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Date May 2017 Marks available 1 Reference code 17M.2.HL.TZ2.7
Level Higher level Paper Paper 2 Time zone Time zone 2
Command term Describe Question number 7 Adapted from N/A

Question

Yellow light of photon energy 3.5 x 10–19 J is incident on the surface of a particular photocell.

The photocell is connected to a cell as shown. The photoelectric current is at its maximum value (the saturation current).

Radiation with a greater photon energy than that in (b) is now incident on the photocell. The intensity of this radiation is the same as that in (b).

Calculate the wavelength of the light.

[1]
a.i.

Electrons emitted from the surface of the photocell have almost no kinetic energy. Explain why this does not contradict the law of conservation of energy.

[2]
a.ii.

Radiation of photon energy 5.2 x 10–19 J is now incident on the photocell. Calculate the maximum velocity of the emitted electrons.

[2]
b.

Describe the change in the number of photons per second incident on the surface of the photocell.

[1]
c.i.

State and explain the effect on the maximum photoelectric current as a result of increasing the photon energy in this way.

[3]
c.ii.

Markscheme

wavelength = «\(\frac{{hc}}{E} = \frac{{1.99 \times {{10}^{ - 25}}}}{{{\text{3}}{\text{.5}} \times {\text{1}}{{\text{0}}^{ - 19}}}} = \)» 5.7 x 10–7 «m»

 

If no unit assume m.

a.i.

«potential» energy is required to leave surface

Do not allow reference to “binding energy”.
Ignore statements of conservation of energy.


all/most energy given to potential «so none left for kinetic energy»

a.ii.

energy surplus = 1.7 x 10–19 J

vmax = \(\sqrt {\frac{{2 \times 1.7 \times {{10}^{ - 19}}}}{{9.1 \times {{10}^{ - 31}}}}}  = 6.1 \times {10^5}\) «m s–1»

 

Award [1 max] if surplus of 5.2 x 10–19J used (answer: 1.1 x 106 m s–1)

 

b.

«same intensity of radiation so same total energy delivered per square metre per second» 

light has higher photon energy so fewer photons incident per second

 

Reason is required

c.i.

1:1 correspondence between photon and electron

so fewer electrons per second

current smaller

 

Allow ECF from (c)(i)
Allow ECF from MP2 to MP3.

c.ii.

Examiners report

[N/A]
a.i.
[N/A]
a.ii.
[N/A]
b.
[N/A]
c.i.
[N/A]
c.ii.

Syllabus sections

Additional higher level (AHL) » Topic 12: Quantum and nuclear physics » 12.1 – The interaction of matter with radiation
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