The laws list
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Mach number to muon experiment.

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M.
Mach number (E. Mach)
The ratio of the speed of an object in a given medium to the speed of sound in that medium.
Mach's principle (E. Mach; c. 1870)
The inertia of any particular particle or particles of matter is attributable to the interaction between that piece of matter and the rest of the Universe. Thus, a body in isolation would have no inertia.
magnetic constant
See permeability of free space.
magnetic monopole
A hypothetical particle which constitutes sources and sinks of the magnetic field. Magnetic monopoles have never been found, but would only cause fairly minor modifications to Maxwell's equations. They also seem to be predicted by some grand-unified theories. If magnetic monopoles do exist, they do not seem to be very common in our Universe.
Magnus effect
A rotating cylinder in a moving fluid drags some of the fluid around with it, in its direction of rotation. This increases the speed in that region, and thus the pressure is lower. Consequently, there is a net force on the cylinder in that direction, perpendicular to the flow of the fluid. This is called the Magnus effect.
Malus' law (E.L. Malus)
The light intensity I of a ray with initial intensity I0 travelling through a polarizer at an angle theta between the polarization of the light ray and the polarization axis of the polarizer is given by
I = I0 cos2 theta.
Maxwell's demon (J.C. Maxwell)
A thought experiment illustrating the concepts of entropy. We have 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 perfect insulators.

Now imagine there is a very small demon who is waiting at the partition next to a small trap door. He can open and close the door with negligible work. Let's say he opens the door to allow a fast-moving molecule to travel from the left side to the right, or for a slow-moving molecule to travel from the right side to the left, and keeps it closed for all other molecules. The net effect would be a flow of heat -- from the left side to the right -- even though the container was in thermal equilibrium. This is clearly a violation of the second law of thermodynamics.

So where did we go wrong? It turns out that information has to do with entropy as well. In order to sort out the molecules according to speeds, the demon would be having to keep a memory of them -- and it turns out that increase in entropy of the maintenance of this simple memory would more than make up for the decrease in entropy due to the heat flow.

Maxwell's equations (J.C. Maxwell; 1864)
Four elegant equations which describe classical electromagnetism in 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; in differential form,
div E = rho,
where rho is the charge density.
Gauss' law for magnetic fields
The magnetic flux through a closed surface is zero; no magnetic charges exist. In differential form,
div B = 0.
Faraday's law
The line integral of the electric field 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; in differential form,
curl E = -dB/dt,
where d/dt here represents partial differentation.
Ampere's law, modified form
The line integral of the magnetic field 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 differential form,
curl H = J + dD/dt,
where d/dt here represents partial differentiation.

In addition to describing electromagnetism, his equations also predict that waves can propagate through the electromagnetic field, and would always propagate at the same speed -- these are electromagnetic waves; the speed can be found by computing (epsilon0 mu0)-1/2, which is c, the speed of light in vacuum.

mediocrity principle
The principle that there is nothing particularly interesting about our place in space or time, or about ourselves. This principle probably first made its real appearance in the scientific community when Shapley discovered that the globular clusters center around the center of the Galaxy, not around the solar system. The principle can be considered a stronger form of the uniformity principle; instead of no place being significantly different than any other, the mediocrity principle indicates that, indeed, where you are is not any more special than any other.
Meissner effect (W. Meissner; 1933)
The decrease of the magnetic flux within a superconducting metal when it is cooled below the transition temperature. That is, superconducting materials reflect magnetic fields.
metre; meter; m
The fundamental SI unit of length, defined as the length of the path traveled by light in vacuum during a period of 1/299 792 458 s.
Michelson-Morley experiment (A.A. Michelson, E.W. Morley; 1887)
Possibly the most famous null-experiment of all time, designed to verify the existence of the proposed "lumeniferous aether" through which light waves were thought to propagate. Since the Earth moves through this aether, a lightbeam fired in the Earth's direction of motion would lag behind one fired sideways, where no aether effect would be present. This difference could be detected with the use of an interferometer.

The experiment showed absolutely no aether shift whatsoever, where one should have been quite detectable. Thus the aether concept was discredited as was the idea that one measures the velocity of light as being added vectorially to the velocity of the emitter.

See constancy principle.

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 between charged plates. After charging the drop with x-rays, he adjusted the electric field between the plates so that the oil drop was exactly balanced against the force of gravity. Then the charge on the drop would be known. Millikan did this repeatedly and found that all the charges he measured came in integer multiples only of a certain smallest value, which is the charge on the electron.
mole; mol
The fundamental SI unit of substance, defined as the amount of substance that contains as many elementary units (atoms, molecules, ions, etc.) as there are atoms in 0.012 kg of carbon-12.
mu_0
See permeability of free space.
muon experiment
An experiment which demonstrates verifies the prediction of time dilation by special relativity. Muons, which are short-lived subatomic particles, are created with enormous energy in the upper atmosphere by the interaction of energetic cosmic rays. Muons have a very short halflife in their own reference frame, about 2.2 us. Since they are travelling very close to c, however, time dilation effects should become important. A naive calculation would indicate that, without special relativistic effects, the muons would travel on the average only about 700 m before decaying, never reaching the surface of the Earth. Observations reveal, however, that significant numbers of muons do reach the Earth. The explanation is that muon is in a moving frame of reference, and thus time is slowed down for the muons relative to the Earth, effectively extending the halflife of the muons relative to the Earth, allowing some of them to reach the surface.
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