Photons and the photoelectric effectEdexcel A-Level Physics: Subtopic test

1. 1
   A clean zinc plate is fixed to a gold-leaf electroscope that has been given a negative charge. When ultraviolet radiation shines on the plate, the gold leaf slowly falls. When bright visible light shines on the plate, the leaf does not fall, however long it is left. The work function of zinc is 4.3 eV.

   (a)

   Which statement explains why the leaf falls when ultraviolet radiation shines on the plate?

   [1 mark]

   • AThe ultraviolet photons are absorbed by the gold leaf, making it lighter.
   • BElectrons are emitted from the zinc plate, so the negative charge on the electroscope decreases.
   • CThe plate gains electrons from the air, so its negative charge increases.
   • DThe ultraviolet radiation heats the plate, so the leaf expands and falls.

   (b)

   What is the work function of zinc in joules? (e = 1.60 × 10⁻¹⁹ C)

   [1 mark]

   • A2.7 × 10¹⁹ J
   • B4.3 × 10⁻¹⁹ J
   • C3.7 × 10⁻²⁰ J
   • D6.9 × 10⁻¹⁹ J

   (c)

   Use the photon model to explain why increasing the intensity of the visible light does not make the leaf fall.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Caesium has a work function of 2.1 eV. A clean caesium surface in an evacuated cell is illuminated with monochromatic light of wavelength 450 nm, and photoelectrons are emitted.

   (a)

   What is the energy of one photon of this light? (h = 6.63 × 10⁻³⁴ J s, c = 3.00 × 10⁸ m s⁻¹)

   [1 mark]

   • A4.4 × 10⁻¹⁹ J
   • B1.5 × 10⁻²⁷ J
   • C3.0 × 10⁻⁴⁰ J
   • D4.4 × 10⁻²⁸ J

   (b)

   What is the threshold frequency of caesium? (e = 1.60 × 10⁻¹⁹ C)

   [1 mark]

   • A3.2 × 10³³ Hz
   • B6.7 × 10¹⁴ Hz
   • C5.1 × 10¹⁴ Hz
   • D2.2 × 10⁻⁵² Hz

   (c)

   Calculate the maximum speed of the photoelectrons emitted. (Electron mass = 9.11 × 10⁻³¹ kg; use the photon energy from part (a).)

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student illuminates a clean metal surface with radiation of different frequencies and measures the maximum kinetic energy of the emitted photoelectrons. At a frequency of 7.0 × 10¹⁴ Hz the maximum kinetic energy is 1.44 × 10⁻¹⁹ J, and at 10.0 × 10¹⁴ Hz it is 3.43 × 10⁻¹⁹ J.

   (a)

   Use the two sets of results to determine a value for the Planck constant.

   [3 marks]

   (b)

   Calculate the work function of the metal in eV and its threshold frequency. State what happens when the metal is illuminated with intense radiation of frequency 4.0 × 10¹⁴ Hz, and explain your answer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A photocell contains a metal surface with a work function of 2.30 eV. Ultraviolet radiation of wavelength 300 nm falls on the surface and photoelectrons are emitted. Before 1900 light had been successfully modelled as a wave, for example to explain interference and diffraction.

   (a)

   Explain why the observations of photoelectric emission could not be explained by the wave model of light, and how the photon model explains them.

   [6 marks]

   (b)

   Calculate the energy of a photon of the ultraviolet radiation in eV, and the maximum kinetic energy and maximum speed of the photoelectrons. (h = 6.63 × 10⁻³⁴ J s, c = 3.00 × 10⁸ m s⁻¹, e = 1.60 × 10⁻¹⁹ C, electron mass = 9.11 × 10⁻³¹ kg)

   [6 marks]

   Total for question 4: 12 marks

End of questions