PHYSICS (732351), страница 3
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The most commonly seen are the laws of conservation of mass-energy (formerly two conservation laws before A. Einstein), ofelectric charge, of linear momentum, and of angular momentum.There are several others that deal more with particle physics,such as conservation of baryon number, of strangeness, etc., whichare conserved in some fundamental interactions but not others.
Law of reflection
For a wavefront intersecting a reflecting surface, the angle ofincidence is equal to the angle of reflection.
Laws of black hole dynamics
First law of black hole dynamics. For interactions between black holes and normal matter, the conservation laws of total energy, total momentum, angular momentum, and electric charge, hold.
Second law of black hole dynamics. With black hole interactions, or interactions between black holes and normal matter, the sum of the surface areas of all black holes involved can never decrease.
Laws of thermodynamics
First law of thermodynamics. The change in internal energy of a system is the sum of the heat transferred to or from the system and the work done on or by the system.
Second law of thermodynamics. The entropy -- a measure of the unavailability of a system's energy to do useful work -- of a closed system tends to increase with time.
Third law of thermodynamics. For changes involving only perfect crystalline solids at absolute zero, the change of the total entropy is zero.
Zeroth law of thermodynamics. If two bodies are each in thermal equilibrium with a third body, then all three bodies are in thermal equilibrium with each other.
Lawson criterion (J.D. Lawson)
A condition for the release of energy from a thermonuclearreactor. It is usually stated as the minimum value for theproduct of the density of the fuel particles and the containmenttime for energy breakeven. For a half-and-half mixture ofdeuterium and tritium at ignition temperature, nG is between1014 and 1015 s/cm3.
Le Chatelier's principle (H. Le Chatelier; 1888)
If a system is in equilibrium, then any change imposed on thesystem tends to shift the equilibrium to reduce the effect of thatapplied change.
Lenz's law (H.F. Lenz; 1835)
An induced electric current always flows in such a direction thatit opposes the change producing it.
Loschmidt constant; Loschmidt number; NL
The number of particles per unit volume of an ideal gas atstandard temperature and pressure. It has the value 2.68719.1025 m-3.
Lumeniferous aether
A substance, which filled all the empty spaces between matter,which was used to explain what medium light was "waving" in. Nowit has been discredited, as Maxwell's equations imply thatelectromagnetic radiation can propagate in a vacuum, since theyare disturbances in the electromagnetic field rather thantraditional waves in some substance, such as water waves.
Lyman series
The series which describes the emission spectrum of hydrogen whenelectrons are jumping to the ground state. All of the lines arein the ultraviolet.
Mach's principle (E. Mach; 1870s)
The inertia of any particular particle or particles of matter isattributable to the interaction between that piece of matter andthe rest of the Universe. Thus, a body in isolation would have noinertia.
Magnus effect
A rotating cylinder in a moving fluid drags some of the fluidaround with it, in its direction of rotation. This increases thespeed in that region, and thus the pressure is lower.Consequently, there is a net force on the cylinder in thatdirection, perpendicular to the flow of the fluid. This is calledthe Magnus effect.
Malus's law (E.L. Malus)
The light intensity travelling through a polarizer is proportionalto the initial intensity of the light and the square of the cosineof the angle between the polarization of the light ray and thepolarization axis of the polarizer.
Maxwell's demon (J.C. Maxwell)
A thought experiment illustrating the concepts of entropy. Wehave a container of gas which is partitioned into two equal sides;each side is in thermal equilibrium with the other. The walls(and the partition) of the container are a perfect insulator. Now imagine there is a very small demon who is waiting at thepartition next to a small trap door. He can open and close thedoor with negligible work. Let's say he opens the door to allow afast-moving molecule to travel from the left side to the right, orfor a slow-moving molecule to travel from the right side to the left, and keeps it closed for all other molecules. The net effectwould be a flow of heat -- from the left side to the right -- eventhough the container was in thermal equilibrium. This is clearlya violation of the second law of thermodynamics. So where did we go wrong? It turns out that information hasto do with entropy as well. In order to sort out the moleculesaccording to speeds, the demon would be having to keep a memory ofthem -- and it turns out that increase in entropy of the simplemaintenance of this simple memory would more than make up for thedecrease in entropy due to the heat flow.
Maxwell's equations (J.C. Maxwell; 1864)
Four elegant equations which describe classical electromagnetismin all its splendor. They are:
Gauss' law. The electric flux through a closed surface is proportional to the algebraic sum of electric charges contained within that closed surface.
Gauss' law for magnetic fields. The magnetic flux through a closed surface is zero; no magnetic charges exist.
Faraday's law. The line integral of the electric flux around a closed curve is proportional to the instantaneous time rate of change of the magnetic flux through a surface bounded by that closed curve.
Ampere's law, modified form. The line integral of the magnetic flux around a closed curve is proportional to the sum of two terms: first, the algebraic sum of electric currents flowing through that closed curve; and second, the instantaneous time rate of change of the electric flux through a surface bounded by that closed curve.
In addition to describing electromagnetism, his equations alsopredict that waves can propagate through the electromagneticfield, and would always propagate at the same speed -- these are electromagnetic waves.
Meissner effect (W. Meissner; 1933)
The decrease of the magnetic flux within a superconducting metalwhen it is cooled below the critical temperature. That is,superconducting materials reflect magnetic fields.
Michelson-Morley experiment (A.A. Michelson, E.W. Morley; 1887)
Possibly the most famous null-experiment of all time, designed toverify the existence of the proposed "lumeniferous aether" throughwhich light waves were thought to propagate. Since the Earthmoves through this aether, a lightbeam fired in the Earth'sdirection of motion would lag behind one fired sideways, where noaether effect would be present. This difference could be detectedwith the use of an interferometer.
The experiment showed absolutely no aether shift whatsoever,where one should have been quite detectable. Thus the aetherconcept was discredited as was the constancy of the speed oflight.
Millikan oil drop experiment (R.A. Millikan)
A famous experiment designed to measure the electronic charge.Drops of oil were carried past a uniform electric field betweencharged plates. After charging the drop with x-rays, he adjustedthe electric field between the plates so that the oil drop wasexactly balanced against the force of gravity. Then the charge onthe drop would be known. Millikan did this repeatedly and foundthat all the charges he measured came in integer multiples only ofa certain smallest value, which is the charge on the electron.
Newton's law of universal gravitation (Sir I. Newton)
Two bodies attract each other with equal and opposite forces; themagnitude of this force is proportional to the product of the twomasses and is also proportional to the inverse square of thedistance between the centers of mass of the two bodies.
Newton's laws of motion (Sir I. Newton)
Newton's first law of motion. A body continues in its state of rest or of uniform motion unless it is acted upon by an external force.
Newton's second law of motion. For an unbalanced force acting on a body, the acceleration produces is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.
Newton's third law of motion. In a system where no external forces are present, every action is always opposed by an equal and opposite reaction.
Ohm's law (G. Ohm; 1827)
The ratio of the potential difference between the ends of aconductor to the current flowing through it is constant; theconstant of proportionality is called the resistance, and isdifferent for different materials.
Olbers' paradox (H. Olbers; 1826)
If the Universe is infinite, uniform, and unchanging then theentire sky at night would be bright -- about as bright as the Sun.The further you looked out into space, the more stars there wouldbe, and thus in any direction in which you looked your line-of-sight would eventually impinge upon a star. The paradox isresolved by the Big Bang theory, which puts forth that theUniverse is not infinite, non-uniform, and changing.
Pascal's principle
Pressure applied to an enclosed imcompressible static fluid istransmitted undiminished to all parts of the fluid.
Paschen series
The series which describes the emission spectrum of hydrogen whenthe electron is jumping to the third orbital. All of the linesare in the infrared portion of the spectrum.
Pauli exclusion principle (W. Pauli; 1925)
No two identical fermions in a system, such as electrons in anatom, can have an identical set of quantum numbers.
Peltier effect (J.C.A. Peltier; 1834)
The change in temperature produced at a junction between twodissimilar metals or semiconductors when an electric currentpasses through the junction.
permeability of free space; magnetic constant; 0
The ratio of the magnetic flux density in a substance to theexternal field strength for vacuum. It is equal to 4 . 10-7 H/m.
permittivity of free space; electric constant; 0
The ratio of the electric displacement to the intensity of theelectric field producing it in vacuum. It is equal to 8.854.10-12 F/m.
Pfund series
The series which describes the emission spectrum of hydrogen whenthe electron is jumping to the fifth orbital. All of the linesare in the infrared portion of the spectrum.
Photoelectric effect
An effect explained by A. Einstein that demonstrate that lightseems to be made up of particles, or photons. Light can exciteelectrons (called photoelectrons) to be ejected from a metal.Light with a frequency below a certain threshold, at anyintensity, will not cause any photoelectrons to be emitted fromthe metal. Above that frequency, photoelectrons are emitted inproportion to the intensity of incident light. The reason is that a photon has energy in proportion to itswavelength, and the constant of proportionality is Planck'sconstant. Below a certain frequency -- and thus below a certainenergy -- the incident photons do not have enough energy to knockthe photoelectrons out of the metal. Above that threshold energy,called the workfunction, photons will knock the photoelectrons outof the metal, in proportion to the number of photons (theintensity of the light). At higher frequencies and energies, thephotoelectrons ejected obtain a kinetic energy corresponding tothe difference between the photon's energy and the workfunction.
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