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Index (P)

PA - PC

PD - PF

PG - PJ

PK - PN

PO - PR

PS - PV

PW - PZ

Moment of truth
The Sun, with a diameter of 1,392,000 km, is made of 94% hydrogen atoms.


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Planck length Phys.
A theoretical limit below which the distance is irreducible due to the Heisenberg Uncertainty Principle of quantum mechanics. It is a natural unit of length, derived from the fundamental constants of speed of light, c, Planck's constant ħ = h/2p and gravitational constant, G. It is approximately equal to 10-35m

Planck length definition

This is 1020 times smaller than the diameter of a proton. The Einstein's general theory of relativity is thought to fail at this length scale, which will be dominated by quantum effects. It is thought that a quantum theory of gravity will be needed to handle the physics at this length scale where the gravity becomes important.

plasma Phys.
Mixture of unionized molecules or atoms and ionized gaseous discharged formed by superheating gas to remove electrons from nucleus. It is widely considered as the fourth state of matter (in addition to solid, liquid and gas), plasma occurs naturally round the region of black holes and the atmosphere of sun. A fluorescent light is an example of a highly contained plasma.

plasmolysis Biol.
Contraction of cells within plants as a result of the loss of water through osmosis in a hypertonic environment. These cells will eventually wilt and die.

plaster of Paris Chem.
A modified calcium sulfate (hemihydrate form, 2CaSO4.H2O) made by heating mineral gypsum. When ground to a fine powder and mix with water, it reverts back to fully hydrated sulfate and sets hard, due to interlocking of fine gypsum crystals. It is used in casts and mouldings for making ornaments, figures, etc. The use of the plaster was known since ancient by the Egyptians and Greeks, but the name is due to large deposits of gypsum found near Paris and by the construction of plaster walls for houses in Paris during 1700s.

Pleiades Astron.
A group of prominant star cluster located at Taurus the Bull constellation, which forms the Bull's shoulder. Also known as the Seven Sisters or 'Seven Maidens' or 'Seven Little Girls' by other cultures. However, only six stars are readily visible to naked eyes. Using a telescope, about 250 stars can be identified. The cluster is about 410 light years from Earth and spans 20 light years across. The members are thought to consist mainly of young stars of not more than 20 million years.

pleochroic Min.
Pertaining to the behavior of a crystal, gem or crystalline rock that appears to be of different colors, depending on the direction from which it is viewed. It is caused by polarization of light as it passes through an anisotropic medium. Such behavior is exploited in mineralogy for mineral identification since minerals that are otherwise very similar often have different pleochroic color schemes. They are identified under transmitted polarized light in a microscope and the mineral sample is often in thin section form. For instance, sapphirine often shows extreme pleochroism that can change from deep blue to pale light blue under polarized light when the mineral sample is rotated by 90.

PnP Comp.
Abbreviation for Plug and Play, a personal computer (PC) specification jointly developed by Compaq, Intel, Microsoft and Pheonix companies in order to make simple and automatic configuration by the PC. A PC that was built after 1996 usually use PnP system to identify any existing or new devices when inserted. A PnP device contains configuration information which can be identified by the PnP-compatible PC in order to automatically load the appropriate driver.

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