| Date | May 2017 | Marks available | 4 | Reference code | 17M.2.HL.TZ1.9 |
| Level | Higher level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | Explain | Question number | 9 | Adapted from | N/A |
Question
Two observations about the photoelectric effect are
Observation 1: For light below the threshold frequency no electrons are emitted from the metal surface.
Observation 2: For light above the threshold frequency, the emission of electrons is almost instantaneous.
The graph shows how the maximum kinetic energy Emax of electrons emitted from a surface of barium metal varies with the frequency f of the incident radiation.
Explain how each observation provides support for the particle theory but not the wave theory of light.
Determine a value for Planck’s constant.
State what is meant by the work function of a metal.
Calculate the work function of barium in eV.
The experiment is repeated with a metal surface of cadmium, which has a greater work function. Draw a second line on the graph to represent the results of this experiment.
Markscheme
Observation 1:
particle – photon energy is below the work function
OR
E = hf and energy is too small «to emit electrons»
wave – the energy of an em wave is independent of frequency
Observation 2:
particle – a single electron absorbs the energy of a single photon «in an almost instantaneous interaction»
wave – it would take time for the energy to build up to eject the electron
attempt to calculate gradient of graph = «\(\frac{{4.2 \times {{10}^{ - 19}}}}{{6.2 \times {{10}^{14}}}}\)»
\( = 6.8 - 6.9 \times {10^{ - 34}}\) «Js»
Do not allow a bald answer of 6.63 x 10-34 Js or 6.6 x 10-34 Js.
ALTERNATIVE 1
minimum energy required to remove an electron «from the metal surface»
ALTERNATIVE 2
energy required to remove the least tightly bound electron «from the metal surface»
ALTERNATIVE 1
reading of y intercept from graph in range 3.8 − 4.2 × 10–19 «J»
conversion to eV = 2.4 – 2.6 «eV»
ALTERNATIVE 2
reading of x intercept from graph «5.8 − 6.0 × 1014 Hz» and using hf0 to get 3.8 − 4.2 × 10–19 «J»
conversion to eV = 2.4 – 2.6 «eV»
line parallel to existing line
to the right of the existing line
