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# 4.1 Matter and Energy

Atoms are microscopic constituents of matter, composed of a tiny and massive nucleus and a circulating cloud of electrons. The positively charged protons in the nucleus are balanced by an equal number of negatively charged electrons — normal matter is electrically neutral. Atoms combine into molecules of different chemical compounds. An element is defined by the number of protons in its nucleus. The processes in which atoms combine to form molecules, or those in which molecules break into their constituent atoms, are called chemical reactions. During chemical reactions, only the electrons in the outer part of each atom interact; the nuclei remain separate. In normal matter, atoms retain their own electrons or share electrons. At high temperatures, atoms lose their electrons and become ionized. At extremely high temperatures, atomic nuclei can be broken apart or combined by fusion. Most of the universe is extremely cold, but stars are extremely hot.

A depiction of the atomic structure of the helium?atom. The darkness of the?electron cloud?corresponds to the line-of-sight integral over the?probability function?of the 1s?atomic orbital?of the electron. The magnified?nucleus?is schematic, showing protons?in pink and?neutrons?in purple. In reality, the nucleus (and the wave function of each of the?nucleons) is also spherically symmetric and 1s, and the four particles, each with a different quantum number, like the electrons in the helium atom, are all most likely to be found in the same space, at the exact center of the nucleus. Click here for original source URL

Energy is a general term to describe anything that can cause a change. Two important forms of energy in astronomy are kinetic energy — the energy of motion of an object, and gravitational potential energy — the ability of an object to move under the force of gravity. There are other forms of energy too. Throughout the universe, energy changes forms but the total amount of energy is conserved. Another form of energy is the thermal energy due to the constant motions of atoms and molecules. Temperature is just a measure of the kinetic energy or movement of the atoms and molecules of a substance. The zero point of the physical (Kelvin) temperature scale is the absence of all motion. The higher the temperature, the faster are the movements of atoms and molecules. At any given temperature, lighter molecules will move faster than heavy molecules, on the average.

Carnot engine diagram (modern) - where an amount of heat?QH?flows from a high temperature?TH?furnace through the fluid of the "working body" (working substance) and the remaining heat?QC?flows into the cold sink?TC, thus forcing the working substance to do?mechanical work?W?on the surroundings, via cycles of contractions and expansions. Click here for original source URL.

Whenever two regions have different temperatures, heat will flow in a way to try and equalize the temperatures, a situation called thermal equilibrium. Equilibrium can be upset by the addition of new energy or heat that warms up one region, or by the removal of heat that cools a region. The three modes of heat transfer from one region to another are conduction, convection, and radiation. All bodies emit a broad spectrum of thermal radiation, which reflects energy of motion of the individual atoms and molecules. The higher the temperature, the more radiation is emitted and the shorter the wavelength of the peak radiation. For objects at room temperature, the peak of thermal radiation is at infrared wavelengths.