Parallel Circuit  - An electric circuit in which devices are connected to the same two points of the circuit, so that any single device completes the circuit independently of the others.

Pascal - The SI unit of pressure. One pascal (symbol Pa) of pressure exerts a force of one newton per square meter of surface.

Pascal's principle - The statement that changes in pressure at any point in an enclosed fluid at rest are transmitted undiminished to all points in the fluid and act in all directions.

Penumbra - A partial shadow which appears where some of the light is blocked and other light fills it in.

Perigee - The point in an elliptical orbit where an object is nearest the object about which it orbits.

Period - The time required for a complete orbit. Also, the time required for a pendulum to make on to-and-fro swing. In general, the time required to complete one cycle.

Periodic Table - A chart that lists elements by their atomic number and by their electron arrangements, so that elements with similar chemical properties are in the same column.

Perturbation - The deviation of an orbiting object from its normal path, caused by an additional gravitational force.

Photoelectric Effect - The ejection of electrons from certain metals when exposed to light.

Photon - In the particle model of electromagnetic radiation, a particle that travels at the speed of light and whose energy is related to the frequency of the radiation in the wave model.

Pigment - A material that selectively absorbs colored light.

Pitch - Term that refers to how high or low a sound appears to be.

Planck's Constant - The quantity that results when the energy of a photon is divided by its frequency

Plasma - A fourth state of matter, in addition to a solid, liquid, an gas. In the plasma state, which exists only at high temperatures, matter consists of bare atomic nuclei and free electrons.

Polarization - The filtering out of all vibrations in a transverse wave, such as a light wave, that are not in a given direction.

Postulate - A fundamental assumption.

Potential Difference - The difference in electric potential, or voltage, between two points. Charge will flow when there is a difference, and will continue until both points reach a common potential.

Potential Energy - Energy that is stored and held in readiness by an object by virtue of its position. In this stored state it has the potential for doing work.

Power - The rate at which work is done, equal to the amount of work done divided by the amount of time during which the work is done; measure in watts

Pressure - The force per unit of surface area, where the force is perpendicular to the surface; measured in pascals.

Principal Axis - The line joining the centers of curvature of the surfaces of a lens.

Principle - A general hypothesis or statement about the relationship of natural quantities that has been tested over and over again and has not been contradicted. Also known as a law

Principle of Flotation - The statement that a floating object displaces a quantity of fluid of weight equal to its own weight.

Projectile - Any object that is projected by some force and continues in motion by its own inertia

Proton - A positively charged particle that is one of the two kinds of particles found in the nucleus of an atom.

Pulley - A type of lever that is a wheel with a groove in its rim and that is used to change the direction of a force. A pulley or system of pulleys can also multiply force.

Pupil - The opening in the eyeball through which light passes.

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Quantum - An elemental unit; the smallest amount of something. One quantum of light energy is called a photon.

Quantum Mechanics - The branch of physics that is the study of the motion of quanta in the micro world of the atom.

Quantum Physics - The branch of physics that is the general study of quanta in the micro world of the atom.

Quark - One of the elementary particles of which all nucleons are made.

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Radiant Energy - Any energy, including heat, light, and X rays, that is transmitted by radiation. It occurs in the form of electromagnetic waves.

Radiation - (a) The transmission of energy by electromagnetic waves. (b) The particles given off by radioactive atoms such as uranium and radium.

Radioactive - Term applied to an atom with a nucleus that is unstable and that can spontaneously emit a particle and become the nucleus of another element.

Rarefaction - A disturbance in air or other matter in which the pressure is lowered.

Rate - How fast something happens, or how much something changes per unit of time; a change in a quantity divided by the time it takes for the change to occur.

Ray - A thin beam of light.

Ray Diagram - A diagram showing the principal rays that can be drawn to determine the size and location of an image formed by a mirror or lens.

Reaction Force - The force that is equal in strength and opposite in direction to the action force and that acts on whatever is exerting the action force.

Real Image - An image that is formed by converging light rays and that can be displayed on a screen.

Red Shift - A decrease in the measured frequency of light or other radiation from a receding source; called the red shift because the decrease is toward the low-frequency, or red, end of the color spectrum.

Reflection - The bouncing back of a particle or wave that strikes the boundary between two media.

Refraction - The change in direction of a wave as it crosses the boundary between two media in which it travels at different speeds.

Relegation - The phenomenon of ice melting under pressure and freezing again when the pressure is reduced.

Relative - Regarded in relation to something else.

Relative Humidity - A ratio between how much water vapor is in the air, and the limit for the same air temperature.

Relativistic Mass - The total mass of a moving object, taking into account any increase in mass due to its kinetic energy.

Resolution - The process of breaking up a vector into components.

Resonance - A phenomenon that occurs when the frequency of forced vibrations on an object matches the object's natural frequency, and a dramatic increase in amplitude results.

Rest Mass- The mass of an object at rest.

Resultant - The geometric sum of two vectors.

Retina - The layer of light-sensitive tissue at the back of the eye.

Reverberation - The persistence of a sound, as in an echo, due to multiple reflections.

Revolution - Motion in which an object turns about an axis outside the object.

Rotation - The spinning motion that takes place when an object moves about an axis that is located within the object.

Rotational Inertia - The resistance of an object to changes in its state of rotation, determined by the distribution of the mass of the object and the location of the axis of rotation or revolution.

Rotational Speed - The number of rotations or revolutions per unit of time; often measured in rotations or revolutions per second or per minute.

Rotational Velocity - Rotational speed, together with a direction of rotation or revolution

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- The symbol for second.

Saturated - Term applied to a substance, such as air, that contains the maximum amount of another substance, such as water vapor, at a given temperature.

Scalar Quantity - A quantity in physics, such as mass, volume, and time, that can be completely specified by its magnitude, without regard to direction.

Scaling - The study of how size affects the relationship between weight, strength, and surface area.

Scatter - To absorb sound or light and re-emit it in all directions.

Schematic Diagram - Diagram that describes an electric circuit, using special symbols to represent different devices in the circuit.

Scientific Method - An orderly method for gaining, organizing, and applying new knowledge.

Second Postulate of Special Relativity - The statement that the speed of light in empty space will always have the same value regardless of the motion of the source or the motion of the observer.

Semiconductor - Material that can be made to behave as either a conductor or an insulator of electricity.

Series Circuit - An electric circuit in which devices are arranged so that charge flows through each in turn. If one part of the circuit should stop the current, it will stop throughout the circuit.

Shadow - A shaded region that results when light falls on an object and thus cannot reach into the region on the far side of the object.

Shell Model of the Atom - A model in which the electrons of an atom are pictured as grouped in concentric, spherical shells around the nucleus.

Shock Wave - A cone-shaped wave produced by an object moving at supersonic speed through a fluid.

Sine Curve - A curve whose shape represents the crests and troughs of a wave traced out by a swinging pendulum that drops a trail of sand over a moving conveyor belt.

Solar Eclipse - The cutoff of light from the sun to an observer on the earth when the moon is directly between the sun and the earth.

Sonic Boom - The sharp crack heard when the shock wave that sweeps behind a supersonic aircraft reaches the listener.

Space-Time - A combination of space and time, which are viewed in special relativity as two parts of one whole.

Special Theory of Relativity - The theory, introduced in 1905 by Albert Einstein, that describes how time is affected by motion in space at a constant velocity, and how mass and energy are related.

Specific Heat- The quantity of heat required to raise the temperature of a unit mass of a substance by one degree.

Spectroscope - An instrument used to separate the light from a hot gas or other light source into its consistent frequencies.

Spectrum - For sunlight and other white light, the spread of colors seen when the light is passed through a prism. In general, the spread of radiation by frequency, so that each frequency appears at a different position.

Speed - How fast something is moving; the distance moved per unit of time.

Spring Tide - A high or low tide that occurs when the sun, earth, and moon are all lined up, so that the tides due to the sun and moon coincide, making the high tides higher than average and the low tides lower than average.

Stable Equilibrium - The state of an object balanced so that any small rotation raises its center of gravity.

Standing Wave - Wave in which parts of the wave remain stationary and the wave appears not to be traveling. The result of interference between an incident (original) wave and a reflected wave.

State - One of the four possible forms of matter: solid, liquid, gas, and plasma.

Streamline - The smooth path of a small region of fluid in a steady flow.

Strong Force - The force that attracts nucleons to each other within the nucleus, and that is very strong at close distances but decreases rapidly as the distance increases.

Superconductor - Material that has near infinite conductivity at very low temperatures, so that charge flows through it without resistance.

Support Force - Force that completely balances the weight of an object at rest.