AQA AS Physics P3 Quantum Phenomena Kerboodle Answers

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3.1 The photoelectric effect AQA AS Phsyics P3 Quantum Radiation Kerboodle Answers: Page No. 31

1.-Ans  a.The photoelectric effect refers toward the emission, or ejection, of electrons from the surface of, generally, a metal in reaction to incident light. Energy contained inside the incident light is absorbed by electrons within the metal, giving the electrons sufficient energy to be ‘knocked’ out of, that is, emitted from, the surface of the metal. b .There is a minimum energy required to free an electron from the metal so only a photon with greater than or equal to the work function can actually free it. 2.Ans- a .i) 6.7 x 10¹⁴ Hz, 4.4 x 10^-19 J ii )2.0 x 10¹⁴ Hz, 1.3 x 10^-19 J b. shows that the energy of a photon is inversely proportional to its wavelength. This means the 450nm photon has a greater energy than the 650nm photon. The work function of the metal must lie in the energy range\ hence only the shorter wavelength photon has enough energy to cause photoelectric emission. 3Ans-a. 1.7 x 10¹⁴ Hz b.2.7 x 10^-19J 4Ans- a 3.1 x 10^-19 J b 1.6 x 10-¹⁹ J c. 2.5 x 10¹⁴Hz

3.2 More about photoelectricity AQA AS Phsyics P3 Quantum phenomena Kerboodle Answers : Page No. 33

  1.Ans- a The blue light has a higher frequency then the threshold frequency meaning that electrons flow from the cathode to the anode. b However the red light has a lower frequency then the threshold frequency meaning it doesn’t have enough energy to break the work force and leave the metals surface therefore resulting in no photoelectric current 2.Ans-a 1.6 x 10¹² b the no of electrons emitted depends on the no of photons hitting the metal , as electrons emitted current flows , hence current flow proptional to the electrons emitted. 3.Ans- a 3.4 x 10^-19 J b 1.5 x 10¹⁵ c 2.5 x 10¹² 4.Ans- a i)4.0 x 10¹⁴ Hz ii 2.7 x 10^-19 J, b 2.7 x 10^-19J Banner 3

3.3 Collisions of electrons with atoms AQA AS Phsyics P3 Quantum Phenomena Kerboodle Answers: Page No. 35

1.Ans-Similarity; energy is absorbed by the atom; Difference; an electron stays in the atom when excitation occurs but leaves the atom when ionisation occurs. 2Ans- a1.66 x 10^-18 J b.ANS-1.6 eV 3Ans- a.less 4.Ans- a An electron inside the atom moves to an outer shell. b.The electron has insufficient kinetic energy to excite or ionise the atom and so cannot be absorbed. Banner 4

3.4 Energy levels in atoms AQA AS Phsyics P3 Quantum PhenomenaKerboodle Answers : Page No. 38

1.Ans- a 9.0 eV b.Ans-There are 3 energy levels below the 5.7 eV level. There are 3 possible transitions to the ground state, 2 to the first excited state and I to the second excited state. The energy changes for these transitions are all different. Hence there are 6 possible photon energies. 2Ans- a The energy of an electron in the atom increases in excitation and decreases in de-excitation. Excitation can occur through photon absorption or electron collision. De-excitation only occurs through photon emission.

1. There are two excited levels at 1.8 eV and 4.6 eV above the ground state;

ii photon energies I.8 eV, 2.8 eV and 4.6 eV. 3.Ans-a b An electron in the atom absorbs the 3.8 eV photon and moves to an outer shell. lt moves to an inner shell and emits a 3.2 eV photon. An electron moves from this shell to the ground state, emitting a 0.6 eV photon. 4.Ans- When the electricity supply is switched off. excitation by collision of the gas atoms in the tube ceases because the supply of electrons to the tube is cul off. Therefore the mercury atoms no longer emit ultraviolet radiation and so the coating atoms can no longer emit light. Banner 5

3.5 Energy levels and spectra AQA AS Phsyics P3 Quantum Phenomena Kerboodle Answers : Page No. 40

1.Ans-A line spectrum consists of discrete coloured lines whereas a continuous spectrum has a continuous spread of colours. A line spectrum is due to photons of certain energies only, whereas a continuous spectrum is due to a continuous spread of photon energies. 2 .ANS- a 1.28 x 10^-19J b 1.55 x 10^-6 m 3.Ans- a) 3.21 x 10^-19 J b 2.0 eV 4.Ans-The energy levels of the atoms of an element are unique to those atoms. The photons emitted by them have energies equal to the energy differences between rhe levels. Therefore the photon energies arc characteristic of the atoms and can be used to identify the element. Baneer 6

3.6 Wave-particle duality AQA AS Phsyics P3 Quantum Phenomena Kerboodle Answers : Page No. 43

1.Ans- a explain what is meant by the dual wave—particle nature of a light b matter particles, for example, electrons. 2.Ans- a the photoelectric effect b electron diffraction. 3.Ans- a an electron moving at a speed of 2.0 10? Ms^-1 3.6 x 10^-11 m b 2.0 x 10^-14 m 4.Ans- a 1.3 x 10^-27 kgms^-1 1.5 x 10³ ms^-1 b 1.3x 10^-27 kgms^-1, 0.78ms^-1 Banner 7

Practice questions: Page No. 44-45

1 .Ans- (a) the minimum energy required to remove an electron from the surface of a metal (b) (i) Use these results to plot a graph of E against f. (ii) the graph is a straight line which does not pass through the origin EK = hf − ϕ is an equation of the same form as that of the general straight line, y = mx + c m = h, and c = − ϕ (iii) at the intercept f0 on the f-axis EK = 0, so work function ϕ = h f0 intercept value, f0 = 4.3 × 10¹⁴ Hz and ϕ = 6.5 × 10^-34 × 4.3 × 10¹⁴ = 2.8 × 10^-19 J (c) if the test does not provide supporting evidence, the prediction is incorrect so the theory is incorrect and must be changed 2.Ans- (a) Name: work function

• Definition: the minimum energy required to remove an electron from the surface of a metal

(b)

• incident photon has fixed energy
• photon loses all its energy in a single interaction
• electron can lose various amounts of energy in reaching surface of metal
• electrons have a maximum kinetic energy = (photon energy) − work function

(c) (i) ϕ = hf – EK = (6.63×10^-34×1.8×10¹⁵)− 4.2×10^-19 = 7.73 × 10^-19 J (ii) 3.Ans- (b) (i) the line would be parallel to the original line, but the intercept on the f-axis would be at 8.0 × 10¹⁴ Hz (ii)

• the gradient of the line is the Planck constant, h, so it is unchanged 1
• the intersection with the f-axis is doubled because h f = ϕ when the photoelectrons have zero kinetic energy

4.Ans- (a) there must be sufficient distance between collisions for the electrons to gain enough energy for the required excitations to occur or

• the vapour must not completely absorb the electrons

(b)

• the mercury vapour emits ultraviolet radiation
• the ultraviolet radiation excites the atoms of the coating
• the coating then emits electromagnetic radiation of longer wavelengths (or lower frequencies)
• some of which is in the visible region

5.Ans- (a).Ans- (b) from n = 3 to n = 2 6 (a) electrons behave both as waves and as particles Wave behaviour:

• they can be diffracted, or show interference effects

Particle behaviour:

• can be deflected in electric or magnetic fields, or make collisions with atoms

(b) (i) (ii) 7 .Ans (a) Electron diffraction (or interference) (b). (c) (d)

• both experience the same increase in energy (or have same work done on them)
• wavelength is inversely proportional to momentum
• gain in momentum is different for the muons and electrons
• the smaller mass has the largest acceleration (or gain in speed)

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