Physics by Credo Reference: General Information

Change in the velocity of a body with respect to time. Since velocity is a vector quantity, involving both magnitude and direction, acceleration is also a vector. In order to produce an acceleration, a force must be applied to the body.

In physics, number of periodic oscillations, vibrations, or waves occurring per unit of time. The SI unit of frequency is the hertz (Hz), one hertz being equivalent to one cycle per second.

Force of attraction that arises between objects by virtue of their masses. The larger the mass of an object the more strongly it attracts other objects.

Branch of science traditionally defined as the study of matter, energy, and the relation between them; it was called natural philosophy until the late 19th cent. and is still known by this name at a few universities.

From Collins Dictionary of Astronomy
Symbol: vr. The velocity of a star along the line of sight of an observer. It is calculated directly from the doppler shift (see Doppler effect) in the lines of the star's spectrum: if the star is receding there will be a redshift in its spectral lines and the radial velocity will be positive; an approaching star will produce a blueshift and the velocity will be negative.

Longitudinal wave motion with which sound energy travels through a medium. It carries energy away from the source of the sound without carrying the material itself with it.

Physical principle, enunciated by Werner Heisenberg in 1927, that places an absolute, theoretical limit on the combined accuracy of certain pairs of simultaneous, related measurements. The accuracy of a measurement is given by the uncertainty in the result; if the measurement is exact, the uncertainty is zero.

From The Penguin Dictionary of Mathematics
Symbol: v. The rate of change of position with time when the direction of motion is specified. Velocity v is thus a vector quantity; its magnitude v is referred to as speed.

Elementary particle corresponding to an ordinary particle such as the proton, neutron, or electron, but having the opposite electrical charge and magnetic moment. Every elementary particle has a corresponding antiparticle; the antiparticle of an antiparticle is the original particle.

[Gr.,=uncuttable (indivisible)], basic unit of matter; more properly, the smallest unit of a chemical element having the properties of that element.

Internal structure of an atom.

Device used to detect elementary particles and other ionizing radiation. A cloud chamber consists essentially of a closed container filled with a supersaturated vapor, e.g., water in air.

Elementary particle carrying a unit charge of negative electricity. Ordinary electric current is the flow of electrons through a wire conductor (see electricity). The electron is one of the basic constituents of matter.

Atom, or group of atoms, that is either positively charged (cation) or negatively charged (anion), as a result of the loss or gain of electrons during chemical reactions or exposure to certain forms of radiation.

In chemistry and physics, one of two or more atoms having the same atomic number but differing in atomic weight and mass number. The concept of isotope was introduced by F. Soddy in explaining aspects of radioactivity; the first stable isotope (of neon) was discovered by J. J. Thomson.

Elementary particle heavier than an electron but lighter than other particles having nonzero rest mass. The name muon is derived from mu meson, the former name of the particle.

[Ital.,=little neutral (particle)], elementary particle with no electric charge and a very small mass emitted during the decay of certain other particles. The neutrino was first postulated in 1930 by Wolfgang Pauli in order to maintain the law of conservation of energy during beta decay.

Uncharged elementary particle of slightly greater mass than the proton. It was discovered by James Chadwick in 1932. The stable isotopes of all elements except hydrogen and helium contain a number of neutrons equal to or greater than the number of protons.

In physics, device for detecting, measuring, and analyzing particles and other forms of radiation entering it. Such devices play an important role not only in basic research, as in the study of elementary particles, but also in numerous applications of physics, from uses of radioactive tracers in medicine and biology to prospecting for natural ores that exhibit radioactivity.

Study of the particles that make up all atoms, and of their interactions. More than 300 subatomic particles have now been identified by physicists, categorized into several classes according to their mass, electric charge, spin, magnetic moment, and interaction.

Elementary particle having a single positive electrical charge and constituting the nucleus of the ordinary hydrogen atom. The positive charge of the nucleus of any atom is due to its protons. Every atomic nucleus contains one or more protons; the number of protons, called the atomic number, is different for every element.

Danish physicist who established the structure of the atom. For this achievement he was awarded the 1922 Nobel Prize for Physics. Bohr made another very important contribution to atomic physics by explaining the process of nuclear fission.

In 1935 Chadwick won the Nobel Prize for his discovery of the neutron.

He became director of a major part of the Manhattan Project at Chicago and built the first reactor with Fermi (1942), publishing an account in his book Atomic Quest (1958).

Marie Curie was a brilliant physicist who discovered radium and polonium and helped to elucidate the nature of radioactivity. She was the first woman to win a Nobel Prize and the first person to win a second Nobel Prize.

German-born US theoretical physicist who revolutionized our understanding of matter, space, and time with his two theories of relativity. Einstein also established that light may have a particle nature and deduced the photoelectric law that governs the production of electricity from light-sensitive metals.

English physicist and chemist who is often regarded as the greatest experimental scientist of the 1800s. He made pioneering contributions to electricity, inventing the electric motor, electric generator and the transformer, and discovering electromagnetic induction and the laws of electrolysis.

Italian-born US physicist best known for bringing about the first controlled chain reaction (in a nuclear reactor) and for his part in the development of the atomic bomb. He also carried out early research using slow neutrons to produce new radioactive elements, for which work he was awarded the 1938 Nobel Prize for Physics.

British theoretical physicist noted for his research into the origin of the universe. His work influenced the development of the big bang and black hole theories.

German physicist who founded quantum mechanics and the uncertainty principle. In recognition of these achievements, he was awarded the 1932 Nobel Prize for Physics.

Maxwell stated that light represented only a small range of the spectrum of electromagnetic waves available. Hertz confirmed this in 1888 by discovering another part of the spectrum, radio waves, but by this time Maxwell was dead.

Austrian-born Swedish physicist who was one of the first scientists to study radioactive decay and the radiations emitted during this process. Her most famous work was done in 1938, in collaboration with her nephew Otto Frisch, describing for the first time the splitting or fission of the uranium nucleus under neutron bombardment.

English physicist and mathematician who is regarded as one of the greatest scientists ever to have lived. In physics, he discovered the three laws of motion that bear his name and was the first to explain gravitation, clearly defining the nature of mass, weight, force, inertia, and acceleration.

From The Hutchinson Dictionary of Scientific Biography
Dutch physicist who is particularly remembered for the contributions he made to the study of the properties of matter at low temperatures. He was the first to liquefy helium and later discovered superconductivity, gaining the 1913 Nobel Prize for Physics in recognition of his work.

Working on quantum mechanics, he contributed the Pauli exclusion principle (1924), which explained much about atomic structure. The principle requires that no two electrons in an atom can be in the same quantum state.

German physicist who discovered that energy consists of fundamental indivisible units, which he called quanta. This discovery, made in 1900, marked the foundation of the quantum theory that revolutionized physics in the early 1900s.

From The Cambridge Dictionary of Scientists
Röntgen was awarded the first Nobel Prize in physics, in 1901, ‘for the discovery of the remarkable rays subsequently named after him’; in fact they are still known as X-rays. Their study added much to physics, gave a new technique for use in medicine and, after the work of the Braggs in 1915, led to X-ray crystallography as a new and immensely valuable method for the study of crystal and molecular structure.

Rutherford’s studies revealed (1898) that the radioactive emission consisted of at least two kinds of rays; those which were less penetrating he called alpha rays (helium nuclei), and the others beta rays (electrons); 2 years later he discovered a third, and even more penetrating kind, gamma rays (electromagnetic waves).

Austrian physicist who founded wave mechanics with the formulation of the Schrödinger wave equation to describe the behaviour of electrons in atoms.

From 1946 she became expert on beta-decay in radioactive atoms, the process whereby an electron and a neutrino are ejected from a neutron in the nucleus, leaving behind a proton. In 1957 she developed her research on nuclear decay by emission of beta particles by observing that the direction of emission is closely tied to the direction of the spin of the emitting nucleus.

In 1913, Soddy gave the clearest of the statements of the radioactive displacement law that emerged about that time: that emission of an alpha-particle (helium nucleus) from an atom reduces its atomic number by two; whereas the emission of a betaparticle (an electron) increases the atomic number by one.