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Compound (Chemical Compound)
Element (Chemical Element)
Acceleration is any deviation from
uniform motion, in other words a deviation from constant velocity. This can happen in several ways. For example the speed
can be changing, but the direction might not change. So if a car changes its speed from 25 miles per hour to 45 miles
per hour while moving north, it is accelerating. It is also accelerating if it slows from 45 miles per hour to 25
miles per hour. In each case you could
find a number that describes the acceleration by dividing the change in speed
by the time it takes to make that change.
Another form of acceleration is a change in direction at constant speed
such as a car going around a curve in a highway while maintaining a speed of 50
miles per hour. In each of these cases,
you could feel the acceleration. It
would feel like an extra weight pointed opposite the direction in which the
motion is changing. Of course, both the
speed and the direction could change at the same time, and this would also be
acceleration.
An Acid is a compound
that can lose hydrogen ions (H+) when the
compound is put into a water solution. Therefore the hydrogen ions are free in the
solution where they can react with many other substances or objects placed in
the solution. This accounts for the
corrosive effects of acids and the fact that they are dangerous and can produce
burns. This is not the most general
definition of an acid even though it applies to many common acids. For a more general discussion see the web
links under the topic Acids and Bases.
The Amplitude of a wave
is the maximum height of the wave as measured from the middle. For example, if a spring is stretched out on
a table, then it would make a straight line if there were no wave in it. When a wave comes down the spring, the
amplitude is the distance from where the straight line should be to the largest
distance the spring is displaced as the wave comes through.
An Asteroid is often called a minor planet. It is a
small, cold object made of rock and/or metal that revolves around the sun. Most asteroids
revolve between the orbits of Mars and Jupiter. The main distinction between an asteroid and a comet
is the composition: comets are made
mostly of ice with some dust, rock, or metal mixed in with the ice. There are a number of objects of a size and
mass intermediate between asteroids and comets on one hand and planets on the
other. Therefore the exact boundary
between asteroid and planet is subject to some dispute. Although our solar system has nine planets,
the definitions could be revised at some point to take Pluto out of the planet
category. On the other hand, some other
proposed revisions might reclassify some of the larger asteroids or comets as
planets.
An Atom is the smallest particle
of a chemical element, and it consists of a nucleus (consisting of protons and neutrons) and surrounding electrons. Although the original concept of an atom was
that it was the smallest possible particle of matter that could no longer be
subdivided, this turns out to be not true.
For example, electrons can be lost or added to an atom. If the number of electrons equals the number
of protons, it is electrically neutral, but if there are more electrons, it has
a net negative charge. If one or more
electrons are lost, it carries a positive charge. If it is charged either way, it is an example of an ion. It is also possible to divide a
nucleus. A typical atom is about 10-10
meters across; in other words it is about one ten billionth of a meter
across. At that size, about 250 million
of them placed end-to-end would cover one inch.
A Base is a compound
that can sweep up hydrogen ions (H+), react with
them, and take them out of a solution. Since the hydroxide ion (OH-) is
very good at this, then compounds that lose hydroxide ions in a water solution
make up a common class of bases. An
example would be potassium hydroxide (KOH).
Since the hydroxide ion can react easily with many other substances and
objects placed in the solution, bases can be dangerous and can burn or dissolve
organic things like hands and fingers.
For a more general discussion of bases, see the web links under the
topic Acids and Bases.
The Big Bang Theory is a theory about the
evolution of the universe in which all galaxies
are moving away from one another, and the speed with which they move becomes
larger for galaxies that are farther away from one another. When this evolution is traced back in time, it
appears that the entire universe came from a very hot, very dense state in
which even neutrons, protons, and electrons could not exist independently of one another. Several natural consequences of the
existence of this original state of the universe have been calculated and
observed as predicted giving cosmologists a good deal of confidence in the
theory. These consequences include
microwave radiation that should have been left over from the initial state and
which has been observed as predicted. They also include the relative abundance
in the universe of hydrogen and helium (which would have been made from the
hydrogen by nuclear fusion). The current motion of the galaxies away from one another has also
been observed.
A Catalyst is a substance that can make a chemical reaction take place at a faster rate. Frequently chemical reactions are so slow as
to be impractical for industrial use unless a catalyst is used to speed them
up. In general, a catalyst is not used
up itself, so that the same quantity of it can keep on making the reaction take
place at a fast rate for a very long time.
For example, chlorine is a catalyst for destroying ozone (O3)
in the stratosphere thus contributing to the ozone problem.
A Chain Reaction is a nuclear fission process in which one fission reaction makes several
more fissions take place. Each of
these, in turn, induces several more fissions, and the process continues until
there is a large release of energy sufficient to produce
a nuclear explosion. The reason this
can work is that neutrons released by one fission are
absorbed by other nuclei, and these absorbed neutrons
induce the new fissions. Only a few
heavy nuclei will support such a chain reaction. Two of them are Uranium-235 and Plutonium-239, which are used in
nuclear weapons and nuclear reactors.
In the reactors, the chain reaction is not allowed to run away, as
described above. Instead the neutrons
are controlled so that each fission induces only one other fission at a slow
enough rate so that heat can be produced without a nuclear explosion.
A Chemical Bond is a name for the forces that
hold atoms together in molecules
and some crystals.
There are some much weaker forces that hold other crystals
together. The term “Chemical bond” is
reserved for stronger forces that are either covalent bonds,
ionic bonds, or some combination of these two.
A Chemical Equation is a symbolic way to
represent a chemical reaction. For example, to represent the process of
hydrogen and oxygen combining to make water, it is possible to write 2 H2 + O2 à 2 H2O. This
says that two diatomic molecules of hydrogen and one
diatomic molecule of oxygen combine to make 2 molecules of water. This has been written as a balanced chemical
equation. It is called “balanced” because it shows the same number of atoms of each type on each side of the arrow. There are four hydrogen atoms and two oxygen
atoms on each side. Balanced chemical
equations represent the fact that atoms are not created and are not destroyed
during chemical reactions.
A Chemical Reaction is a process in which the
atoms in certain molecules, crystals, or solutions are
rearranged into different molecules, crystals or solutions. Some substances are used and others are
made. Sometimes a chemical reaction (an
exothermic reaction) releases energy, and sometimes
energy must be put into the system to make the reaction work (an endothermic
reaction). If a reaction is to be
considered “chemical” in nature, then atoms cannot be created or
destroyed. Electrons
can be removed from atoms and picked up by other atoms, but the nucleus remains intact.
This means that the number of protons and neutrons remain the same, and chemical elements
are not transformed into other chemical elements. If the nucleus does change, then the reaction is a nuclear reaction.
A Comet is an object made mostly of ice but
with dust, rock, and/or metal mixed with the ice. When they come near enough to the sun, some of
the ice evaporates and takes some dust along thus forming the familiar head and
tail of a comet as seen from the earth.
Comets revolve around the sun, mostly outside the orbit of Neptune. Many are in a region known as the Kuiper
Belt. There is thought to be another
region of orbiting comets very far from the planets at a
significant fraction of the distance to the nearest star. This region is called the Oort cloud. Some Kuiper Belt objects are known to be
large enough to border on the size of small planets although none of them (so
far) is as large as Pluto, the smallest planet. Therefore the boundary between comet and planet is under some
dispute, and Pluto might at some point be reclassified out of the planet
category. On the other hand, some other Kuiper Belt objects and maybe some asteroids might be reclassified into the planet category.
A Compound is a substance that is composed of
more than one element provided that the different
elements are always present in definite proportions. For example, water is a compound that always has two hydrogen atoms for each oxygen atom.
The atoms of a compound are joined together into molecules
or crystals and sometimes both. They can be separated into their constituent
elements or transformed into other compounds by means of chemical reactions.
The concept of Conservation of Energy
tells us that energy is neither created nor
destroyed. Instead, if something gains
a certain amount of energy, then something else must have lost the same amount of
energy. The energy had to have come
from somewhere such as fuel or falling water, or something. This is of great value in figuring out how
things work and in understanding what is possible and what is not
possible. For example, the sun could not possibly shine by burning coal or oil because the
energy released is not nearly enough to produce the amount of energy that the
sun does, in fact, produce. Nuclear fusion reactions, on the other hand, do produce enough
energy. In using conservation of energy
to help understand natural phenomena, we have to keep in mind that there are
many forms of energy which can contribute to whatever process we are studying. Energy can change from one form to another such
as from the potential energy of a raindrop high in the
air to its kinetic energy as it falls. It is even possible to get energy by making mass disappear, as Einstein’s relativity theory predicts, and
as observed in nuclear reactions.
A Covalent Bond is one type of chemical
bond between atoms.
It comes about when two atoms share a pair of electrons,
usually one electron from each atom.
The shared electrons spend most of their time in the space between the
two atoms. Since the electrons have a
negative charge, then the rest of each atom, having partially lost an electron,
has a positive charge. Thus the shared
electrons form sort of a negative “glue” that can hold the two positive atoms
together. In some cases, there are double or triple bonds between
two atoms. Some crystals,
such as diamond, are held together by covalent bonds. It is often written as a dash (-) as in the diatomic hydrogen
molecule H-H.
A Covalent Compound is a chemical compound that is held together by covalent
bonds.
The Crust of the earth is the thin layer of
rock at the surface of the earth. The
densities of individual crustal rocks usually range from about 2.5 to about 3
times the density of water, although some are outside this range. One type of rock from volcanoes, called
pumice, is less dense than water and will therefore float on water. The crust, being less dense that the mantle below it, is floating on the mantle. Its temperature ranges from surface
temperature (on top) to about 1150 oF (at the bottom). There are two general types of crust called
continental crust and oceanic crust.
Under the oceans, the crust is from 5 to 10 km (3 – 6 miles) thick in
addition to the depth of the ocean.
Under the continents, it ranges from about 25 to 90 km (16 to 56 miles)
thick with the thickest part under mountains.
A Crystal is a large group of atoms
(or molecules) held together by chemical bonds in a
regular structure. Usually this means a
very large group of atoms indeed – in fact often large enough to be seen with
the eye and once in a while several meters from one side to another. To qualify as a crystal, the structure must
consist of some regularly repeating pattern.
For example, in some crystals, the atoms all sit on the corners of a
cube, and this pattern repeats throughout the crystal. Ordinary ice is crystalline as is salt and
diamond.
A Deferent is the circle whose center is on
or near to the earth in Ptolemy’s geocentric model of
the solar system. The center of an epicycle moves around the deferent, and either a planet or, in some cases, another epicycle moves around the
first epicycle. For an example of
motion involving a deferent and an epicycle look at the following slide show.
(This link will take you out of the “terms” file. To return, finish the slide show and use
your browser’s “back” button.)
A Double Bond is a chemical bond
caused by the action of two pairs of electrons instead
of just one pair. It is often written
as two lines (=) as in O=O for the diatomic oxygen molecule.
An Electron is a very small, light particle that carries a negative charge that is exactly
equal and opposite to the charge of the proton. It would take about 1800 electrons to equal
the mass of a proton or a neutron, and this is why the
protons and neutrons make up most of the mass of an atom. Its size is unknown, but it is somewhere
between a thousandth of the proton diameter and a geometrical point. Electrons can move around easily, and atoms
frequently gain or lose electrons. The
electrons do not really move in planet-like orbits around the nucleus,
as is often depicted. They do exist in
the space surrounding the nucleus, but their motion is governed by the laws of
quantum mechanics and is very complicated.
An Element (sometimes called a chemical
element) is a substance whose atoms all have the same
number of protons.
For example, hydrogen has one proton, helium has two protons, and
uranium has 92 protons in each nucleus. The only way to decompose an element into
other substances is to change the nucleus.
Such changes are nuclear reactions, not chemical reactions. So elements cannot be decomposed by chemical
reactions.
An Ellipse is a curve that looks something
like a flattened circle. You can draw
one by sticking two pins in a piece of cardboard and placing a closed loop of
string around both pins. Take a pencil,
put it inside the loop of string, and pull the string tight. Then, while keeping the string tight, draw a
curve all the way around the pins. The
curve you produce by this method is an ellipse. If the pins are relatively far apart, the ellipse will be long
and thin. If the pins are close
together, the ellipse will be nearly, but not quite, a circle. If you could put both pins at the same
point, you would get a circle. Each of
the points where the pins were is called a “focus”. The planets move around the sun
in ellipses that are nearly circles with the sun at one focus.
The concept of Energy is one of the most
fundamental in science because of the fact that energy
is conserved. Roughly speaking, energy is the ability to move something such as
an atom, a train, or the rock and dirt blasted out of a
crater by the energy of a meteor that strikes it. A somewhat more precise way to describe energy is as the ability
to do work, where work is a precisely defined scientific
concept that means, roughly speaking, “to move something”. You have energy if you are a moving particle that is about to hit the atom and move it, or if
you are the fuel that is going to move the train, or if you are a meteor
hovering over the ground and also moving toward the ground. This implies that there are many forms of
energy. They include energy that you
have because you are moving (kinetic energy) like the particle and
meteor. They also include energy that
can be released in chemical reactions from the fuel, and the so-called potential
energy of the meteor that is has because of its height above the
ground. There are ways to measure such
energy so that a precise amount that is available can be known. Particles and waves are two ways to move energy from one place to another.
An Epicycle is a circle upon which a planet moves and whose center is on another circle. The center of the epicycle moves around the
other circle, whose center is on (actually, near to) the earth in Ptolemy’s geocentric model of the solar
system. The center of the other
circle is near the sun in the Copernicus heliocentric model. The other circle is called the deferent in these cases.
In some versions of these models, however, there were several epicycles
piled on the deferent. Of course, the
models of the solar system involving epicycles are no longer taken seriously
having been replaced by Kepler’s three laws of planetary motion. For an example of motion on an epicycle look
at the following slide show. (This link will take you out of the “terms”
file. To return, finish the slide show
and use your browser’s “back” button.)
An Experiment is a procedure in which a
scientist carefully arranges to observe some kind of
behavior of nature. Usually it is
testing some hypothesis, but an experiment can
sometimes be designed to “go fishing” for data in an effort to form a
hypothesis or a model.
In a good experiment, the scientist arranges things to observe how the
behavior in question depends on other circumstances and conditions. For example, if you were observing the
period of a pendulum (the time for one complete swing), you might keep the mass
of the pendulum the same while you keep changing the length to see how the
period depends on the length. Then you might keep the length the same while
you keep changing the mass. If you
could you might keep the length and mass the same and try the pendulum on
different planets, but some experiments are pretty difficult to do.
A Fact ( in the scientific sense) is a
circumstance that can be reliably observed on a
repeated basis. Every time I release an
object in the earth’s gravitational field, it falls toward the center of the
earth. So the statement that objects
fall is a scientific fact.
Nuclear Fission is a nuclear
reaction in which a heavy nucleus splits into two
roughly equal pieces along with a relatively large release of energy. Some neutrons are
also released. The two main products of
a fission reaction are chemical elements of roughly half
the weight of the original nucleus.
Most heavy nuclei either undergo occasional spontaneous fission or can
be made to fission by hitting them with a neutron. However, only a few will support a fission chain reaction, which
is the basis for nuclear weapons and nuclear reactors.
Force is a push or a pull, and it causes acceleration.
If there is no net force on some object, which means that all forces
balance, then it either remains at rest or remains in uniform
motion, according to Newton’s First Law of Motion. If the forces do not balance, then there is
a net force that causes acceleration, according to Newton’s Second Law of
Motion. In the case where there is a
net force, the object upon which the force acts will have a greater
acceleration if it has a smaller mass.
The Frequency of some repetitive motion is
the number of times that a complete cycle of this motion is completed in one
second. In the case of a wave,
it is the number of wavelengths by which the wave
moves forward in one second. In other
words, imagine any point along side of the path of the wave. If 60 wavelengths pass this point in a
second, the frequency is 60 cycles per second.
If 1000 wavelengths pass this point in a second, the frequency is 1000
cycles per second. A cycle per second
is usually called a hertz, so the frequencies in these two examples are 60
hertz and 1000 hertz. A larger
frequency is usually called a “higher” frequency, and a smaller frequency is
called a “lower” frequency. A higher
frequency corresponds to a shorter period.
Nuclear Fusion is a nuclear
reaction in which two light nuclei join together to
form a heavier nucleus. For light
nuclei, this reaction releases a relatively large amount of energy. If enough fusion reactions can be made to
take place in a very short time, then this process can power a nuclear
explosion, and some nuclear weapons are partially based on fusion (although fission is also involved in these weapons). Nuclear fusion also powers stars
such as the sun, but so far efforts to control these
reactions well enough to make fusion reactors on earth have not been
successful.
Our Galaxy, which is called the Milky Way
Galaxy, is a group of several hundred billion stars arranged
in a disk shape with a central bulge and a structure including several bright,
curved arms of stars outside the central bulge. These arms are called spiral
arms, and this is an example of a spiral galaxy. There are other types of galaxy in the universe
with different shapes including elliptical galaxies, irregular galaxies, and
others. The disk is at least 80
thousand light years across, and the central bulge is about 10,000 light years
across. Space between galaxies is
relatively empty of matter, or at least of matter that shines like stars. There might be quite a bit of dark matter,
though.
A Geocentric Theory is a theory
or model of the solar system that places the earth in the
center and assumes that the sun, moon, and the other planets revolve around the
earth in some manner. This motion
around the earth usually involved one or more epicycles
revolving around the earth on a deferent. The most famous and long lasting of the
geocentric theories is the one published by Ptolemy.
A Hypothesis is a specific result that a
scientist expects from an experiment or observation. In
such a procedure, the scientist is usually trying to test a broader theory or a model. The hypothesis a more specific result derived
from the theory or model that can somehow be observed. If the expected result is not observed, then
the theory or model needs revision. If
the expected result is obtained, the general theory or model is not proven
because any real experiment would only test a small part of it. Besides another theory or model might also
explain the same result. Thus testing a
specific hypothesis only supports a theory but never proves it. In other cases,
the hypothesis is much more speculative than described above; it might just be
an educated guess made by someone who is in the initial stages of building a
model.
A Heliocentric Theory is a theory
or model of the solar system that places the sun in the
center and assumes that the planets revolve around the sun. Although Aristarchus proposed a heliocentric
system during the time of the ancient Greek astronomers, the idea did not
become accepted until after the time of Copernicus, Kepler, and Galileo.
Inertia is a resistance to any change in motion. In
other words it is a resistance to acceleration. The more inertia an object has, then the
larger the force necessary to produce a specific
acceleration. Mass
is associated with inertia. Larger
masses have more inertia, and smaller masses have less inertia.
The inner core of the earth is a sphere of solid
iron at the center of the earth of radius about 1220 km, or about 760
miles. It is one of the major
subdivisions of the interior of the earth, the others being the outer core, the mantle, and the crust. The density is very high, about 13 times the
density of water, partly because iron is very dense to begin with and partly
because the material at the core is highly compressed by the high
pressure. Although the temperature of the
inner core is around 8100 oF, the iron remains solid because of the
high pressure near the center of the earth.
Besides iron, it contains up to 10% nickel.
An Ion is a particle with an electric charge, either positive or
negative. This normally means a
particle of atomic or molecular size with a
charge. Since electrons are very light
and highly mobile, an ion is often created by taking one or more electrons away
from an atom or by adding one or more electrons to an
atom. Other ions consist of groups of
atoms similar to a molecule except with more or fewer electrons than protons.
An Ionic Bond is one type of chemical
bond between atoms.
It comes about when an atom completely loses an electron,
and another atom gains the electron.
The atom that lost an electron becomes positively charged, and the atom
that gained an electron also gains a negative charge. The two oppositely charged ions then can
attract one another thus forming the ionic bond. There are many crystals, such as Sodium
Chloride (table salt), that are held together by ionic bonds.
An Ionic Compound is a chemical compound that is held together by ionic
bonds.
A Scientific Law is a short summary of a large body of
experience that has been well verified by experiment and observation. All known scientific laws are incomplete
because they work only under certain conditions. Kepler’s First Law of Planetary Motion states that planets move
around the sun in ellipses with the sun at one focus. This statement is exact only if the gravity of the sun is the
only thing that influences the planet.
The gravity of one planet pulling on another would make the orbit differ
from an ellipse. However, the
difference is extremely small because the gravity of planets is so much smaller
than the gravity of the sun.
A Light Year is the distance that light
travels in a year at the speed of 300,000,000 meters per second (186,000 miles
per second). That makes a light year
equal to 5.9 x 1012 miles (5.9 trillion miles) or 9.5 x 1015
meters (9.5 thousand trillion meters). By this way of figuring it, the sun is about eight light minutes away, the planet
Pluto is about 5 light hours away, and the nearest star (other
than the sun) is about four light years away.
The Magnitude of a star
is a number that tells how bright it seems to us on the earth (this is actually
“apparent visual magnitude” – astronomers have several kinds). A magnitude 2 star seems dimmer than a
magnitude 1 star. A magnitude 3 star
seems dimmer than a magnitude 2. Each
time the magnitude increases by 1, the star seems about 2.5 times dimmer. The magnitude scale also extends to
magnitude 0, which is about 2.5 times brighter than a magnitude 1. A magnitude –1 is about 2.5 times brighter
than a magnitude 0.
The Mantle of the earth is the thick layer of
dense rock that fills the region between the outer core
and the crust. Its
density is somewhat higher than the density of the crust but lower than the
density of the outer core. Therefore
the mantle floats above the outer core, and the crust floats above the
mantle. It is about 2850 km (1770
miles) thick, and it can be further subdivided into smaller layers, especially
near the top. The densities of the
upper layers range from just over 3 to about 4 times the density of water, and
the lower mantle has a density of around 5.5 times the density of water. This figure of 5.5 is also the average
density of the entire earth. The
temperature ranges from about 1150 oF near the top to about 5000 oF
at the bottom. It is mostly solid, but
there is a layer at the top that is at least nearly molten. Whether something is solid or molten depends
on both the temperature and pressure, and the pressure is low enough at the top
to allow at least partial melting of typical upper mantle rock. It is this layer that allows the continents
to drift around the earth.
Mass can be described roughly as “how much stuff there
is in something”. However, it also
means much more than that. Mass
produces inertia, where inertia means resistance to any
change in motion. So it is much harder to accelerate a
large mass than a small mass. The large
mass would require a greater force to produce the same
acceleration. Mass also is necessary to
produce a gravitational force, or a weight. Any mass is capable of attracting another
mass with a gravitational force.
According to Einstein’s Theory of Relativity, the mass of an object is a
measure of its energy.
This is expressed in his equation E = mc2, which expresses
the fact that giving an object more energy also gives it more mass.
A Mixture is more than one element
or compound mingled together. The elements and
compounds making a mixture are not required to be in definite proportions to
one another as they are in a compound.
Sand and sugar stirred together make up a mixture, as do the gasses in
the earth’s atmosphere.
A Model is a picture, a set of ideas, a set
of equations, or some other kind of description of whatever a scientist is
trying to study. This makes a model
similar to a theory, but the word “model” is generally
used when knowledge about the system being studied is rather crude, and it is
suspected that the description of it is incomplete. Ptolemy’s earth-centered
description of the solar system was a model as is the idea that an atom has electrons circling around it
in smooth orbits. There are complex
systems of equations stored in large computers that model the climate.
A Molecule is a group of atoms
held together by chemical bonds. The atoms can be of different types such as in the water (H2O)
molecule, or they can be of the same type such as in a diatomic molecule, for
example oxygen (O2).
A Neutron is a very small, electrically
neutral particle usually found in the nucleus. Its size and mass are about the same as the proton, and together the protons and neutrons make up just
about all of the mass of the atom. They are so small, however, that they take
up practically none of the volume of an atom.
For light, stable nuclei the number of neutrons tends to be about the
same as the number of protons, but for heavier stable nuclei there are more
neutrons. Free neutrons (that is, neutrons
not in a nucleus) are unstable, and they decay by beta decay with a half-life
of about 1000 seconds (about 17 minutes).
In some unstable nuclei, a neutron might also decay by beta decay.
The Nucleus is located in the center of the atom, and it carries almost all of the atom’s mass. However, it is very small being between a
ten thousandth and a hundred thousandth of the diameter of the atom (depending
on the type of atom). The nucleus
consists of protons and neutrons,
and the number of protons determines which chemical element
it is. It is held together by the
strong nuclear force, which overcomes the tendency of the positively charged
protons to tear the nucleus apart due to the repulsion of one positive charge
from another. Depending on the number
of protons and the number of neutrons, the nucleus may be stable or unstable. An unstable one will decay at some random
time by means of one of several known types of radioactive decay.
A Nuclear Reaction is a process in which one
or more nuclei can change into other nuclei. This
involves a gain or loss of protons and/or neutrons. Therefore
chemical elements can change into other chemical
elements, something that never happens in chemical
reactions. It is also possible, in
a nuclear reaction, for the type of nucleus to stay the same, but for the energy contained within the nucleus to increase or
decrease. There are many specific types
of nuclear reaction. A few examples are
fission, fusion, alpha decay, beta
decay, gamma decay, and the capture of a neutron.
An Observation is a procedure in which a
scientist watches passively (but carefully) while nature performs in whatever
way it wants. It is often an attempt to
verify some hypothesis, but it might be just general
data gathering in an attempt to build a hypothesis or model. An observation could be a part of an experiment, or it could be just a passive look at a part
of nature that cannot be controlled.
Astronomers are often reduced to passive observations of objects that
are many light years away, or even billions of light years away. Nevertheless scientific observations can be
planned carefully to display the natural effects of interest.
Opposition of a planet occurs
when the planet is on the opposite side of the earth from the sun. In other words, they line up in the
following order: Sun, Earth,
Planet. At opposition, the planet
appears high in the sky in the middle of the night. Only the planets farther from the sun than the earth can have an
opposition; Mercury and Venus cannot.
An Organic Molecule is a molecule
that contains one or more carbon atoms. Most organic molecules contain a large
number of carbon atoms, and such molecules are the building blocks of living
things. It is, however, possible to
have organic molecules that were made by chemical reactions
not involving life.
The Outer Core of the earth is a thick layer
of molten iron and up to 10% molten nickel that starts just outside the inner core. It
extends further outward for a distance of about 2260 km (about 1400 miles)
until it reaches the lower boundary of the mantle. Its temperature is about 5760 oF,
which is lower than the solid inner core.
The outer core remains molten because it is under less pressure than the
inner core. Following a pattern that
the subdivisions of the earth become less dense the farther they are from the
center, the outer core has a density ranging from about 10 times the density of
water on top to about 12 times the density of water on the bottom. Since it is a liquid, it can flow in complex
currents, and these currents are responsible for the earth’s magnetic field.
Parallax refers to the apparent motion of some object
caused by a real motion of the observer. Suppose you stand to one side of a road and
look at a tree on the other side as well as some woods in the distance. If you walk parallel to the road (but do not
enter the road), the tree will seem to move in the other direction. This apparent motion is parallax. However, the distant woods will not seem to
move much. Another tree half way to the
distant woods will seem to move less than the nearby tree but more than the
woods. If you use some geometry, the
amount of parallax can be used to measure the distance to a tree or to another
object. Hipparchus measured the
distance to the moon using parallax. Stars that are relatively nearby show enough parallax due to
the earth’s motion around the sun to allow measurement of
the distance to these stars. This is
however very difficult to measure, and it was impossible to observe with the
technology available before the early 1800’s.
Therefore the apparent lack of parallax of stars was used as an argument
against people like Copernicus and Galileo when they first proposed a heliocentric model in which the earth moves.
A Particle is a small bit of matter such as
an atom, a molecule, or a proton. Whether you
call something a particle depends partly on how big a “box” it is in. For example, from the point of view of the
whole solar system, the earth might be thought of as
a particle. Only a few particles are Elementary
Particles, which means a particle that cannot, so far as we know, be
subdivided into other particles. A
proton is not elementary because it can be subdivided into quarks. An electron is,
however, elementary. A moving particle
carries kinetic energy from one place to another.
The Period is the time it takes to complete
one cycle of a repetitive motion of some kind.
For a planet it is the time it takes for the
planet to complete one trip around the sun (one year for the earth). For a wave, it is the
time it takes for the wave to move a distance equal to one wavelength. In other words, if you pick any point along
side of the wave, one wavelength will move past the point in a time equal to
one period. A longer period corresponds
to a lower frequency.
The Phases of the moon include new moon,
first quarter (when half of it is lit as seen from the earth), full moon, and
last quarter (when the other half is lit as seen from the earth). Other phases include other terms that
indicate how much of the moon seems to be lit as seen from the earth. The planets Mercury,
Venus, and Mars can also show different phases as seen from the earth, but a
telescope is required to see them.
Galileo’s analysis of the phases of Venus produced a strong argument
that Venus revolves around the sun and not the earth.
A Planet is a relatively small, relatively
cold object that revolves around a star. The earth is a planet. The distinction between a planet, a star, an
asteroid, and a comet has become somewhat confused in
recent years because of the discovery of intermediate objects. However, a planet is intermediate in mass
between a star and an asteroid.
A Precipitate is a solid substance that forms
within a liquid. Frequently chemical reactions that take place in a water solution produce a precipitate because at least one of the
products of the reaction cannot dissolve in water. A precipitate can also be a liquid that forms within a gas, such
as rain.
A Proton is a very small particle
with a positive charge usually found in the nucleus. The number of protons determines which
chemical element the atom is. A hydrogen nucleus contains just one
proton. So a free proton is a hydrogen ion, or a hydrogen atom without its electron. The proton is about 10-15 meters
in diameter, so a hundred thousand of them placed side-by-side would
approximately cover the diameter of an atom.
It would take about 6X1025 of them to make a mass of a
kilogram and is similar to the neutron in size and
mass. Free protons are stable, or at
least if they decay it is with a lifetime much longer than the lifetime of the
universe. Within some nuclei, however,
a proton can decay by positron decay.
A Radical is a group of atoms
that resemble a molecule except that there is a
unfinished bond that can easily attach to another radical
or to an atom. In other words it is
sort of an “unfinished” molecule. An
example is the hydroxide radical (OH-), which can attach to a
Hydrogen ion (H+) to form water (H2O).
Retrograde Motion of a planet
is apparent motion as seen from the earth in which the planet seems to move
opposite its usual direction. For Mars,
for example, the usual apparent motion as seen from the earth is to the east
relative to the stars.
Once approximately every two years, it moves west relative to the stars
for a few weeks. This westward motion
is retrograde motion. For an example of
retrograde motion and an explanation, go to the following slide show. (This will take you out of the “terms” file. To return, finish the slide show and use
your browsers “back” button.)
A Salt is an ionic
compound made of a positive radical or other ion and a negative radical or other ion. However, to qualify as a salt, the ions must
be something other than the Hydrogen ion (H+) and the hydroxide ion
(OH-). With just one these
ions, the compound would be an acid or a base. If the compounds contain both hydrogen and
hydroxide and nothing else, then it is a common substance that you can probably
identify, but not a salt. A salt is
formed by a neutralization reaction when an acid and a
base are combined. Common table salt,
sodium chloride, is an example of a salt, but far from the only example.
The word Science is the process of learning
about nature by scientific means: in other words by formulating models and theories and then carefully
testing them by experiment and observation. If the experiment or observation does not
support the theory or model, then the theory or model must be adjusted or maybe
even discarded entirely. This assumes
that the work is all carefully checked, analyzed, and exposed to criticism and
is able to pass all of this testing.
The word “science” also means the set of scientific facts,
laws, theories, and models that has been slowly built up
over the centuries by this procedure.
A Scientific hypothesis,
theory, or model is one that is subject
to being contradicted by means of an experiment or observation. If
a scientist with a new theory is doing the work
correctly, then he/she will work as hard as possible to prove the theory
wrong. This means deriving a hypothesis
from the theory that could be observed and which, if not found, would discredit
the theory. An example of a hypothesis
that is not scientific would be that the ghost of Albert Einstein is causing a
banging sound in my house. If I look
and do not see the ghost nothing is proven because ghosts are invisible and
cannot be seen or are so good at hiding that I could never find it. Makers of unscientific hypotheses have all
bases covered so that nothing can discredit them. A scientific hypothesis about the same phenomenon might be that
the banging is caused by temperature changes in the radiator. I could design an experiment in which I
record the temperature over a long period of time and correlate the changes
with the banging. If there is no
correlation, then the hypothesis is discredited. You could make the ghost hypothesis scientific by inventing a
reliable method of detecting ghosts.
The Solar System consists of the sun, planets, asteroids,
and comets with all of the other objects revolving around
the sun. Most of the planets and at
least some asteroids have satellites (or, moons) of their own that revolve
around the planet or asteroid. It is
now known to be heliocentric although before the
1500’s it was thought to be a geocentric system.
A Solution is formed when something is
dissolved in water making a homogeneous mixture.
“Homogeneous” means that things are so thoroughly mixed that the properties of
the mixture are the same in every part of the mixture. Some dissolved substances will break apart
into positive and negative ions, but this does not happen in
all solutions. This is actually just
one class of solution. Substances can
be dissolved in other liquids besides water.
It is possible to have a gaseous solution (air is an example). It is also possible to have a solid solution
such as an alloy (two or more metals mixed together).
Speed is the rate at which the position of an object is
changing without regard for the direction it is moving. It tells how many feet you are moving each
second. This could be the number of
miles you are moving each hour, or it could be described in terms of any other
units of distance and time. To describe
velocity, you need to give the speed and the direction
the object is moving.
A Star is a ball of gas, mostly hydrogen, in
outer space that is massive enough to heat and compress the hydrogen in its
core enough to ignite nuclear fusion reactions. Thus a star shines with light and heat
generated by these reactions in the core.
It is more massive than a planet, but the
difference has become somewhat confused in recent years because of the
discovery of objects of intermediate mass.
Therefore the exact boundary between star and planet is subject to some
dispute. There is no doubt, however,
that the sun is the only star in our solar
system. Most of the points of light
seen in our night sky are, of course, stars except that five of the ones seen
with the unaided eye are planets.
The Sun is the star at the center of our solar system.
A Theory is an organized set of ideas, often
described by means of mathematical equations, which describes some part of
nature. Normally different scientific laws will be connected logically to one another by a
theory. For example, Newton’s Theory of
Motion can be used to derive Kepler’s Laws of Planetary Motion from Newton’s
three Laws of Motion. It is something
like a model, but a theory generally covers more of the
natural world and is generally better organized in a logical sense. Good theories are well verified by experiment or observation but
are nevertheless still called theories.
Examples of good theories are the Theory of Relativity, the Theory of
Conservation of Energy, and the Theory of Evolution. On the other hand, a theory might be
contradicted by experiment or observation, which would make it a bad
theory. Sometimes physicists dream about
a “Theory of Everything”, but no one has found such a thing. All scientific theories are incomplete; they
cover only some part of nature. People
not trained in science sometimes use the word “theory”
when they really mean something like a speculative hypothesis.
A Triple Bond is a chemical bond
caused by the action of three pairs of electrons
instead of just one. It is often
written as three lines (=) as in the molecule H
– C = C – H.
Uniform Motion means motion at constant velocity. Since
the velocity involves both the speed and the direction, then
uniform motion means motion at constant speed in a straight line.
The Universe is the entire collection of galaxies that we can observe and that we think exist even if
we have not seen them yet. It would also
include anything else that there is between galaxies such as clouds of hydrogen
gas and probably more. We have probably
not seen everything in the universe because it is not old enough for the light
from the most distant galaxies to have reached us. Is appears to be evolving according to the Big
Bang Theory.
Velocity is the distance something travels divided by
the time it takes. In other words, it
tells how fast the position of the object is changing. The concept of velocity also includes the
direction that the object is moving. So
a rocket might be moving at a speed of 3 miles per second
up, or a parachute might be moving at 5 miles per hour down. If the velocity is constant, the motion is uniform and there is no acceleration.
A Wave is a disturbance in a medium that can
travel from one place to another without actually moving matter with it. Just the disturbance moves, but it can carry
energy with it.
An example would be a disturbance in a spring. Such a wave can range from a single pulse to a continuous train
of waves. Some waves are fairly simple
looking and consist of alternating crests and troughs. These can be described by the concepts of amplitude, frequency, wavelength, and period. Others can look very complicated. If more than one wave arrives at the same
place at the same time, the sum of the waves will actually be observed, and
such a sum can take many forms that usually look very complicated.
The Wavelength of a wave
is the distance from one crest to the next crest. The crest is the highest point on the wave. This same distance is the distance from one
trough to the next trough, where a trough is the lowest point on the wave. The wavelength can also be measured in other
ways. See the various simulations of
waves that are available in the Clinton Community College computer labs for
examples. If you are watching a wave go
past you, one wavelength will pass your position in a time of one period.
Weight is the pull of a planet’s
gravity (the force of gravity) on you or on any other object. Usually we mean the pull of the earth’s
gravity, or your weight on earth.
However, if you went to the moon, the pull of the moon’s gravity would
be smaller so you would have less weight there. You would have still another weight on Mars. Mass is necessary for
producing weight, since any object with mass produces a gravitational
attraction on other objects with mass.
However, mass and weight are not the same. Your mass would be the same on all of the planets mentioned above
as well as on any other planet or in space removed from any planet.
The concept of Work, as it is used in science, is something that takes place when you push
something through a certain distance with a certain force. If you multiply the force by the distance,
you get the amount of work done. This
will also be the amount of energy transferred from one
place to another by the process.