Physics Dictionary (Contd...)

Anelasticity

In some engineering materials, there also exists time dependent elastic strain component. That is elastic deformation will continue even after stress application and upon load release, some finite time is required for complete recovery. This time dependant elastic behavior is known as anelasticity.    ther particles and with electric and magnetic fi

Anemometer

The term is derived from Greek word “anemos”, meaning wind. It is device used for measuring speed of air flow in wind tunnels, and in other gas flow applications.

Angle

The amount of space between two straight lines having a common end point usually measured in degrees.

Angle of Incidence

Angle between incidence ray and normal to the surface is called angle of incidence.

Angle of Minimum Deviation

When the angle of incidence in a prism is increased, the value of angle of deviation first decreases and then increases. The minimum value of deviation is called the angle of deviation.

Angle of Reflection

It is the angle between reflected ray and normal to the surface, from which light is reflected.

Angle of Refraction

It is the angle between refracted ray and the surface from which light is refracted.

Angstrom

It is the unit of length usually used to express sizes at atomic scale. It is equal to 10^-10 meter.

Angular Acceleration

It is defined as the rate of change of angular velocity.

Angular Aperture

Angular aperture of an objective is largest angular extent of wave surface which it can transmit.

Angular Displacement

When a particle describes circular path, the angle described by the radius vector in a given time interval is called angular displacement. Its direction is perpendicular to plane of rotation along the axis.

Angular Momentum

It is the ability of a body by virtue of which it imparts rotatory motion to other bodies. Angular momentum is product of linear momentum and perpendicular distance of direction of linear momentum from center of rotation.

Angular velocity

It is defined as rate of change of angular displacement.

Anisotropy

It is the phenomenon of exhibiting different values of a physical property in different crystallographic directions.

Annealing

It is the process of heat treatment of metal by which microstructure and consequently properties of a material are altered. Annealing is generally heating of metal to specific temperature, for certain duration and then allowed to cool slowly thus softening by re-crystallizing. Cold worked metals usually undergo this for relieving strain.

Anode Rays

Rays of positively charged particles formed inside gas discharge tubes. These rays were discovered by Goldstein in 1886.

Anode

It is the electrode in an electrochemical cell that experiences oxidation, or gives up electron.

Anodizing

It is an electrochemical metal finishing process used for increasing thickness and toughness of the naturally occurring protective oxides on surface of metals. The process is called anodizing, as the part to be chemically treated forms the anode electrode of electric circuit. The process increases non-corrosiveness & non-conductivity. 

Anomalous Dielectric Dispersion

The fall in permittivity of a dielectric material with increasing frequency of external electric field is a phenomenon called anomalous dielectric dispersion. 

Anomalous Expansion of Water

When temperature of water is raised from 0 ^oC, its volume decreases up to 4 ^oC and above 4 ^oC its volume increases. This is called anomalous expansion. It is due to formation of more number of Hydrogen bonds.

Anomalous Zeeman Effect

When spectral lines of an atom split into three or more unequally spaced lines in the presence of magnetic field then the phenomenon is called as anomalous Zeeman effect. This usually happens for the systems where net spin i.e. spin quantum number is not zero.

Antenna

A type of transducer that converts RF fields into alternating current or vice versa. A receiving antenna intercepts RF energy and delivers oscillating electric current to electronic equipment, and the transmitting antenna is fed with electric current to generate RF field. They consist of metallic conductors, electrically connected to transistor or receiver.

Anti Ferromagnetism

Phenomenon of magnetic moment (spin moments) of neighboring atoms or ions aligned in exactly opposite directions.

ELECTROMAGNETIC SPECTRUM DETAILS

GAMMA RAYS
	Wavelength range: 10-14 to 10-10  meter  Frequency range: 3x1018 to 3 x 1022 Hz Production: Nuclear origin. Emitted on disintegration of Nuclei of atoms. Properties: Highly penetrating and uncharged. Exhibits Fluorescence, Phosphorescence, ionization and chemical reaction on photographic plates. Uses: gives information about structure of nuclei.
X-rays
	Wavelength range: 10-10 to 10-8  meter  Frequency range: 3x1016 to 3 x 1018 Hz Production: by striking high speed electrons on heavy target. Properties: All properties of gamma rays but less penetrating. Uses: helpful in medical diagnosis, study of crystal structure.
U V REGION
	Wavelength range: 10-8 to 4x10-7  meter  Frequency range: 8x1014 to 3 x 1016 Hz Production: by sun, arc, spark and ionized gases. Properties: All properties of gamma rays but less penetrating. They can produce photoelectric effect. Uses: used in medical applications. Detection of finger prints, forged documents.
VISIBLE
	Wavelength range: 4x10-7 to 7.8x10-7  meter  Frequency range: 4x1014 to 8 x 1014 Hz Production: radiated from ionized gases and incandescent bodies. Properties: exhibit reflection, refraction, interference, diffraction, polarization, photoelectric effect, sensation of sight. Uses: used in LASER technology.
INFRARED
	Wavelength range: 7.8x10-7 to 10-3  meter  Frequency range: 3x1011 to 4 x 1014 Hz Production: by hot bodies Properties: heating effect on thermo piles and bolometer. Exhibit reflection, refraction, diffraction and photographic action. Uses: used in industry, astronomy & medicine etc.
HERTZIAN WAVES/ MICROWAVES
	Wavelength range: 10-3 to 1 meter.  Frequency range: 109 to 3 x 1011 Hz Production: by spark discharge, by electronic devices such as Klystron & Magnetron. Properties: Reflection, Refraction & Diffraction. Produces spark in gaps and receiving circuits. Uses: used in Radar and MASERs and also to reveal finer details of atomic and molecular structure.
RADIO WAVES
	Wavelength range: 1 TO 104 meter. Frequency range: 10 KHz to 30 GHz Production: oscillating circuits and electronic devices. Properties: exhibits Reflection, Refraction & Diffraction. Uses: used in television and radio broadcast system.

How does mass depend on speed?

In order to answer this question, we have to consider two situations:

a) The object could be at rest in our reference frame.

b) The "object" is never at rest in any (physically realizable) frame of reference.

An electron and a tennis ball belong to situation (a), where as a photon belongs to situation (b)

Let us assume that two masses exist, the mass when object has zero speed 'm0' and mass 'm' when object has speed 'V' as observed in our frame of reference.

When the object has zero speed, it is at rest (as observed by us or by someone accompanying the object), and so 'm0' is called the object's rest mass. It is an intrinsic property of the object.

For instance every electron has rest mass m0=9.11 x 10^-31 Kg.

If we set into motion a tennis ball that is initially at rest, the ball acquires kinetic energy, the energy associated with motion.Its energy increases.

From the equation  E=mC2; it implies that increase in energy will correspond to increase in mass too. We deduce that ball's inertia increases. Thus the mass 'm' of moving ball is greater than the rest mass 'm0'. Hence m>m0 holds for any object with kinetic energy.

In 1905, Einstein summarized his theoretical discovery with sentence,

" The mass (i.e. the inertia) of a body is a measure of its energy content.  

 

What is wave motion?

Wave motion, in general refers to transfer of energy from one point to other point of medium.

For transfer of energy through a medium, the medium must possess the properties of
i) Elasticity ii) Inertia & iii) Negligible frictional resistance

and now the question is what propagates in wave motion?

It is not the matter that is propagated but it is only state of motion of matter that is propagated.
It may be said that in wave motion momentum and energy are transferred or propagated.

What is grounding?

Grounding is one of primary ways to minimize unwanted noise and pickup.

There are two basic objectives involved in designing good grounding systems.

i) To minimize noise voltage generated by currents from two or more circuits flowing thorugh a common ground impedance.

ii) To avoid creating ground loops which are susceptible to magnetic fields and differences in ground potential.

Grounding, if done improperly however, can become a primary means of noise coupling.

In most general sense a ground can be defined as equipotential point or plane which serves as a reference voltage for a circuit or system. By equipotential point we mean the point where voltage does not change regardless of the amount of current supplied to it or drawn from it. 

If the ground is connected to earth through a low impedance path, it can then be called an earth ground.

There are two common reasons for grounding a circuit:

i) For safety

ii) To provide an equipotential reference for signal voltages

Safety grounds are always at earth potential where as signal grounds are usually but not necessarily at earth potential.

In many cases, a safety ground is required at a point which is unsuitable for a signal ground, and this may complicate noise problem. 

  

Discovery of X-rays

Wilhelm Roentgen was professor of physics at university of Wurzburg, Germany when he discovered X-rays in 1985. The discovery was entirely serendipitous; Roentgen was merely studying a beam of electrons in a highly evacuated glass vessel. When the electrons, moving at great speed slammed into glass wall, they produced a very high penetrating radiation - a wholly unexpected occurrence. Roentgen first noticed the radiation when it caused a paper coated with Barium Platino-cyanide to glow. The chemical compound was a standard detector of UV light which causes the chemical to fluorescence i.e. to emit visible light after it has absorbed UV light. But Roentgen's evacuated vessel was tightly covered with black cardboard and so no UV light could emerge from it. The glow must be some other kind of radiation.

When he announced the discovery of the new radiation, Roentgen wrote:

"I posesss, for instance, photographs of ............the shadow of bones of hand, the shadow of a covered wire enclosed in a box.........."

Earlier in the paper, he noted that "the darker shadow of bones is seen with in the slightly dark shadow image of hand itself.

The new radiation quickly became a diagnostic tool in hospitals all over the world. Roentgen could not determine what the rays are made of and thus rays are named as X-rays.