The lowest possible temperature according to the kinetic theory; all molecular motions cease.
Acceleration
The change in instantaneous velocity divided by the time interval over which the change occurs.
Acceptable risk
The cost in human life is justified, given the economic benefit.
Action and reaction, law of
(Newton's third law) When A exerts a force on B, B will exert a force on A equal in magnitude but opposite in direction to the force which A exerts on B.
Aether
The medium hypothesized by nineteenth-century physicists for the propagation of light; relative to an observer at rest with respect to the aether, the speed of light was thought to be c = 3 x 108 m/sec.
Alternating current (AC)
An electrical current that changes its direction of flow with a certain periodicity. For example, 60-cycle AC is an electrical current that changes its direction of flow 60 times per second. Also loosely used as a synonym for any current or field that varies with time.
A statement which says nothing about the physical world; e.g., "The fleen is prang" or "A triangle contains 180 degrees."
Angstrom
A unit of length often used for wave-lengths; 1 A = 10-10 meters.
Angular acceleration
The change in the instantaneous angular velocity divided by the time over which the change occurs.
Angular displacement
The change in angle of an object moving in a circle; usually measured in radians.
Angular momentum
Defined by the equation L = r x mv; in a circle. we have the simple form L = mvr.
Angular speed
The scalar part of angular velocity.
Angular velocity
The angular displacement divided by the time over which the displacement occurs; direction given by a right-hand rule.
Antimatter
A general term for the collection of antiparticles.
Antinode
A point of maximum wave displacement in a standing wave pattern..
Antiparallel
Pointing in opposite directions, usually referring to vectors, e.g., north and south, east and west.
Antiparticle
General term for a particle with internal quantum numbers which are the negative of those of the corresponding particle.
A posteriori statement
A statement whose truth or falsity depends on appeal to experience.
A priori statement
A statement whose truth or falsity depends on an appeal to reason, not experience.
Area rule
On a velocity-vs.-time graph, the area under the curve gives the distance traveled.
Atom
The smallest particle of matter into which an element can be resolved by chemical means; retains the same chemical properties as the original element.
Atomic number
The number of protons or positive charges in a nucleus; appears as a subscript to the left of the element symbol.
Atomic weight
The average of the mass numbers of the isotopes found in a large, naturally occurring sample of an element.
Average (temporal) power
The time-averaged rate of energy transfer.
Average value of a physical variable
The change in the variable over a finite time period; if the variable changes uniformly over the time interval, the average value is equal to the instantaneous value of the variable.
Averaging time
The appropriate time period over which exposure is averaged for purposes of determining compliance with RF exposure limits.
Avogadro's hypothesis
Equal volumes of gas at the same temperature and pressure contain the same number of molecules or atoms.
Axiom
A basic assumption underlying a theory or branch of mathematics.
The biological cycle that usually occurs at 24-hour intervals and is also known as the biological clock. Believed to be tied to the Schumann Resonance.
Circuit
A system of electric conductors that will allow a flow of current.
Closed system
A system of interacting entities such that nothing either enters or leaves the system.
Commutative
Two operators or operations are said to be commutative if the result of the first applied before the second is the same as that of the second applied before the first.
Commutator
[A, B] = AB - BA.
Component of a vector
One of a pair of (perpendicular) vectors whose resultant is the given vector.
Conservation law
A general law requiring that a physical variable have the same value at a later time as at some earlier time.
Constant
(a) A fixed number, (b) unchanging or uniform.
Continuous
A collection is continuous if for any given member of the set there does not exist another element "closest" to the original member; e.g., the real numbers from 0 to 1.
Continuous exposure
Exposure for durations exceeding the corresponding averaging time. Applicable in applying exposure standards.
Correspondence principle
Principle that a new theory must make the same predictions as an older theory in those circumstances in which the older one is known to be correct (due to Niels Bohr).
Cosine
The ratio of the side adjacent to an angle to the hypotenuse of a right triangle; corresponds to the x-component of a unit vector.
Cosmological principle
Principle which states that what we observe of the expansion of the universe will also be observed by any other observer in the universe.
Direction in which a positive test charge would move if placed at the point in question.
Direction of magnetic field
Direction in which the north pole of a compass would point if placed at the point in question.
Discrete
A collection is discrete if for any given element there exists another member of the set "next to" the original element; e.g., the whole numbers.
Displacement
The change in position of an object (a vector).
Distribution line
A power line used to distribute power locally to a Utilities end-users.
Doppler effect
The phenomenon in which the frequency of a wave is changed by the motion of the source and/or the observer.
Duty factor
The ratio of pulse duration to the pulse period of a periodic pulse train. Also, may be a measure of the temporal transmission characteristic of an intermittently transmitting RF source such as a paging antenna by dividing average transmission duration by the average period for transmissions. A duty factor of 1.0 corresponds to continuous operation.
The flow of charges; measured in amperes, (amps, A).
Electric field
A vector defined as the electric force exerted on a positive test charge divided by the magnitude of the positive test charge.
Electric field strength (E)
A field vector quantity that represents the force (F) on an infinitesimal unit positive test charge (q) at a point divided by that charge. Electric field strength is expressed in units of volts per meter (V/m).
Electric force
The attraction or repulsion of charged objects, usually elementary particles; the mathematical formula for electric force is Coulomb's law.
Electric potential
The work which must be done to move a positive test charge from one point to another in an electric field divided by the magnitude of the test charge; measured in volts.
Electric potential energy
The work we must do to move a positive test charge from one point to another in an electric field; for point charges, PE = KqQ/r, where r is the distance between centers and K = 9 x 109 nt-m/coul2.
Electromagnetic field (EM field)
A force field that extends or radiates out away from any moving electrical current. This electromagnetic field has a direction of movement away from the electrical current and contains both a magnetic field and an electric field.
Electromagnetic spectrum (EM spectrum)
A way of organizing electromagnetic fields on the basis of their frequency of oscillations. The non-ionizing electromagnetic spectrum starts with zero (no oscillation, or DC) and extends up to visible light with trillions of oscillations per second. Frequencies of oscillation above light are considered to be ionizing and include X-rays and cosmic rays. The EM spectrum is divided into regions based on frequency and usage. See also Extra low frequency, Very low frequency, Radio frequency, and Microwave.
A wave which travels at the speed of light and is characterized by oscillations of combined electric and magnetic fields; examples are light, radio waves, x-rays.
Electron volt
The energy change experienced by an electron when passing through a potential difference of one volt; also applicable to any particle with a charge of the same magnitude as the electronic charge, the smallest unit of charge; denoted by eV; 1 eV = 1.6 X 10-19 joule.
Ellipse
A geometric figure defined by the distance of any point on the curve from two fixed points, called the foci; if o and b are the two distances, then o + b = constant.
EMF
Electromagnetic field
Empiricism
Philosophical position that all knowledge is derived from experience.
EMR
Electromagnetic radiation
Energy
A physical variable measured in joules; we say that an object gains or loses energy when work is done on the object; conversely, an object has energy if it can do work on another object. Energy is the ability to do work. Kinetic energy is due to the motion of a mass, as when you throw a ball. Potential energy is stored energy, as in a coiled spring. The units for both kinetic and potential energy are the same as for work. (See Work.)
Energy density (electromagnetic field)
The electromagnetic energy contained in an infinitesimal volume divided by that volume.
Energy level
The discrete values of energy for a quantized system.
Energy level diagram
A diagram in which the energy levels for a quantum system are displayed; levels represented by straight horizontal lines arranged vertically.
Epicycle
The small circle rotating on the deferent on which a planet moves in the geocentric system of astronomy.
Epidemiology
The study of the distribution of disease in populations.
Equal areas, law of
Kepler's second law of planetary motion: the line joining a planet to the sun sweeps out equal areas in equal times.
Equation of motion
An integral part of any dynamical system which relates the acting forces to the motion of the system.
Equivalence principle
The assumption that an accelerated observer in a gravity-free region is equivalent to an observer at rest in a gravitational field; due to Einstein.
Euclidean geometry
The usual geometry of a flat surface; plane geometry.
Event
A point in space-time defined by two numbers, its spatial distance and its temporal distance from a reference event, usually the origin 0 of the space-time diagram.
Expanding universe
A theory that the red-shift in the spectrum of galaxies is due to the recession of the galaxies from our location in the universe.
Exposure
Exposure occurs whenever and wherever a person is subjected to electric, magnetic or electromagnetic fields other than those originating from physiological processes in the body and other natural phenomena.
Exposure, partial-body
Partial-body exposure results when RF fields are substantially nonuniform over the body. Fields that are nonuniform over volumes comparable to the human body may occur due to highly directional sources, standing-waves, re-radiating sources or in the near field. See RF "hot spot".
Extra low frequency (ELF)
The portion of the electromagnetic spectrum extending from zero to 1000 cycles per second. This includes the 60-cycle power frequency in the United States, the 50-cycle European power frequency, and the U.S. Navy's submarine communication system at 45 and 75 cycles per second.
The Celsius scale, formerly known as the centigrade scale, invented by A. Celsius, has 100 divisions between 0° for the freezing point of water and 100° for the boiling point. The Fahrenheit scale, invented by G. D. Fahrenheit, is still used for weather observations and general purposes. On this scale, the freezing point is 32° while the boiling point is 212°, with 180 divisions between. To convert from one scale to the other - TC = 5(TF-32°)/9, and TF = (9TC/5) + 32° The Kelvin or absolute temperature scale was devised by Lord Kelvin. On this scale, the zero point is absolute zero, 273° below 0°C. At 0 K, any material loses all its thermal energy. The divisions of the °C scale and the K scale are the same. (Note: The SI unit for temperature is K for Kelvin, without the degree symbol.) To convert from °C to K, just add 273°. Thus, 0°C equals 273 K. To convert from K to °C, subtract 273°. Then 0 K equals -273°C. Average room temperature is generally considered about 20 to 25°C. This equals 68 to 77°F. On the absolute scale, the corresponding temperatures are 293 to 298 K.
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denoting the change in some physical variable - final value minus initial value.
