# AS Physics Unit 3 Exam Questions Ks5 AS Physics AQA 2450 Mr D Powell.

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AS Physics Unit 3 Exam Questions Ks5 AS Physics AQA 2450 Mr D Powell

Exam Questions June 06 7 (a) State what is meant by the photoelectric effect and explain how two observations made in photoelectric experiments suggest that electromagnetic radiation behaves like a stream of particles rather than a wave. (7 marks + 2 QWC) (b) Photons with energy 1.1 × J are incident on a metal surface. The maximum energy of electrons emitted from the surface is 4.8 × J. Planck constant = 6.6 × J s mass of an electron = 9.1 × kg (i) Calculate the work function of the metal. (ii) Calculate the wavelength of the de Broglie wave associated with the emitted electrons. (leave for later!) (4 marks)

Answers EM radiation of certain wavelengths causes electrons to be emitted from the surface of a metal such as zinc. It is found that the frequency has no effect on ejecting electrons if below a certain frequency called the “threshold frequency”  This occurs even if the amplitude or intensity of the wave if high or low. This leads us to the idea of quanta or chunks of energy of a size big enough to kick out an electron. The emissions start immediately even when the radiation is very weak in intensity. Suggesting no need for a build up of energy. There is a maximum KE of electrons this is not consistent with the idea of waves delivering energy proportional to amplitude squared. Also Ek = hf -  so Ek  hf

Does every photon produce a photoelectron?
Assuming we’re above the threshold frequency, there are other things to think about when the photon arrives. Photons can easily pass between or even through atoms without hitting (interacting with) an electron. So when they do finally hit an electron, there are a number of possible scenarios. So in fact a very small proportion of the photons that arrive at the metal will cause photoelectrons to be emitted.

What happens then….. The photon hits an electron at the surface of the metal. The electron uses the energy that it has gained to leave the atom and head off to freedom. The photon hits an electron at the surface of the metal. The electron leaves the atom but heads off deeper into the metal and never escapes The photon passes deep into the metal before it hits an electron. The electron leaves the atom and heads towards the surface and escapes! The photon passes deep into the metal before it hits an electron. The electron leaves the atom and heads towards the surface but it doesn’t have enough energy to push its way past all the other atoms to get to the surface so it grinds to a halt still inside the metal and never escapes. The photon passes deep into the metal before it hits an electron. The electron leaves the atom and heads off in the wrong direction and never escapes.

Exam Question 1 Work function is the amount of energy which is required to release a photoelectron from the surface of the metal. It can be thought of that the electron is trapped inside an energy well. See graph and also. You can see from the graph that the there is an intercept which relates to the work function . Ek =hf- where  is the energy taken away from the energy of the incoming photon. The Ek term relates to the energy of the outgoing photon or photoelectron. h is equivalent to the gradient. iii) Anything near to 6.63 x 10-34Js is fine. c) Constants such as h need really to be tested on lots of metals and circumstances to obtain an accurate figure. Also they should be tested using different light frequencies etc.. to see if the results are true in any circumstances.

Exam Question 2a work function (1)
One quantity in the photoelectric equation is a characteristic property of the metal that emits photoelectrons. Name and define this quantity. work function (1) minimum energy to remove an electron from the surface of a metal (1)

Exam Question 2b A metal is illuminated with monochromatic light. Explain why the kinetic energy of the photoelectrons emitted has a range of values up to a certain maximum. You may be awarded marks for the quality of written communication in your answer. incident photon energy is fixed [or photoelectron receives a fixed amount of energy] (1) photon loses all its energy in a single interaction (1) electron can lose various amounts of energy to escape from the metal (1) electrons have a maximum energy = photon energy − work function (1)

Exam Question 2c A gold surface is illuminated with monochromatic ultraviolet light of frequency 1.8 × 1015 Hz. The maximum kinetic energy of the emitted photoelectrons is 4.2 × J. Calculate, for gold, the work function, in J, the threshold frequency.

Exam Question 3 Ek =hf- So graph shows Ek = y & f = x. Hence m = h & intercept is - Also we know that v = f or c = f (for speed of light photons) For this case f = 10x1014 Hz when Ek = 4 x10-19J To get the work function  So Ek – hf = -  = 4 x10-19J - (6.63 x 10-34Js x 10x1014 s-1) = 4 x10-19J – 6.63 x-19J  = x 10-19J Also we can see the threshold frequency from the graph is fT = 4x1014 Hz So c = f or c/f = 3 x 108ms-1 / 4x1014 Hz = 750nm = 750 x 10-9m E = hc/ so if energy is double and h and c are constants E  1/  so  must half or frequency must double in each case to make the graph transpose. With the same gradient since. c = f 

Exam Question 4 A fluorescent tube is filled with mercury vapour at low pressure. In order to emit light the mercury atoms must first be excited. a) Why is the mercury vapour at a low pressure? (1) (b) What is the purpose of the coating on the inside surface of the glass in a fluorescent tube? (3 marks) a) If there are too many mercury atoms the electrons cannot pass through it to the anode to create an electric current. b) the powder absorbs light/photons (emitted from the mercury) (1) powder atoms are excited and emit light/photons (1) of different wavelengths (to those received) (1) any other relevant statement such as, electrons cascade down energy levels, emitting many wavelengths, or the spectral lines are broadened (1)

Exam Question 5 The lowest energy levels of a mercury atom are shown below. The diagram is not to scale. a) Calculate the frequency of an emitted photon due to a transition, shown by an arrow, from level n = 4 to level n = 3. (2 marks) b) Draw a line on the diagram to show a transition which emits a photon of a longer wavelength than that emitted in the transition from level n = 4 to level line joining level n = 3 to level n = 2 with arrow pointing down

Exam Question 6a electrons behave as waves and as particles (1)
waves - diffraction/interference (1) particles - collisions with atoms/deflection in an electric or magnetic field (1)

Exam Question 6b

Exam Question 7 electron diffraction or interference patterns (1)
Electrons travelling at a speed of 5.00 × 105 ms-1 exhibit wave properties. (a) What phenomenon can be used to demonstrate the wave properties of electrons? Details of any apparatus used are not required. (1 mark) (b) Calculate the wavelength of these electrons. (2 marks) electron diffraction or interference patterns (1)

Exam Question 7 Electrons travelling at a speed of 5.00 × 105 ms-1 exhibit wave properties. (c) Calculate the speed of muons with the same wavelength as these electrons. Mass of muon = 207 × mass of electron. (3 marks)

Exam Question 7 Electrons travelling at a speed of 5.00 × 105 ms-1 exhibit wave properties. d) Both electrons and muons were accelerated from rest by the same potential difference. Explain why they have different wavelengths. (2 marks)

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