Physics Paper 1 November 2021
Elisa Rathrock
The Electron Microscopy and Analysis Group held a Conference on Electron Microscopy at Cambridge from 2nd to 6th July 1963. The Conference included papers on electron optics, instrumentation, specimen preparation techniques and applications to physics and physical metallurgy. The last two sessions were devoted to a Symposium on Radiation Damage.
At the Conference on High Magnetic Fields, Oxford, July 1963, the following topics were discussed: (i) the production of high magnetic fields using normal conductors, (ii) super-conductors and high magnetic fields, (iii) metals and magnetic materials in high magnetic fields and (iv) semiconductors in high magnetic fields.
Recent developments in the use of superconductors for the generation of high magnetic fields are reviewed. The physical bases for high-field superconductivity are discussed relative to type II superconductors, metal-insulator composites and metal-metal composites. Some characteristics of supermagnets are described, and manifestations and possible causes of current-degradation phenomena are considered. Finally, advanced supermagnet concepts which might offer some prospect of circumventing degradation problems are explored.
This paper reviews the magnetic fields available in pulsed coils and in continuous operation, using normal conductors. The new laboratory now coming into operation at the Massachusetts Institute of Technology is described. This involves primarily four generators designed to produce 10 000 A continuously, and current pulses up to 40 000 A each for times diminishing to a few seconds, and at voltages of 200-250 v. The power and corresponding cooling water can be made available at ten locations by remote control. The generators may be operated independently and simultaneously or in any series or parallel combination desired. The thinking that has led to a 250 kG magnet and a 100 kG high precision magnet is reviewed. A magnet to produce in excess of 400 kG in a 2 second pulse, and requiring 32 MW, is under consideration.
This is a survey paper in which the author discusses the energy level structure of metals in high magnetic fields and the nature of the eigen functions, and relates the various phenomena to one another within the one conceptual framework of the quantum mechanics of electrons moving in a lattice.
The effect of a magnetic field on the energy levels of atomic electrons, and on electrons in band states, is briefly reviewed. The effect of the magnetic energy on the properties of paramagnetic salts, magnetically ordered materials and semiconductors is discussed. Criteria for interesting high field effects are given by comparing the magnetic energies with other energies in the problem, e.g. level splitting, level widths, exchange and anisotropy energies, exciton binding energies, plasma quanta and kT. There is a brief indication of the sort of experiments that have been, or might be, performed.
The problem at the moment is to combat the increased level of noise resulting from increased urbanization using modern methods of construction, usually less effective insulators than traditional ones. Statutory standards of sound insulation for dwellings are under consideration and the Wilson Committee recommend financial aid. The menace of noise from road and air traffic suggests the prescriptive siting of new buildings. The coincidence effect which explains discrepancies in sound insulation is discussed both in theory and practice. High insulation of the two-leaved partition is shown by two examples. Modern partitions are proving inadequate sound barriers in important parts of the frequency range. The new acoustic laboratories at the Building Research Station are described and illustrated. An increased use of scientific measurement of sound is apparent. Work begun by the Building Research Station and developed in the United States uses subjective judgements in assessing concert hall acoustics. Assessment of room acoustics by computers may lead logically to electronic control of natural acoustics. Noise control is probably the most urgent task.
A theory of irradiation damage in graphite is developed, based on the interactions of three types of defect, single sessile vacancies, small mobile interstititial clusters and large sessile interstititial clusters. Reactions between the mobile and sessile defects occur at random, and lead to expressions for recombination, nucleation and cluster growth, and to equations for the concentration of the various types of defect as a function of temperature, dose and flux. These equations are consistent with experimental data on total stored energy and cluster size reported by other workers.
An account is given of an experimental investigation of the Barkhausen effect in manganese zinc, nickel zinc, and magnesium manganese ferrites. The durations and number-moment level distributions of the Barkhausen discontinuities during magnetization reversal have been measured. Also measured were the distributions of Barkhausen moment changes of a given size per unit field. A model is proposed from which the Barkhausen durations may be calculated and which predicts values in reasonable agreement with those found. The number-moment level distributions follow complex exponential laws and the equations of these distributions were obtained. The irreversible induction change was calculated and the sum of this component and the measured reversible induction was found to be in general agreement with the total change. Similarly the reversible and irreversible inductions per unit field were examined and each has the expected relationship to the slope of the traversed limb of the hysteresis loop.
An expression for the dispersion of a compressible fluid in turbulent motion in a pipe has been derived. The solution is based on Taylor's theory for dispersion during incompressible fluid flow but allows for the continuous increase in linear velocity which can occur during turbulent flow of a compressible fluid.
The theory has been checked experimentally by injecting radioactive 85Kr into air flowing turbulently in a smooth pipe. The width 2S of the tracer at distance L along a pipe of diameter 2a and Fanning friction factor γ has been shown to be given by the equation
Amorphous polyethylene terephthalate was stretched below the glass transition temperature to various degrees of orientation. The resulting material had transverse symmetry and, according to classical elasticity theory, its elastic behaviour could be described by five independent constants. Three could be calculated from Young's modulus experiments and the remaining two from torsional measurements. Such experiments were carried out using a 25 second (square) stress cycle.
Increasing orientation (birefringence) caused an increase in Young's modulus in the stretch direction to about five times the isotropic value. The transverse modulus and the 45 modulus were little affected by orientation. The shear modulus involved when twisting about the symmetry direction also showed only small variations while the value of the other shear modulus decreased to about half the isotropic modulus.
Calculations of the volume and linear compressibilities showed that oriented polyethylene terephthalate behaves under hydrostatic pressure essentially as the isotropic polymer of the same density. This observation implies two further relations between the elastic constants and thus the number of independent constants for the oriented material reduces from five to three.
In the course of determining the potentials within a mass spectrometer source, simple rules have been discovered which aid the calculation of potentials about a thin slit. These rules, of which there are four, make possible the approximation with ruler and compass of the equipotentials formed by opening a long, narrow slit in a thin conductor separating regions of uniform electric field. A simple iteration formula is included which enables the approximate potential at any point to be refined to any desired degree. Proofs of these rules are developed with the aid of conformal mapping in the complex plane.
The theory of triple-axis spectrometers is discussed, and expressions are given for peak height and integrated intensities which show that the instrument possesses focusing properties which serve to sharpen line widths and increase peak heights. Five focusing conditions are derived and an operational procedure is suggested to exploit these effects. The use of focusing promises to give considerable improvements in the accuracy of measurements of phonon and magnon line widths.
The shape of a stress test specimen was so chosen that a uniform two-dimensional tensile stress field could be produced in it. The ratio of the two principal stresses of the field can be varied arbitrarily between 0 and 1, i.e. any desired tensile stress state is obtainable. The accuracy of the stress produced in the specimen was checked by means of photoelastic models.
Two methods of measuring coating thickness in an electron probe microanalyser without the aid of standards are described, and thickness calibration curves in terms of the x-ray intensity from coating or substrate material are illustrated. Agreement between the micro-analysis methods and others is within 15%. For 29 kv electrons, the range of measurable thickness is 2 10-4 to 1.5 mg cm-2. It is shown that the coating intensity method is more accurate for coatings up to about a quarter of the electron range while the substrate method is the better for the thicker coatings.
It is pointed out that there should be a modification to both the friction factor and Metzner's generalized Reynolds number before polymeric solutions may be correlated on the usual Reynolds number-friction factor graphs for pipe flow. The existence of the correction factors for both these numbers in fiscoelastic flow may explain, in part at least, why laminar flow apparently exists to much higher Reynolds numbers calculated on a basis which does not account for rotation of the stress ellipsoid.
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