1...mō´dem) (n.) Short for modulator-demodulator. A modem is a device or program that enables a computer to transmit data over, for example, telephone or cable lines. Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves. A modem converts between these two forms.
2.. FM broadcasting is a broadcast technology invented by Edwin Howard Armstrong that uses frequency modulation (FM) to provide high-fidelity sound over broadcast radio.
3.... freshly polished, negatively charged zinc plate looses its charge if it is exposed to ultraviolet light. This phenomenon is called the photoelectric effect.
Careful investigations toward the end of the nineteenth century proved that the photoelectric effect occurs with other materials, too, but only if the wavelength is short enough. The photoelectric effect is observed below some threshold wavelength which is specific to the material. Especially the fact that light of large wavelengths has no effect at all even if it is extremely intensive, appeared mysterious for the scientists.
Albert Einstein finally gave the explanation in 1905: Light consists of particles (photons), and the energy of such a particle is proportional to the frequency of the light. There is a certain minimum amount of energy (dependent on the material) which is necessary to remove an electron from the surface of a zinc plate or another solid body (work function). If the energy of a photon is bigger than this value, the electron can be emitted. From this explanation the following equation results:
Ekin = h f – W
Ekin ... maximal kinetic energy of an emitted electron
h ..... Planck constant (6.626 x 10-34 Js)
f ..... frequency
W ..... work function
4...acid rain
Acid deposition is a general term that includes more than simply acid rain. Acid deposition primarily results from the transformation of sulphur dioxide (SO2) and nitrogen oxides into dry or moist secondary pollutants such as sulphuric acid (H2SO4), ammonium nitrate (NH4NO3) and nitric acid (HNO3). The transformation of SO2 and NOx to acidic particles and vapours occurs as these pollutants are transported in the atmosphere over distances of hundreds to thousands of kilometers. Acidic particles and vapours are deposited via two processes - wet and dry deposition. Wet deposition is acid rain, the process by which acids with a pH normally below 5.6 are removed from the atmosphere in rain, snow, sleet or hail. Dry deposition takes place when particles such as fly ash, sulphates, nitrates, and gases (such as SO2 and NOx), are deposited on, or absorbed onto, surfaces. The gases can then be converted into acids when they contact water.
5..A set is a collection of objects called elements of the set. Why not use the word ``collection'' and eliminate the word ``set'', thereby having fewer words to worry about? ``Collection'' is a common word whose generic meaning is understood by most people. The use of the word ``set'' means that there is also a method to determine whether or not a particular object belongs in the set. We then say that the set is well-defined. For example, it is easy to decide that the number 8 is not in the set consisting of the integers 1 through 5. After all, there are only five objects to consider and it is clear that 8 is not one of them by simply checking all five.
A basic problem here is now to indicate sets on paper and verbally. As seen above, a set could be described with a phrase such as ``the integers 1 through 5'' and the speaker hopes that it is understood. Symbollically, we use two common methods to write sets. The roster notation is a complete or implied listing of all the elements of the set. So and are examples of roster notation defining sets with 4 and 20 elements respectively. The ellipsis, `` '', is used to mean you fill in the missing elements in the obvious manner or pattern, as there are too many to actually list out on paper. The set-builder notation is used when the roster method is cumbersome or impossible. The set B above could be described by . The vertical bar, ``|'', is read as ``such that'' so this notation is read aloud as ``the set of x such that x is between 2 and 40 (inclusive) and x is even.'' (Sometimes a colon is used instead of |.) In set-builder notation, whatever comes after the bar describes the rule for determining whether or not an object is in the set. For the set the roster notation would be impossible since there are too many reals to actually list out, explicitly or implicitly.
6... Hydrogen bonding differs from other uses of the word "bond" since it is a force of attraction between a hydrogen atom in one molecule and a small atom of high electronegativity in another molecule. That is, it is an intermolecular force, not an intramolecular force as in the common use of the word bond.
When hydrogen atoms are joined in a polar covalent bondwith a small atom of high electronegativity such as O, F or N, the partial positive charge on the hydrogen is highly concentrated because of its small size. If the hydrogen is close to another oxygen, fluorine or nitrogen in another molecule, then there is a force of attraction termed a dipole-dipole interaction. This attraction or "hydrogen bond" can have about 5% to 10% of the strength of a covalent bond.
Hydrogen bonding has a very important effect on the properties of water and ice. Hydrogen bonding is also very important in proteins and nucleic acids and therefore in life processes. The "unzipping" of DNA is a breaking of hydrogen bonds which help hold the two strands of the double helix together
7...The equations of modern atomic theory are difficult to solve. Fortunately, many of the results can be obatined by following some simple rules. These rules are known as the Aufbau principle. However, we first need to discuss quantum numbers, shells, subshells and orbitals.
The principal quantum number n - the shell
Quantum numbers abound in quantum theory. These quantum numbers serve the purpose of keeping track of the various quantum possibilities that emerge. Perhaps the most important quantum number is the "principal" quantum number n. The principal quantum number n can take on the values 1, 2, 3, 4, 5, 6, ... . Associated with each n is a principle energy level known as a shell. Thus, shell 1 has n=1, shell 2 has n=2 etc. and so on associated with it.
Each shell has subshells associated with it
Depending upon its quantum number, each shell can have one or more subshells associated with it. For the n=1 shell there is only one subshell - the s subshell. For the n=2 shell there are two subshells - the s and p subshells and so on. The number of subshells within a shell is equal to n.The physical and chemical properties of elements is determined by the atomic structure. The atomic structure is, in turn, determined by the electrons and which shells, subshells and orbitals they reside in. The rules af placing electrons within shells is known as the Aufbau principle. These rules are:
1. Electrons are placed in the lowest energetically available subshell.
2. An orbital can hold at most 2 electrons.
3. If two or more energetically equivalent orbitals are available (e.g., p, d etc.) then electrons should be spread out before they are paired up (Hund's rule).
8..
When deciding how electrons are arranged in an orbital diagram for a given atom or ion, Hund's rule is used.
When several orbitals of equal energy are available, electrons enter singly with parallel spins
In other words, add one electron to each orbital of equal level (such as the five d orbitals) all with the same spin.
9...TotalInternal Reflection
An unusual observation - a discrepant event - was observed in a recent lab - the Index of Refraction lab. The refraction of light through a glass block in the shape of an isosceles triangle was investigated (at least, so we thought). In the lab, a ray of light entered the face of the triangular block at a right angle to the boundary. This ray of light passed across the boundary without refraction since it was incident along the normal (recall the Secret of the Archer Fish). The ray of light then traveled in a straight line through the glass until it reached the second boundary. Only instead of transmitting across this boundary, the entirety of the light seemed to reflect off the boundary and transmit out the opposite face of the isosceles triangle. This discrepant event bothered many as they spent several minutes looking for the light to refract through the second boundary. Then finally, to their amazement, they looked through the third face of the block and could clearly see the ray.
10....
Right-angled triangles
Here is a triangle. There are three angles inside it: one we've called "theta"; one is left empty and one has a small square drawn in it. This small square indicates that that angle is a right-angle, which is 900 or p/2 in radians. The triangle is therefore known as a right-angled triangle.
The longest side of a right-angled triangle is called the "hypotenuse" as shown. The other two sides are labelled in terms of their position to the angle q. (So if we were considering the other angle, rather than q, their labels would be swapped over).
We are going to define two new quantities called sine and cosine. They depend on the angle q: if q changes the they will also change.
The sine of q, written as sin(q), is defined as the length of the opposite side divided by the length of the hypotenuse.
Similarly, the cosine of q, written as cos(q), is defined as the length of the adjacent side divided by the length of the hypotenuse.
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