AS Unit F321: Atoms, bonds and groups

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Sample Student Answers
AS Unit G482: Electrons, waves and photons
Module 5: Quantum physics
Question 1
Total marks: 15
(a) The four statements below may or may not be true about a photon. Write the statements out,
and next to each statement state whether it is true or false.
A photon has a negative charge.
A photon travels at the speed of 3.0 × 108 m s−1 in a vacuum.
A photon is a quantum of electromagnetic radiation.
An X-ray photon has less energy than a photon of radio waves.
Marks available: 2
Student answer:
(a)
A photon has a negative charge.
A photon travels at the speed of 3.0 × 108 m s−1 in a vacuum.
A photon is a quantum of electromagnetic radiation.
An X-ray photon has less energy than a photon of radio waves.
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Examiner comments:
(a) Correct.
(b) Describe and explain the photoelectric effect in terms of photons interacting with the surface of
a metal.
Marks available: 6
Student answer:
(b) The photoelectric effect is the emission of electrons from a metal surface when
electromagnetic radiation is shone onto the metal. For this to happen the frequency of the
electromagnetic radiation must be above a certain minimum frequency known as the
threshold frequency. If the frequency is below this threshold, then no matter how bright the
radiation is, the electrons are not given off. Above this frequency the kinetic energy of the
electrons increases as the frequency increases. Also, the rate at which the electrons are
given off depends on how bright the light is, providing it is above the threshold frequency.
Examiner comments:
(b) This is a good answer and you have taken great care to order your thoughts before writing
them down, but you have carefully avoided any equations. For an even better answer you
could have included the following:
Energy of a photon = hf
Minimum energy required to remove an electron is the work function Φ.
If hf > Φ then electrons emitted.
Energy is conserved in the interaction.
Photon energy = work function + maximum KE of electron.
hf = Φ + KE(max).
© Pearson Education Ltd 2008
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(c) An X-ray machine in a hospital emits X-rays of wavelength 4.0 × 10−10 m and of power 1.4 W.
(i) Calculate the energy of each X-ray photon
1 in joules
2 in electronvolts (eV).
(ii) Calculate the number of photons emitted per second from the X-ray machine.
Marks available:
(i) 4 (ii) 3
Student answer:
(c) (i) 1. E = hf = hc/λ
= (6.63 × 10−34) × (3.0 × 108)/(4.0 × 10−10)
= 4.97 × 10−16 J
2. E = (4.97 × 10−16)/(1.6 × 10−19)
= 3.1 × 103 eV
(ii) Number per second = 1.4/4.97 × 10−16
= 2.8 × 1015
Examiner comments:
(c) Correct calculations, showing what you are doing each time.
© Pearson Education Ltd 2008
This document may have been altered from the original
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Module 5: Quantum physics
Question 2
Total marks: 9
(a) The table below shows four statements that may or may not be true about the wave nature of
the electron. Redraw the table and show which are true and which are false.
Electrons can be diffracted by matter. This confirms their wave nature.
The wavelength of the electron is given by the de Broglie equation.
The wave associated with a moving electron is an electromagnetic wave.
The kinetic energy of the electron is given by the equation E = hf.
Marks available: 3
Student answer:
(a)
Electrons can be diffracted by matter. This confirms their wave nature.
The wavelength of the electron is given by the de Broglie equation.
The wave associated with a moving electron is an electromagnetic wave.
The kinetic energy of the electron is given by the equation E = hf
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Examiner comments:
(a) Correct.
(b) Calculate the speed of a carbon atom of mass 2.0 × 10−26 kg travelling in space with a de Broglie
wavelength of 6.8 × 10−11 m.
Marks available: 3
Student answer:
(b) λ = h/mv
v = 6.63 × 10−34/(2.0 × 10−26 × 6.8 × 10−11)
= 488 m s−1
Examiner comments:
(b) Correct.
© Pearson Education Ltd 2008
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(c) Electrons travelling at 2.0 × 106 m s−1 have a de Broglie wavelength of approximately
3.6 × 10−10 m. State and explain what would happen if a stream of these electrons were directed
towards a fluorescent screen and a thin foil sample, in which the interatomic spacing is
4.0 × 10−10 m, were to be placed in the stream.
Marks available: 3
Student answer:
(c) The atoms act as a diffraction grating where the spacing between the atoms is
approximately equal to the de Broglie wavelength of the electrons. The electrons are
therefore diffracted and produce a diffraction pattern.
Examiner comments:
(c) Good start to an answer that could be improved upon by mentioning that the threedimensional nature of the atomic array in the foil produces a diffraction pattern of concentric
circles.
© Pearson Education Ltd 2008
This document may have been altered from the original
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