REVISED GCE AS & A Level Notes for Guidance Physics

Transcription

1 REVISED GCE AS & A Level Notes for Guidance Physics Unit AS 1: Forces, Energy and Electricity Unit AS 2: Waves, Photons and Medical Physics Unit A2 1: Momentum, Thermal Physics, Circular Motion, Oscillations and Atomic and Nuclear Physics Unit A2 2: Fields and their Applications For first teaching from September 2008 For first award of AS Level in Summer 2009 For first award of A Level in Summer 2010 Issued February 2009

2 CONTENTS 1. Introduction Unit AS 1: Forces, Energy and Electricity Use the principle of moments to solve simple problems Demonstrate knowledge and simple understanding of superconductivity Understand the use of the potential divider as a source of variable p.d Unit AS 2: Waves, Photons and Medical physics Defections of vision and their correction Use the formula Intensity level / db = 10 lg 10 I/I Interpret, qualitatively, graphs of frequency and intensity response for the ear Describe the flexible endoscope in terms of structure, technique and applications Describe ultrasonic A-scans and B-scans in terms of physical principles, basic equipment, technique and applications Describe Computed Tomography scans in terms of physical principles, basic equipment, technique and applications Describe MRI scans in terms of physical principles, basic equipment, technique and applications Provide a simple explanation of laser action Describe electron diffraction Unit A2 1: Momentum, Thermal Physics, Circular Motion, Oscillations and Atomic and Nuclear Physics Describe evidence for the existence of atomic nuclei, to include alphaparticle scattering Know and interpret the variation of nuclear radius with nucleon number Use the equation r = r 0 A 1/3 to estimate the density of nuclear matter Understand the conditions required for fusion Estimate the temperature required for fusion Describe the following methods of plasma confinement: gravitational, inertial and magnetic Appreciate the difficulties of achieving fusion on a practical terrestrial scale Describe the JET fusion reactor State the D-T reaction and appreciate why this is most suitable for terrestrial fusion Unit A2 2: Fields and their Applications Describe the basic principles of operation of linear accelerator, cyclotron and synchrotron Compare and contrast the three types of accelerator... 8

3 5.7.3 Understand the concept of antimatter and that it can be produced and observed using high-energy particle accelerators Describe the process of annihilation in terms of photon emission and conservation of energy and momentum Explain the concept of a fundamental particle Identify the four fundamental forces and their associated exchange particles; classification of gauge bosons Classification of particles: leptons and hadrons (mesons and baryons) Describe the structure of hadrons in terms of quarks Understand the concept of conservation of charge, lepton number and baryon number Describe β-decay in terms of the basic quark model... 10

4 1. Introduction These notes are intended to assist teachers in judging the depth of treatment required in sections of the specification with which they may be unfamiliar. Some of the notes have previously appeared in the Notes for Guidance which accompanied the previous specification. The notes refer to certain sections of the GCE AS and Advanced GCE specification for courses starting in September The notes should be read in conjunction with the relevant specification section and taken with the specimen examination papers to give guidance to teachers on what will be expected of candidates by the Examiner. They are not intended to indicate a particular teaching approach. 2. Unit AS 1: Forces, Energy and Electricity 1.6 Principle of moments Use the principle of moments to solve simple problems Candidates will be expected to be familiar with the concept of the centre of gravity in the context of moments Resistance, resistivity Demonstrate knowledge and simple understanding of superconductivity Candidates should be aware that superconductivity is a phenomenon that occurs in some materials, at very low temperatures. It is the abrupt drop to zero resistance when a material is cooled below its critical or transition temperature that characterises a superconductor. Whilst superconductivity is displayed by some simple elements such as tin and aluminium it is the discovery of ceramic materials at relatively high temperatures, in excess of 90 kelvin, that has stimulated interest and extended the possible applications. The application in the production of powerful electromagnets with no generation of internal energy as used in MRI scanners should be known Direct current circuits Understand the use of the potential divider as a source of variable p.d. Candidates will be expected to perform calculations on a loaded potential divider circuit using basic series and parallel equations. The specific effect of a change in the p.d. on loading will not be examined. Examples should include the use of thermistors and LDRs in potential divider circuits. 1

5 3 Unit AS 2: Waves, Photons and Medical Physics 2.3 Lenses Defections of vision and their correction Candidates should be familiar with the terms long sight (hypermetropia) and short sight (myopia). They should be able to explain how these defects occur, and how they are corrected with spectacle lenses. They should be able to calculate the focal length and power of the lens required to correct the defect of long sight only. 2.6 Sound Use the formula Intensity level / db = 10 lg 10 I/I 0 Candidates should understand what is meant by the intensity of a sound wave, and should be able to express intensity level on the decibel scale Interpret, qualitatively, graphs of frequency and intensity response for the ear Candidates should know that the physical response of the ear to sound is one of resonance, i.e. the vibrations of the sound match the natural frequencies of vibration of parts of the ear. The combination of these various resonances gives the ear its characteristic frequency range: candidates should know that the range detectable by an average ear is from about 20 Hz to about 20 khz. The brain discriminates frequencies by distinguishing the location in the inner ear from where the electrical impulses originate. Candidates should be able to recall and interpret a graph of the intensity response with frequency of the average ear, such as that shown in Fig 1. Fig 1 2

6 Candidates should understand the meaning of the term threshold of hearing, and should recall that the threshold is about 1 x W m 2 at a frequency of about 2 khz. They should be aware that the ability to discriminate between frequencies varies across the range: between about 60 Hz and 1 khz a difference of 3 Hz can be distinguished, but this ability falls away as frequency increases, and above 10 khz discrimination is very poor. 2.7 Imaging techniques Describe the flexible endoscope in terms of structure, technique and applications Candidates should be aware of the need to make visual observations inside the body of a patient. The idea of the orderly (coherent) arrangement of a bundle of optical fibres, so that an optical image can be transmitted from one end to the other, should be contrasted with that of a noncoherent bundle for illumination. They should be aware of the provision of irrigation and operations channels and control cables. Examples of the application of the endoscope for both examination and treatment of patients should be known Describe ultrasonic A-scans and B-scans in terms of physical principles, basic equipment, technique and applications The principles of ultrasonic scanning should be understood, and the functions of the main components of the scanner should be highlighted. Candidates should be made aware of some medical uses for the scans. The information given below indicates the level of knowledge required. Ultrasonic pulses from a transducer are transmitted into the body. The resulting pulse echoes received from the body are converted into an electrical signal giving diagnostic information. This information is normally displayed on a CRT screen, and is also stored on computer. The maximum mean ultrasound power delivered during this procedure is 10 4 W, and the frequency range is between 1 MHz and 15 MHz. It is common practice for a film of oil, or a water-based cellulose jelly, to be used as a coupling medium to prevent excessive reflections which would occur at a solid-air boundary. These reflections would effectively block further transmission. The lungs and bowel contain air, and so are not imaged using ultrasound. The A-scan (amplitude-scan) technique is used to determine ranges, and functions in the following manner. The c.r.o. sweep is triggered by the time-base and simultaneously an electronic pulse is generated. This energises the piezoelectric transducer which emits a short (1 μs) pulse of ultrasound which enters the first medium. The first spike on the c.r.o. screen represents the reflection of this pulse from the surface of the body. This signal has been transmitted via the transducer and amplifier to the Y-plates of the c.r.o. The echo signals from the deeper surfaces will be weaker than those from surfaces close to the transducer. To overcome this problem, the signals from the deeper surfaces are amplified more. 3

7 The position of a spike on the x-axis indicates the time taken for the ultrasound to travel to the surface and back. This time is influenced by the medium through which the pulse is travelling, and can be used to obtain the thickness of an organ. The amplitude of the spike depends on the attenuation of the medium. Soft tissues reflect back only about 1% of the incident intensity, but a bony surface reflects a larger proportion of the sound and gives a signal of higher intensity. Applications of A-scans include detailed measurements of the eye before ophthalmic surgery, and also the identification of tumours. In the B-scan (brightness-scan) technique the amplitude of the reflect signal is shown by the brightness of a spot. From this came the idea that a pictorial display of the organs could be obtained. To create such a display the transducer is moved, thereby allowing the body to be viewed from several angles. The views build to a set of spots which are correlated with information regarding probe position and orientation. As the probe moves the display is recorded using a storage c.r.o. Operator skill is very important, as the probe must remain in contact with the skin. Applications of B-scans include monitoring the growth of the foetus and the location of tumours. Candidates should be able to compare ultrasonic imaging with other types of imaging referred to in the specification Describe Computed Tomography scans in terms of physical principles, basic equipment, technique and applications This is a powerful X-ray based diagnostic tool, capable of producing images of any crosssection within the body. Candidates should understand the physical principles of production of X-rays. They should know that X-rays are produced when electrons accelerated to a high voltage strike a metal target in an X-ray tube. There are two distinct methods of production: a) rapid deceleration of electrons after passing the nucleus results in photon emission and b) tightly bound inner electrons being knocked out of the atom by the incident high-energy electrons, with subsequent photon emission when electrons in upper states drop down into vacated lower states. They should be able to describe the structure of the rotating X-ray tube used in this type of imaging, and identify the different types of detector used in different situations. Candidates should be able to illustrate with reference to linear tomography how the image in one plane in the body can be produced by motion of both the X-ray source and the detector. Candidates should appreciate that computed tomography relies on powerful computers and complex software to analyse data from an X-ray source and detection system which rotates around the patient; no further detail of the computer principles is required. They should explain the need for a collimated beam, and discuss the uses, advantages and drawbacks of CT scanning, including cost, reliability, quality of image, and the ability to store and reconstruct images. Candidates should be able to compare this type of imaging with others referred to in the specification Describe MRI scans in terms of physical principles, basic equipment, technique and applications Candidates should appreciate that Magnetic Resonance Imaging (MRI) is a powerful diagnostic tool, requiring complex analysis by computer of data based on atoms (usually hydrogen) in the human body which have experienced nuclear magnetic resonance (NMR). Only a simple descriptive treatment of nuclear magnetic resonance is required. Candidates should recall that a 4

8 carefully selected external r.f. signal from the scanner can cause the hydrogen atoms in the body to change their orientation in the magnetic field, allowing changes in the magnetic field due to atoms in the body to be detected. A simple description of the magnetic resonance system should include the function of the main components (scanner magnet, field gradient coils, r.f. transmitter, r.f. receiver). Candidates should appreciate the difficulties and costs in producing and maintaining a magnetic field strength of the order of 2 T in a volume large enough for the patient to lie in. They should contrast the earlier copper coil scanner magnets with more recent superconducting materials, where currents of the order of 700 A can be maintained without significant power loss. They should be aware of the design features of a building suitable for accommodating this magnet, and for providing shielding for people working in the area. Candidates should be aware of precautions taken when working with strong magnetic fields: the removal of credit cards, jewellery and loose metal objects, and the exclusion of patients with pacemakers or other implanted metal devices. Candidates should be able to compare this type of imaging with others referred to in the specification. 2.9 Quantum physics Provide a simple explanation of laser action Candidates should have a basic understanding of laser action as a specific example of energy emission due to electron transition between permitted energy levels. They should be aware of the concept of having more excited than unexcited atoms which are stimulated to de-excite simultaneously by incident photons, producing light from each atom that is in phase. Some uses of lasers should be known, for example, in CD players and in endoscopes for micro surgery Wave-particle Duality Describe electron diffraction Candidates should be aware of the experiment to demonstrate diffraction of electrons by a thin film of graphite, and of the pattern of rings obtained from this. They should be aware that the wavelength of the electrons depends on their velocity, according to the de Broglie relation. They should have a qualitative awareness that the radius of the circular diffraction rings depends on the electron de Broglie wavelength and consequently the electron velocity. 4 Unit A2 1: Momentum, Thermal Physics, Circular Motion, Oscillations and Atomic and Nuclear Physics 4.5 The nucleus Describe evidence for the existence of atomic nuclei, to include alpha-particle scattering Candidates should be aware of the experimental arrangement used by Geiger and Marsden (students of Rutherford in 1910) when alpha-particles in a vacuum were directed towards a thin gold foil, only a few hundred atoms thick. They should be able to describe the experimental arrangement used to detect the alpha-particle deflections: in the original experiment, the alpha- 5

9 particles exiting the gold foil were incident on a fluorescent screen, and scintillations were observed by eye. They should appreciate that although most of the alpha-particles passed straight through the gold film, some were scattered appreciably, and a very few were reflected back towards the alpha-particle source. They should appreciate the significance of this backscattering, and how the results led to Rutherford s model of the atom Know and interpret the variation of nuclear radius with nucleon number Candidates should be introduced to a graph of nuclear radius r against nucleon number A (Fig 2), showing a uniform increase of r with A. It can be deduced from the data that r is proportional to A 1/3. Fig Use the equation r = r 0 A 1/3 to estimate the density of nuclear matter Candidates should be able to deduce that ρ = 3m/4πr 0 3, where m is the mean mass of a nucleon. They should appreciate that the density of nuclear matter (about 2 x kg m 3 ) is many orders of magnitude greater than that of solids because of the close-packing of the particles within the nucleus. 6

10 4.9 Nuclear fusion Understand the conditions required for fusion To overcome the electrostatic repulsion between nuclei, they need to have sufficient kinetic energy. This requires temperatures of the order of 10 8 or 10 9 K. At such temperatures matter exists in a fourth state known as plasma. The atomic electrons break free from the nucleus, and the gas-like fluid is a mixture of electrons, positive ions and free nuclei Estimate the temperature required for fusion Given the energy needed to bring two nuclei together the candidate should be able to calculate the temperature using E = 1.5 kt where k is the Boltzmann constant Describe the following methods of plasma confinement: gravitational, inertial and magnetic Stars have sufficient mass to provide a gravitational force sufficient to hold a plasma together. Inertial confinement relies on intense beams of laser light or ions to compress a fuel pellet while heating it. Magnetic confinement is employed in the JET fusion reactor Appreciate the difficulties of achieving fusion on a practical terrestrial scale Candidates should be aware of the difficulties involved in achieving the high temperatures and plasma confinement, and of then extracting the heat energy produced Describe the JET fusion reactor Magnetic confinement is employed in the JET fusion reactor: it is necessary to hold the plasma away from the container walls, as otherwise the container would vaporise. Charged plasma particles circulate endlessly in helical paths around the magnetic field lines produced by watercooled toroidal field coils. Temperatures of 10 7 K have been achieved for fractions of a second State the D-T reaction and appreciate why this is most suitable for terrestrial fusion Deuterium + Tritium Helium + n MeV The tritium is generated when lithium is converted by neutron absorption in the Tokamak design. Both the lithium and the deuterium are readily available on earth. The D-T reaction is a single-stage reaction which produces a much greater energy release than others of similar type, e.g. D-D fusion. Another advantage of this reaction is that there would be no long term storage issues with radioactive waste products. But note neutron irradiation will occur. 5 Unit A2 2: Fields and their Applications 5.7 Particle accelerators 1 0 7

11 5.7.1 Describe the basic principles of operation of linear accelerator, cyclotron and synchrotron Candidates should appreciate that accelerators can supply charged particles with the energy needed to create new matter in collisions. The linear accelerator has a series of tubular electrodes supplied by a high-frequency alternating voltage. Alternate electrodes are connected to the same terminal. The frequency is carefully chosen so that the charged particle is accelerated towards each electrode, and as the particle passes through the electrode the voltage supply reverses so that it repels the charged particle out of the electrode while the next electrode attracts it. In the cyclotron, charged particles from a source at the centre follow a spiral path of increasing radius. Two semi-circular electrodes (the dees ) are used with an alternating voltage of fixed frequency applied between them. A magnetic field is applied to maintain the circular paths. The particles pass back and forward across the same gap, increasing their kinetic energy each time. In the synchrotron, a large number of electromagnets keep the particles in a curved path of fixed radius. As the speed increases, the magnetic field strength is increased. Two or more radiofrequency electrodes are used to accelerate the charged particles. As with the linear accelerator, the alternating voltage is applied to an electrode so that the particles are accelerated towards it and repelled as they leave. Candidates will not be expected to carry out detailed calculations relating to particle accelerators. However, the forces acting on charged particles in electric and magnetic fields are met in A2 2, 5.6, and centripetal acceleration and forces in A2 1, 4.3, so some quantitative work on the linear accelerator and the cyclotron could be introduced in a question Compare and contrast the three types of accelerator The advantages of the linear accelerator are that it has high beam intensity, with a well-focused beam with small energy loss. Its disadvantage is that because each pair of electrodes accelerates the particle only once, the machine is very long. It provides particles with energies up to 30 GeV. The advantages of the synchrotron are that higher energies are obtained than with the cyclotron, and a uniform magnetic field is required only over the circumference of the orbit. Its disadvantage is that the energy loss due to a charged particle being accelerated in a circular path (synchrotron radiation) is high, especially for electrons. It provides particles with energies up to 1000 GeV (1 TeV) Understand the concept of antimatter and that it can be produced and observed using high energy particle accelerators Candidates should be aware that most types of particle have a corresponding antiparticle, and that these have the same (rest) mass, but at least one other property which is opposite to that of 8

12 the particle. In high energy collisions particles and antiparticles may materialise from the energy supplied Describe the process of annihilation in terms of photon emission and conservation of energy and momentum When a particle and an antiparticle meet they may annihilate each other. Their mass is converted into energy as given by E = mc 2. For example, the annihilation of an electron and a positron may produce a pair of gamma photons; candidates should appreciate that the production of a single photon is impossible, as momentum must be conserved. Annihilation is rare, as there are far more particles than antiparticles. Candidates should be able to calculate the energy, frequency and wavelength of the photons. 5.8 Fundamental particles Explain the concept of a fundamental particle A particle that has no internal structure is said to be a fundamental particle, i.e. it cannot be broken down into any further constituents. Candidates should have knowledge of which particles are fundamental and which are not Identify the four fundamental forces and their associated exchange particles; classification of gauge bosons Candidates should be aware of the four fundamental forces: strong nuclear, electromagnetic, weak nuclear and gravitational. It is believed that the fundamental forces are carried by virtual exchange particles. For example, electrons repel each other by exchanging virtual photons. These virtual particles are called gauge bosons. Candidates should be able to state the gauge boson that carries each of the fundamental forces: Force Strong nuclear Electromagnetic Gravitational Gauge boson Gluon Photon Graviton Weak nuclear W + W Z Classification of particles: leptons and hadrons (mesons and baryons) Candidates should be aware that as a result of the study of the bombardment of targets by beams of high-energy charged particles in accelerators, and of the study of cosmic rays, many subnuclear particles were discovered. Most of these are short-lived. 9

13 They should appreciate that matter particles can be divided into two main groups, hadrons that experience the strong nuclear force, and leptons that do not. Hadrons can be sub-divided into baryons and mesons. There are three generations of leptons, but only the first (the electron and its neutrino) occurs in ordinary matter. Leptons have no internal structure and are therefore fundamental Describe the structure of hadrons in terms of quarks Candidates should be able to explain that the properties of hadrons can be accounted for by assuming that each particle is a combination of others, called quarks. These have fractional charge of + 2 / 3 e, for an up quark and 1 / 3 e for a down quark. For each type of quark there is an antiquark. Baryons contain 3 quarks or antiquarks, whereas the mesons contain 2, a quark and an antiquark. Candidates should be able to account for the formation of baryons, such as the proton and neutron and mesons such as the pion +, from quarks and antiquarks. Quarks combine together to make particles with a total relative charge of 0 or ±1. The most important feature of the quark model is that all mesons and baryons can be understood in terms of appropriate combinations of quarks, and all matter can be expressed in terms of quarks and leptons. There are six types or flavours of quark in total. For examination purposes, only knowledge of the up and down quark is required Understand the concept of conservation of charge, lepton number and baryon number Candidates should be introduced to the use of numbers associated with particle properties that are conserved in interactions. This may then be applied to test whether or not a given event is allowed or possible Describe β-decay in terms of the basic quark model Candidates should be able to describe β-decay in terms of quarks. β-decay occurs when an udd baryon (neutron) converts to an uud baryon (proton). The weak nuclear force is responsible for this process. The W boson is the virtual particle emitted. This decays, creating an electron and an antineutrino in the process. 10

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