Huygens' construction; Huygens' principle (C. Huygens)
The mechanics propagation of a wave of light is equivalent toassuming
that every point on the wavefront acts as point source ofwave emission.
Ideal gas constant; universal molar gas constant; R
The constant that appears in the ideal gas equation. It is equalto
8.314 34.
Ideal gas equation
An equation which sums up the ideal gas laws in one simpleequation. It
states that the product of the pressure and thevolume of a sample of ideal
gas is equal to the product of theamount of gas present, the temperature of
the sample, and theideal gas constant.
Ideal gas laws
Boyle's law. The pressure of an ideal gas is inversely proportional to
the volume of the gas at constant temperature.
Charles' law. The volume of an ideal gas is directly proportional to
the thermodynamic temperature at constant pressure.
The pressure law. The pressure of an ideal gas is directly
proportional to the thermodynamic temperature at constant volume.
Joule-Thomson effect; Joule-Kelvin effect (J. Joule, W. Thomson)
The change in temperature that occurs when a gas expands into aregion
of lower pressure.
Joule's laws
Joule's first law. The heat produced when an electric current flows
through a resistance for a specified time is equal to the square of the
current multiplied by the resistivity multiplied by the time.
Joule's second law. The internal energy of an ideal gas is independent
of its volume and pressure, depending only on its temperature.
Josephson effects (B.D. Josephson; 1962)
Electrical effects observed when two superconducting materials
areseparated by a thin layer of insulating material.
Kepler's laws (J. Kepler)
Kepler's first law. A planet orbits the Sun in an ellipse with the Sun
at one focus.
Kepler's second law. A ray directed from the Sun to a planet sweeps out
equal areas in equal times.
Kepler's third law. The square of the period of a planet's orbit is
proportional to the cube of that planet's semimajor axis; the constant of
proportionality is the same for all planets.
Kerr effect (J. Kerr; 1875)
The ability of certain substances to differently refract lightwaves
whose vibrations are in different directions when thesubstance is placed in
an electric field.
Kirchhoff's law of radiation (G.R. Kirchhoff)
The emissivity of a body is equal to its absorptance at the
sametemperature.
Kirchhoff's rules (G.R. Kirchhoff)
The loop rule. The sum of the potential differences encountered in a
round trip around any closed loop in a circuit is zero.
The point rule. The sum of the currents toward a branch point is equal
to the sum of the currents away from the same branch point.
Kohlrausch's law (F. Kohlrausch)
If a salt is dissolved in water, the conductivity of the solutionis the
sum of two values -- one depending on the positive ions andthe other on the
negative ions.
Lambert's laws (J.H. Lambert)
Lambert's first law. The illuminance on a surface illuminated by light
falling on it perpendicularly from a point source is proportional to the
inverse square of the distance between the surface and the source.
Lambert's second law. If the rays meet the surface at an angle, then
the illuminance is also proportional to the cosine of the angle with the
normal.
Lambert's third law. The luminous intensity of light decreases
exponentially with the distance that it travels through an absorbing
medium.
Landauer's principle
A principle which states that it doesn't explicitly take energy
tocompute data, but rather it takes energy to erase any data,since erasure
is an important step in computation.
Laplace's equation (P. Laplace)
For steady-state heat conduction in one dimension, the
temperaturedistribution is the solution to Laplace's equation, which
statesthat the second derivative of temperature with respect todisplacement
is zero.
Laue pattern (M. von Laue)
The pattern produced on a photographic film when high-
frequencyelectromagnetic waves (such as x-rays) are fired at a
crystallinesolid.
Laws of conservation
A law which states that, in a closed system, the total quantity
ofsomething will not increase or decrease, but remain exactly thesame. For
physical quantities, it states that something canneither be created nor
destroyed.
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 t 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
