Presentation on theme: "2.5.2 The Photoelectric Effect Wave-Particle Duality"— Presentation transcript:
1 2.5.2 The Photoelectric Effect 2.5.3 Wave-Particle Duality 5 Quantum MechanicsG482 Electricity, Waves & Photons2.5.1 Energy of A Photon2.5.2 The Photoelectric Effect2.5.3 Wave-Particle Duality2.5.4 Energy Levels in AtomsKs5 OCR Physics H158/H558IndexMr Powell 2012
2 Introduction....The aim of this module is to introduce the concept of quantum behaviour. How do we know that light is a wave?The evidence for this comes from diffraction of light. However, this wave-like behaviour cannot explain how light interacts with electrons in a metal.A revolutionary model of light (photon model), developed by Max Planck and Albert Einstein, is needed to describe the interaction of light with matter.Physicists expect symmetry in nature. If light can have a dual nature, then surely particles like the electron must also have a dual nature. We study the ideas developed by de Broglie.The final section looks briefly at the idea that electrons in atoms have discrete bond energies and they move between energy levels by either absorbing or by emitting photons.There are many opportunities to discuss how theories and models develop with the history of wave-particle duality.
3 Practical Skills are assessed using OCR set tasks. The practical work suggested below may be carried out as part of skill development. Centres are not required to carry out all of these experiments.This module does not lend itself to many experiments carried by the students. However, it does contain many revolutionary ideas and engaging students in discussions is vital whendemonstrating some of the experiments.Use a GM tube to ‘count’ gamma ray photons.Determine the wavelength of light from different LEDsDemonstrate the photoelectric effect using a photocell or a negatively charged zinc plateconnected to an electroscope.Observe ‘diffraction rings’ for light passing through a tiny hole.Demonstrate the diffraction of electrons by graphite.Observe emission line spectra from different discharge tubes. (A hand-held optical spectrometer can be used to observe Fraunhofer lines in daylight. Caution: Do not look directly at the Sun.)
4 SOWActivities...Class experiment: Observe line spectra from gas discharge tubes with diffraction gratings, hand- held optical spectrometers or spectrometer.Discuss implications of fixed specific colours for each element – relate colour to energy of photons and then consider how they could be producedRelate energy levels to specific energies and jumps between levels to photon energy, hence hf = E1 – E2Discuss emission spectra and absorption spectra. Illustrate with stellar spectra and test identification with spectra from different starsResources....Gas discharge tubes e.g. hydrogen, sodium, mercury. Spectrometer or several hand held spectrometersStudents to take role of electrons, standing at different positions in lab with distance in one direction to represent energy. They have to decide how they can behave to give line spectraThis can be extended to providing energy in one direction and producing absorption spectraEnd of topic testPoints to Note…Ideally hand held spectrometers for immediacy (caution: do not look directly at sun)Absorb Physics for A levelQuantum Physics – Emission spectraThere is much scope for chemistry students to relate this topic to their course with electron orbitals, the periodic table and flame test identification of elementsThe story of Helium being discovered in the Sun before on Earth is relevant and an interesting part of HSW
5 2.5.4 Energy Levels in Atoms (p180-185) Assessable learning outcomes....(a) explain how spectral lines are evidence for the existence of discrete energy levels in isolated atoms, ie in a gas discharge lamp;(b) describe the origin of emission and absorption line spectra;(c) use the relationships hf = E1 – E2 and hc/ = E1 – E2Students can use a diffraction grating to observe the emission spectral lines from different gasdischarge tubes. Students can discuss how different elements can be identified in stars using spectra. (HSW 2)
11 What are we talking about? When we talk about “Line Spectra” for an atom we simply mean that an atom can absorb or emit radiation at certain frequencies due to electron levels.656nm
12 Chemical Composition of Our Sun ElementAbundance (% of total number of atoms)Abundance (% of total mass)Hydrogen91.271.0Helium8.727.1Oxygen0.0780.97Carbon0.0430.40Nitrogen0.00880.096Silicon0.00450.099Magnesium0.00380.076Neon0.00350.058Iron0.0300.014Sulfur0.0150.040You see that hydrogen is by far the most abundant element in the Sun, followed by helium.Those two together make up 99.9 percent by number of the total atoms in the Sun!This is also what we find in the composition of the Universe as a whole.
16 Task..Write down some example gases and their emission spectral line positions. You should be able to see clear patterns.Explain how a Geissler tube works with EHT (2.5kV)Explain how this tells us that the bohr model is correct and improves on the Rutherford exp.NeonHydrogen
19 TASK: Define each type of spectral diagram and how it is created. Spectrum Types..Light bulbA continuous spectrum results when gas pressures are higher (very hot). Generally, solids, liquids, or dense gases emit light at all wavelengths when heated.Emission spectra are produced by hot low pressure gases in which the atoms do not experience many collisions (because of the low density). The emission lines correspond to photons of discrete energies i.e. low pressure sodium lamp or mercury lampAn absorption spectrum occurs when light passes through a colder, low pressure gas and atoms in the gas absorb at characteristic frequencies; since the re-emitted light is unlikely to be emitted in the same direction as the absorbed photon, this gives rise to dark lines (absence of light) in the spectrum. The atmospheres of stars act as a cooler blanket around the hotter interior of a star so that typical stellar spectra are absorption spectra.Geissler onSun/ Geissler offTASK: Define each type of spectral diagram and how it is created.Same gas
20 Energy Levels?The Rutherford Bohr Model of the atom is simply based on the idea that electrons can only exist on certain fixed levels. These levels relate to the amount of energy an electron has.Hydrogen is the most simple atom with only one proton and electron and the level n = 1 is referred to as the ground state of hydrogen (lowest energy) and has a value of -13.6eV for hydrogen.The ground state is negative of the amount of energy that must be added to the atom to completely remove the electron or ionise the atom. Other states which have been “excited” can leap a level to n= 2, 3, 4 etc.
21 Transitions? E = hf = E3 – E2 hf So when we talk about electrons moving levels we call them transitions.Electrons can move from 1 to 2 or 1 to 3 and vice versa. Every atom has a number of combinations. Each resulting in a different absorption or emission of energy.This energy is again considered in a type of “packet” or “quanta” which we define in either joules or eV as;hfE = hf = E3 – E2Show:hydrogen-atom.jarGas tubes
22 Photon has to be exactly the right energy Excitation using photonsAn electron in an atom can absorb an incident photon’s energy if the photonsenergy exactly matches the quantum of energy required for excitation.KEY PointPhoton has to be exactly the right energy
23 De-excitationAn excited atom is unstable and the vacancy in the shell is soon filled by anelectron from an outer shell dropping to a lower energy level emitting a photon.De excitation of a mercury atommay proceed indirectly to theground state via an intermediatestate.5.7 eV4.9 eVO eV0.8 eV photon4.9 eV photon
24 Can you fill in the gaps…. Mini Check Spectrums….Can you fill in the gaps….ItemContinuousAbsorptionEmissionExample sourceGas PressureWhat do they look like?Harder
25 hf = E2 - E1Calculations?For Hydrogen bohr worked out that the energy levels followed a more complex formulae of E = -13.6eV / n2TASK: Take a note of this and work out the transition for n=3 to 2
26 TASK: Make a note of the levels & two forms of measuring energy SummaryThe line spectrum can only be explained by a theory that suggests that electrons in atoms can only exist in well-defined energy levels.Since different atoms produce different line spectra it follows that each atom has a unique set of energy levels. The diagram shows the energy level diagram for atomic hydrogen. The energies are. given in both J and eVThe levels are like rungs on a ladder you cannot stand between the rungs. But the higher energy levels get closer together. Also the energy is defined as negative as it is the energy required to remove and electron. Each shell has a number.The line spectrum can only be explained by a theory that suggests that electrons in atoms can only exist in well-defined energy levels.Since different atoms produce different line spectra it follows that each atom has a unique set of energy levels. The diagram shows the energy level diagram for atomic hydrogen. The energies are. given in both J and eVThe levels are like rungs on a ladder you cannot stand between the rungs. But the higher energy levels get closer together. Also the energy is defined as negative as it is the energy required to remove and electron. Each shell has a number.TASK: Make a note of the levels & two forms of measuring energy
28 Electron loses some energy and carries on Ionisation- any process of creating ions( removing an electron from an atom creates a positive ion)* alpha beta gamma radiation creates + ve ions* electrons colliding with atoms of gas in a tube + ve ionsKEY PointElectron loses some energy and carries on
29 Excitation from collision *atoms can absorb energy from the incident electron in discrete or quantisedamounts without being ionised called excitation energies.The colliding electron having lost its KE does not reach the anodeand the current fallsKEY PointElectron loses energy = excitation energy of atom and carries on
30 Mini Check Ions / Excitation…. Can you fill in the gaps….ItemElectron Collision IonisationElectron ExcitationPhoton Emission/De ExcitationWhat happensAny comments on Energies involved?Harder
31 Electrons gain KE qV which can be equated to KE =1/2mv2 Measuring EnergyElectrons are emitted from a hot filament in a tube of gas at low pressure and are attracted to an anode at the other endInitially the current is small as just a few electrons reach the anodeAs the pd is increased the electrons’ speed increases until ionisation occurs near the anode and the current rises considerably.From; V = WqW = work done on eachelectron of charge eby filament= KE of each electron= Ionisation of gas atom= eVKEY PointElectrons gain KE qV which can be equated to KE =1/2mv2Rearrange to find velocity
32 h E λ p E1 m E2 v Φ f c KEmax Quick think? Using the symbols below can you make any formulae....hEλpE1mE2vΦfcKEmax
35 ConnectionConnect your learning to the content of the lessonShare the process by which the learning will actually take placeExplore the outcomes of the learning, emphasising why this will be beneficial for the learnerDemonstrationUse formative feedback – Assessment for LearningVary the groupings within the classroom for the purpose of learning – individual; pair; group/team; friendship; teacher selected; single sex; mixed sexOffer different ways for the students to demonstrate their understandingAllow the students to “show off” their learningConsolidationStructure active reflection on the lesson content and the process of learningSeek transfer between “subjects”Review the learning from this lesson and preview the learning for the nextPromote ways in which the students will rememberA “news broadcast” approach to learningActivationConstruct problem-solving challenges for the studentsUse a multi-sensory approach – VAKPromote a language of learning to enable the students to talk about their progress or obstacles to itLearning as an active process, so the students aren’t passive receptors