is valued for being light, corrosion-resistant and cheap, which is why it goes into conductors, foil and aircraft. Like copper it holds its electrons too tightly for visible light to free them.

In the photoelectric effect, light striking a metal surface ejects electrons, but only if the frequency of the light exceeds a threshold set by that metal's work function. Below the threshold no electrons come out however bright the light is; above it, the number of electrons grows with intensity and their kinetic energy grows with frequency. Einstein explained this in 1905 by treating light as packets of energy, and it earned him the Nobel Prize in Physics for 1921.

The stem's wording deserves a comment, since it is what makes the item feel harder than it is. A photoelectric cell does not really have a filament; it has a photocathode, a curved metal surface coated with the photoemissive material, facing a collecting anode. The paper's use of 'filaments' is loose, and it points a candidate straight at tungsten. Answer the physics rather than the vocabulary: of the four metals offered, only rubidium is an alkali metal, and only an alkali metal holds its outer electron loosely enough to release it under ordinary light. School texts most often name caesium as the photocell metal because its work function is the very lowest, and rubidium sits immediately beside it in the same group — so a candidate who has learnt 'caesium' still arrives at rubidium here by elimination.

• Photoemission needs light above a threshold frequency; below it, no current flows no matter how bright the source.
• Rubidium's first ionisation energy is about 403 kJ per mole, among the lowest of any element, and it is listed among its uses as a photocell component.
• Caesium and rubidium are the standard photocathode metals; both are heavy alkali metals with very low work functions.
• Tungsten's role is the opposite — it emits electrons when heated white-hot, which is thermionic and not photoelectric emission.

• Reading the word 'filament' and answering tungsten, the incandescent-lamp metal.
• Assuming the best electrical conductor must also be the best photoemitter.
• Believing a very bright source can always produce photoelectrons — below the threshold frequency it cannot.

NDA sets one photoelectric item most papers, either naming the metal, naming the phenomenon, or asking what raising the intensity does as against raising the frequency.

• practice — not a real PYQ

  In the photoelectric effect, increasing the intensity of the incident light while keeping its frequency unchanged increases

  • (a)the maximum kinetic energy of the emitted electrons
  • (b)the number of electrons emitted per second
  • (c)the threshold frequency of the metal
  • (d)the work function of the metal

  Answer(b) the number of electrons emitted per second — kinetic energy depends on frequency, not on brightness.

• practice — not a real PYQ

  The emission of electrons from a hot metal surface, as in a lamp filament, is called

  • (a)photoelectric emission
  • (b)thermionic emission
  • (c)field emission
  • (d)secondary emission

  Answer(b) thermionic emission — heat, not light, supplies the energy.