O
- observation
information obtained through the senses (p. 8)
- occluded front
a front that occurs when a warm air mass is caught between two cooler air masses (p. 768)
- Ohm's law
the relationship of voltage, current, and resistance: V = IR (p. 607)
- Oort cloud
thought to be a very sparse sphere of comets encircling the solar system out to a distance of about 50,000 AU (p. 815)
- opaque
a description of a material that either absorbs or reflects all of the light that strikes it so nothing can be seen through it (p. 547)
- orbital
a region of space around the nucleus where an electron is likely to be found (p. 117)
- organic compounds
compounds that contain carbon and hydrogen, often combined with a few other elements such as oxygen or nitrogen (p. 262)
- output arm
the distance between the fulcrum in a lever and the output force (p. 428)
- output distance
the distance an output force acts through in a machine (p. 420)
- output force
the force exerted by a machine (p. 420)
- oxbow lake
a lake formed when a river forms a new path by cutting off a meander loop from the rest of the river (p. 716)
- oxidation-reduction (redox) reaction
a chemical reaction in which electrons are transferred from one reactant to another (p. 204)
- ozone layer
a region of ozone concentration in the stratosphere (p. 750)
P
- Pangaea
an ancient supercontinent formed about 300 million years ago, which later gave rise to today's continents (p. 677)
- parallax
an apparent change in position of an object with respect to a distant background when viewed from different locations (p. 835)
- parallel circuit
an electric circuit with two or more paths through which charge can flow (p. 610)
- pascal (Pa)
the SI unit of pressure, equal to 1 newton per square meter (N/m2) (p. 391)
- penumbra
a region of the moon's shadow that surrounds the umbra and is less dark than the umbra (p. 800)
- period
a row in a periodic table of elements (p. 131); the time required for one complete cycle of a periodic motion (p. 504); a unit of geologic time into which geologists divide eras (p. 734)
- periodic law
the pattern of repeating properties displayed by elements in the periodic table (p. 131)
- periodic motion
any motion that repeats at regular time intervals (p. 504)
- periodic table
an arrangement of elements in columns, based on a set of properties that repeat from row to row (p. 127)
- permeable
a description of a material that water can easily pass through (p. 707)
- pH
a measure of the hydronium ion concentration of a solution (p. 247)
- phase change
a reversible physical change that occurs when a substance changes from one state of matter to another (p. 84)
- phases
the different shapes of the illuminated side of the moon as seen from Earth (p. 798)
- phosphor
a solid material that emits light by fluorescence (p. 559)
- photoelectric effect
the emission of electrons from a metal caused by light striking the metal (p. 537)
- photon
a packet of electromagnetic energy (p. 537)
- photosphere
the thin innermost layer of the sun's atmosphere, which is the visible surface of the sun (p. 831)
- photosynthesis
a process in which plants chemically combine carbon dioxide and water into carbohydrates, a process requiring light and chlorophyll (p. 282)
- physical change
a change that occurs when some properties of a material change, but the substances in the material stay the same (p. 51)
- physical property
any characteristic of a material that can be observed or measured without changing the composition of the substances in the material (p. 45)
- physics
the study of matter and energy and the interactions between the two through forces and motion (p. 4)
- pigment
a material that selectively absorbs certain colors of light and reflects other colors (p. 553)
- pipe
a narrow vertical channel through which magma rises to Earth's surface (p. 691)
- pitch
the frequency of a sound as a listener perceives it (p. 515)
- plane mirror
a mirror with a flat surface (p. 571)
- planetary nebula
a glowing cloud of gas surrounding a dying low-mass star (p. 842)
- planetesimals
asteroid-sized bodies in the protoplanetary disk from which the planets formed by accretion (p. 819)
- plasma
a state of matter in which atoms have been stripped of their electrons (p. 315)
- plate tectonics
the theory that pieces of Earth's lithosphere, called plates, move about slowly on top of the asthenosphere (p. 676)
- plucking
a process in which glacial ice widens cracks in bedrock beneath a glacier, which carries away the loosened pieces of rock (p. 720)
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