PHYSICS DICTIONARY

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Stiffness Constant

Any one of the coefficients of the relations in the generalized Hooke's law used to express stress components as linear functions of the strain components.

Stimulated Emission (LASER)

If in the excited state of the atom, a photon of frequency same as the frequency of photon which the atom in passing from the excited state to the ground state will emit, strikes the atom, then that atom come to ground state very soon and stimulates the light incident on it, by emitting photon of exactly the same frequency. This is called stimulated emission.

Stochastic Process

Stochastic process appears in probability theory, a process involving the operation of chance. For example, in radioactive decay every atom is subject to a fixed probability of breaking down in any given time interval. More generally, a stochastic process refers to a family of random variables indexed against some other variable or set of variables.

Stoichometry

The word stoichometry derives from two Greek words:  Stoicheion (meaning "element") and Metron (meaning "measure"). Stoichometry deals with calculations about the masses (sometimes volumes) of reactants and products involved in a chemical reaction. It is a very mathematical part of chemistry. The rules for determining stoichometric relationships are based on the laws of conservation of mass and energy and the law of combining weights or volumes.

Stokes Lines

The smaller frequency component in Raman spectrum are called stokes lines.

Stopping Power

The ratio of differential energy loss within the absorber to the corresponding differential path length for a charged particle is called stopping power.

or

Stopping power or linear energy transfer (LET) is the energy lost per unit path length.

STP (Standard Temperature Pressure)

 STP is commonly used to define standard conditions for temperature and pressure which is important for measurements of chemical and physical processes. As per IUPAC (International Union of Pure and Applied   Chemistry), it is defined as air at 0 ^oC and 10⁵ Pascals. STP in imperial and USA system of units is defined as air at 60^oF and 14.696 Psi.

Straggling

Energy lost by identical charged particles passing through a medium is statistical in nature if the medium is inhomogeneous in nature, this would lead to variation in “range” of same energetic incident particles in that medium. This is called straggling.

Strain (Engineering)

 The change in gauge length of a specimen (in the direction of an applied stress) divided by its original gauge length.

Strain Energy

When a body is deformed, the work done is stored in the form of potential energy in the body. This potential energy is called strain energy.

Strange Particles

Kaons (K-mesons) and Hyperons are known as strange particles. They are produced in strong interactions but they decay slowly and are hence called as strange particles.

Stratopause

The boundary between the stratosphere and the mesosphere is called the stratopause.

Stratosphere

The stratosphere is a layer of Earth's atmosphere. The stratosphere is the second layer, as one move upward from Earth's surface, of the atmosphere. The stratosphere is above the troposphere and below the mesosphere. The top of the stratosphere occurs at 50 km (31 miles) altitude. 

Stream Line Flow

It is the path of liquid such that tangent to which at any point gives the direction of flow of liquid at real point.

or

When a liquid flows such that each particle of liquid passing a point moves along the same path and has some velocity as preceding particle, its flow is called streamline flow.

Stress

Restoring force per unit area.

Strong Interaction

Interaction which exists between nucleons and holds them is called as strong interaction. This interaction overcomes Coulomb’s force. It has short range of order of 10^-13 cm. It holds quarks together in the Proton and Neutron and also holds protons and neutrons together in nucleus of an atom. The strong interaction force is carried by spin -1 particle called Gluon which interacts only with itself and with quarks.

Strong Nuclear Force

The force which holds nucleus together. It has short range of order 10^-13 cm. It holds quarks together in the proton and neutron and also holds protons and neutrons together in nucleus of an atom. It is believed that this force is carried by another spin -1 particle called Gluon which interacts only with itself and with the quarks. 

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Standard Model

All the particles and their interactions observed to date can be described by a quantum field theory called the Standard Model. The Standard Model has 40 species of elementary particles (24 Fermions, 12 vector Bosons, and 4 scalars), which can combine to form composite particles, accounting for the hundreds of other species of particles discovered since the 1960s.

Star

A star is a massive, luminous sphere of plasma held together by its own gravity. A star is so massive that its core is extremely dense and hot. At the high core temperatures of a star, atoms move so fast that they sometimes stick to other atoms when they collide with them, forming more massive atoms and releasing a great amount of energy. This process is known as nuclear fusion. A star usually consists of Hydrogen and Helium.

Stark Effect

The splitting of atomic spectral lines as a result of an externally applied electric field was discovered by Stark, and is called the Stark effect. It is analogue effect of Zeeman Effect. The Stark effect has been of benefit in the analysis of atomic spectra, and a major tool for molecular rotational spectra.

State Function

If the change in a property of a system depends only on initial and final states and not on path of process carried out in going from initial to final state, then the property is said to be state function of system.

States of Matter      

States of matter in physics are the distinct forms that different phases of matter take on.

Static Electricity

It is electric charge that has accumulated on an object. Static electricity is often created when two objects that are not good electrical conductors are rubbed together, and electrons from one of the objects rub off onto the other. 

Static Friction

The friction that acts due to tendency of relative motion between two surfaces in contact is called static friction.

Stationary Waves

When two simple harmonic waves of same amplitude, frequency and time period travel in opposite directions in a straight line, the resultant wave obtained is called stationary or a standing wave. In a stationary wave, nodes and antinodes are formed alternatively. All particles except at nodes vibrate simple harmonically with the time period equal to that of each component wave.

Statistical Equilibrium

An ensemble is said to be in statistical equilibrium if the probabilities of finding the phase points in the various regions of phase space and the average values of properties of its systems are “independent of time”.

Statistical Mechanics

It is the branch of science which establishes the interpretation of macroscopic behavior of system in terms of its microscopic properties. It doesn’t deal with motion of each particle but it takes into account the average or most probable properties of system without going into interior details of characteristics of its constituents. The larger is the number of particles in physical system considered, the more nearly correct are statistical predictions. 

Steady State Diffusion

The diffusion condition for which there is no net accumulation or depletion of diffusing species. The diffusion flux is independent of time.

Stefan – Boltzmann’s Law

 Loss of heat per second from unit surface area of a black body at an absolute temperature ‘T’ surrounded by atmosphere at an absolute temperature ‘T_o’, is directly proportional to difference in fourth power of absolute temperature of that body and surrounding atmosphere.

(or)

Total amount of radiant energy emitted by a black body per second per unit area is directly proportional to fourth power of its absolute temperature. E α T⁴; E = σ T⁴; σ is Stefan’s constant.

Stefan’s Law

The radiant energy emitted per second per unit area by a perfectly black body is directly proportional to the fourth power of the absolute temperature of the body.

Step-Down Transformer

Transformer in which secondary voltage is less than its primary voltage.

Step-Up Transformer

Transformer in which secondary voltage is greater than its primary voltage.

Stern – Gerlach Experiment

In 1921, an experiment conducted by Otto stern and Walter Gerlach has shown that each electron, in addition to its orbital angular momentum L, possess an intrinsic angular momentum referred to as spin ‘S’. The Stern-Gerlach experiment has proved spatial quantization by showing that electron spin in quantized into two states and provided a major impetus for development of quantum theory of atom.

Stethoscope

A medical instrument for listening to the sounds made with in body, typically consisting of a disc that transmits the sound through hollow tubes to ear pieces. 

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Spectrum

 It is colored pattern obtained on screen after dispersion of light.

Speed

Distance per unit time.

Speed of Light

It is denoted by ‘C’ and its value in vacuum is 2.99792458 x 10⁸ m/sec.

Sphere of Influence

 A sphere drawn with a molecule as centre and molecular range as radius is called sphere of influence.

Spherical Aberration

Spherical mirrors have an aberration. There is an intrinsic defect with any mirror that takes on the shape of a sphere. This defect prohibits the mirror from focusing the entire incident light from the same location on an object to a precise point. The defect is most noticeable for light rays striking the outer edges of the mirror. Rays that strike the outer edges of the mirror fail to focus in the same precise location as light rays that strike the inner portions of the mirror. While light rays originating at the same location on an object reflect off the mirror and focus to a point, any light rays striking the edges of the mirror fail to focus at that same point. The result is that the images of objects as seen in spherical mirrors are often blurry.

Spin

It is a quantum mechanical phenomenon attributed to elementary particles, which is intrinsic form of angular momentum. All elementary particles of a given kind have the same magnitude of spin angular momentum, which is indicated by assigning the particle a spin quantum number. However, in a technical sense, spins are not strictly vectors and they are instead described as a related quantity: a Spinor.

Spin Magnetic Moment

Magnetic moment associated with spinning electron.

Spinor

Spinors were discovered by Élie Cartan in 1913. Later, spinors were adopted by quantum mechanics in order to study the properties of the intrinsic angular momentum of the electron and other fermions. In mathematics and physics, in particular in the theory of the orthogonal groups (such as the rotation or the Lorentz groups), Spinors are elements of a complex vector space introduced to expand the notion of spatial vector. Unlike tensors, the space of Spinors cannot be built up in a unique and natural way from spatial vectors. However, Spinors transform well under the infinitesimal orthogonal transformations (like infinitesimal rotations or infinitesimal Lorentz transformations). Under the full orthogonal group, however, they do not quite transform well, but only "up to a sign". This means that a 360 degree rotation transforms a Spinor into its negative, and so it takes a rotation of 720 degrees for a Spinor to be transformed into itself. Specifically, Spinors are objects associated to a vector space with a quadratic form (like Euclidean space with the standard metric or Minkowski space with the Lorentz metric), and are realized as elements of representation spaces of Clifford algebras.

Spin Quantum Number                                    

It is one of the four quantum numbers associated with energy levels of electrons in an atom. It has either +1/2 or -1/2 value.

Spin-Orbit Coupling

See L-S coupling

Spontaneous Fission

Spontaneous fission is a form of radioactive decay where an atom's nucleus splits into two smaller nuclei and generally one or more neutrons, without any external interference.

Spontaneous Emission

An atom in an excited state can decay down to a lower state by emitting a photon with energy equal to the difference between the initial higher energy level and the final lower energy level. When this process takes place naturally, rather than being initiated by disturbing the atom somehow, it is called spontaneous emission.

Sputtering

The process in which a surface is bombarded with energetic particle to cause ejection of surface atoms. The technique is used to make thin films on a substrate.

Square Wave

A square wave is a type of periodic waveform where the signal has only two levels. The signal switches between these levels at regular intervals and the switching is instantaneous. These qualities mean a graph of the wave over time will produce shapes with square corners. The square wave has practical uses in digital circuits.

Stabilized Transistor

The process of making operating point independent of temperature changes or variations in transistor parameters is known as stabilization.

Standard Deviation

The standard deviation is the square root of the variance.

or

The square root of arithmetic mean of the squares of deviation is called as standard deviation. 

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Specific Gravity

The relative density of a solid or liquid, usually when measured at a temperature of 20 ^oC, compared with maximum density of water (at 4^o C) or of a gas to an equal volume of air or Hydrogen under prescribed conditions of temperature and pressure.   

Specific Gravity of Fluid

 It is the ratio of weight of unit volume of fluid to unit volume of an equal volume of standard fluid, taken as water at 4 ^oC.

Specific Heat at Constant Pressure

Amount of heat required to raise the temperature of 1 mole of gas by 1^oC at constant pressure.

Specific Heat at Constant Volume

Amount of heat required to raise the temperature of 1 mole of gas by 1^oC at constant volume.

Specific Heat

Amount of heat required to raise the temperature of unit mass of material through one Kelvin.

C = Q/ M∆T; Q is heat required to raise temperature of mass M; and ∆T is raise in temperature. 

Specific Ionization

Specific ionization is that number of ion pairs produced per centimeter of travel of charged particle through matter. Specific ionization is dependent on the mass, charge, energy of the particle, and the electron density of matter. The greater the mass of a particle, the more interactions it produces in a given distance.

Specific Latent Heat

 The amount of heat required to change the state of unit mass of a substance without any change in temperature.   

Specific Latent Heat of Fusion

The quantity of heat necessary to convert unit mass of a solid substance into a liquid at constant temperature is called specific heat of melting or fusion. It is measured in JKg^-1 in MKS or Calgm^-1 in CGS and denoted by L.

Specific Latent Heat of Vaporization

The quantity of heat necessary to evaporate unit mass of a liquid without any change in its temperature is known as specific heat of vaporization.

Specific Strength

The ratio of tensile strength to specific gravity for a material.

Specific Volume

Specific volume represents the space occupied by a unit weight of dry air, in ft³/lb, and is equal to 1/air density.

Spectral Emissive Power

The amount of radiant energy emitted in a range of unit wavelength at a wavelength ‘λ’ per second per unit area of a surface is called spectral emissive power at wavelength ‘λ’ of that surface.

Spectral Energy Density

The radiant energy enclosed per unit volume in a unit wavelength range for a particular wavelength is called spectral energy density.

Spectral Line

An isolated bright or dark line in a spectrograph produced by emission or absorption of light of a single wavelength is called as spectral line. A spectral line is like a fingerprint that can be used to identify the atoms, elements or molecules present in a star, galaxy or cloud of interstellar gas.

Spectral Resolution

 Ability of optical system to distinguish spectral lines corresponding to two very close wavelengths of a light source in its spectrum is called spectral resolution.

Spectral Series Series of lines in the spectrum of light emitted by excited atoms of an element. Each line being related to the others in the series by a simple numerical equation and identified with a particular energy level of an atom  of  the element.

Spectrometer

The basic function of a spectrometer is to take in light, break it into its spectral components, digitize the signal as a function of wavelength, and read it out and display it through a computer. The first step in this process is to direct light through a fiber optic cable into the spectrometer through a narrow aperture known as an entrance slit. The slit splits the light as it enters the spectrometer. In most spectrometers, the divergent light is then collimated by a concave mirror and directed onto a grating. The grating then disperses the spectral components of the light at slightly varying angles, which is then focused by a second concave mirror and imaged onto the detector. Once the light is imaged onto the detector the photons are then converted into electrons which are digitized and readout through a USB (or serial port) to a computer. The software then interpolates the signal based on the number of pixels in the detector and the linear dispersion of the diffraction grating to create a calibration that enables the data to be plotted as a function of wavelength over the given spectral range. 

Spectrophotometer

A spectrophotometer is employed to measure the amount of light that a sample absorbs. The instrument operates by passing a beam of light through a sample and measuring the intensity of light reaching a detector. According to Beer's law, the amount of light absorbed by a medium is proportional to the concentration of the absorbing material. In general, it is an instrument for producing or recording a spectrum and measuring the photometric intensity of each wavelength present.