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The Conservation Laws of Physics
THE CONSERVATION LAWS OF PHYSICS
(C) Copyright 1991 Max Pandaemonium
In modern physics, there are a large number of conservation laws.
Conservation laws, in their basic form, state the following:
In a closed system, a "certain quantity" will not change (increase
or decrease) with time.
Obviously this definition hinges on the certain quantity in question,
and the definition of a closed system. Physics makes a distinction
between "systems" -- that is, regions of space-time -- they can either
be open or closed. An open system exchanges energy or mass (they are
the same according to Einstein, anyway) with its surroundings; a
closed system does not. In a closed system, there is no net energy
(or mass) change in the system. In other words, there isn't a net
change (either an increase or a decrease) in the total energy of the
system.
Note that a closed system is a rather ideal situation; there will
in fact be no absolutely closed systems in the universe, save the
universe itself [footnote 1]. The Earth-Moon system is certainly not
a closed system, due to the flux of energy from the Sun. The solar
system itself is not a closed system, due to interactions from other
stars. Our galaxy itself isn't one either; there's gravitational
effects and light from other galaxies. And so on.
But physics (and mathematical models used in physics) require
simplicity. The universe itself is too difficult to understand in its
entirety -- one has to isolate certain parts of the universe and learn
how they work bit by bit. And so the idea of a closed system may be
rather construed, but it is very useful nevertheless.
Back to the conservation laws. As I said before, the general
conservation law states that in a closed system, the net change in "a
certain quantity" will be zero. The "in a closed system" clause is
important: certainly if the quantity in question is energy, and the
system in question is not closed, then energy will not be conserved!
Here are the current conservation laws that scientists hold dear:
CONSERVATION OF ANGULAR MOMENTUM. There is a quantity in physics
called angular momentum. It is analogous to linear momentum: it
measures the inability to stop something once it is rotating.
CONSERVATION OF ELECTRIC CHARGE. Charge cannot be created or
destroyed.
CONSERVATION OF ENERGY AND MASS. Energy and mass cannot be created or
destroyed. According to Einstein [footnote 2], energy and mass are
the same thing, so these two once separate conservation laws are
merely reflections of each other (and in some cases, the separate
conservation laws would be wrong, because energy can change into mass
and vice versa). The conservation of mass alone is useful in tackling
chemical problems -- particularly equations. In all of chemistry,
only electrons are effected. You can't destroy protons, neutrons, or
electrons in chemistry, and so a chemical equation must be balanced on
both sides: they must have equal numbers of protons, neutrons, and
electrons.
CONSERVATION OF LINEAR MOMENTUM. Momentum is a measure of a body's
resistance to being stopped. It is defined as mass times velocity;
the more massive something is, the harder it is to stop, and the
faster something is moving, the harder it is to stop as well. This
conservation law is particularly useful in collisions of particles;
when we know momentum is conserved, we can determine in which way the
rebounding particles will move.
There are also a number of fundamental particle conservation laws,
such as baryon number, lepton number, nucleon number, and strangeness
[footnote 3]. These are of narrower interest, and are rather arcane
to begin with.
The conservation laws have helped physics and science in general a
great deal, and has led to new discoveries. One example follows.
A radioactive phenomena called beta decay involves an electron
being emitted from the nucleus of an atom [footnote 4]. Upon
measuring the energies of the atom and emitted particles before and
after the decay, scientists realized that some energy was missing --
the emitted electron didn't have enough energy to account for the loss
in energy of the nucleus. Upon closer inspection, the physicists also
determined that both linear and angular momentum were also not being
conserved. This was a dreadful event -- could the conservation laws
that physicists hold so dear be wrong?
In 1930, a physicist named Wolfgang Pauli proposed a solution.
Maybe there was a new particle, that physics did not know about, that
was carrying off this energy, angular, and linear momentum. This
particle must be very difficult to detect (otherwise it would have
been already). Another great physicist, Enrico Fermi, eventually
named it the _neutrino_ ("little neutral one"), and wrote a paper
detailing its existence. In the 1950s, experiments gave more evidence
for the existence of the particle.
----------
Footnote 1. There is some debate as to whether the universe is closed
or not. Most physicists agree that it is -- no evidence has been
gathered to show that it is not.
Footnote 2. This idea that energy and mass are equivalent is
demonstrated in Einstein's famous equation (and probably one of the
most famous equations in all of science), E = mc^2. E represents
energy, m represents mass, and c^2 is a constant (it is the speed of
light squared).
Footnote 3. Physicists are well-known for their whimsicality when
naming particles and phenomena. A great deal of this silliness has
gone into the naming and workings of quarks, tiny subatomic
particles that make up protons, neutrons, and other particles in the
same class, called baryons. (The term _quark_ itself comes from a
phrase in novel by James Joyce, _Finnegan's Wake_: "Three quarks for
Muster Mark!") There are six "flavors" (kinds) of quark: up, down,
top, bottom, strange, and charmed. The conservation of strangeness
asserts that the "strangeness" of particles cannot be created or
destroyed. Strangeness is an integer quantity, like charge, and can
be either positive or negative one.
Footnote 4. Beta decay in fact involves more that just the emission
of an electron. In beta decay, a neutron decays into its
constituents: a proton and an electron. The electron is emitted, but
the proton stays behind in the nucleus. Thus the atomic number of the
decays element is increased by one!
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