PHYSICS DICTIONARY

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Nano Materials

Materials whose dimensions are less than 100 nm.

Natural Frequency

 Frequency of system when set in free vibration.

Natural Logarithm

A logarithm to the base ‘e’ which is irrational and whose value is equal to 2.71829. It is represented as log _eX_.. It is also called as Napier logarithm.

Nebula

 A visible, thinly spread cloud of interstellar gas and dust. Some nebulae are remnants of supernova explosion others are gravity induced condensations of gases in interstellar medium which in certain cases may become a site for formation of new stars. The term was formerly used of any hazy , seemingly cloud like object, including what are now recognized as other galaxies beyond milky way, it is restricted now to actual clouds of gas and dust within our own galaxy.

Negative Feedback

When feedback energy is out of phase with input signal and opposite to it, then it is called negative effect.

Negatron

Term used for electron when it is necessary to distinguish between electrons and positrons.

Neptunium Series

It is a radioactive series that does not now occur in nature and that begins with Plutonium of mass number-241 and continues to Americium to the longer lived member of series i.e. Neptunium of mass number 237, and eventually to stable end product Bismuth. 

Neutrino

An elementary particle introduced by Pauli in 1952. It was introduced in weak interactions involving beta decay to avoid violation of law of conservation of energy and angular momentum. It has solved mystery of variable energy of beta particles in beta decays.

 

Neutron Diffraction

 It is a non destructive technique that is used to probe the structure of materials at the atomic level. The sample is placed within a neutron beam and the angles at which the neutrons are deflected or scattered by material are recorded to generate a  “diffraction pattern” from which structural information can be extracted. Diffraction occurs when the neutrons encounter atomic nuclei or magnetic diploes within the sample. With an effective wavelength of 0.1-20 nm, thermal neutrons are ideally suited for probing atomic scale structures with high resolution. As neutrons do not possess an electric charge, they readily pass through matter regardless of local charge distribution. In consequence, a neutron beam is capable of penetrating well beyond surface of a sample, to a depth of few centimeters in most condensed phases. The technique is widely used in engineering for stress mapping and to examine mechanical behavior of materials and engineering components.

Neutron Flux

The neutron flux (φ) measures the intensity of neutrons passing through a cubic centimeter of material. It is given by: φ= nv; where n is the density of neutrons (the number of neutrons per cm3) and v is the speed of the neutrons. The unit for neutron flux is neutrons.cm^-2.sec^-1.

In physical terms, the quantity ‘φ’ is the total distance traveled in one second by all the neutrons in the one cm³ volume, since it is obtained by multiplying the number of neutrons in that cm³ by the speed each is travelling. This is equivalent to the total length of all the neutron tracks laid down in one cm³ in one second. The expression for neutron flux applies to any neutron energy. When applied to thermal neutrons the product is known as the thermal neutron flux.  

Newton

It is SI unit of force. It is force required to accelerate a mass of one kilogram one meter per second per second. 

Newton’s First Law

Everybody continues in its state of rest or of uniform motion in a straight line unless it is compelled by an external force to change that state.

Newton’s Law of Cooling

The rate of loss of heat of a hot body (or rate of cooling of body) is directly proportional to mean excess of temperature of hot body over that of its surroundings.

Newton’s Second Law

The rate of change of momentum of a body is directly proportional to external force acting on it and takes place in the direction of force.

Newton’s Third Law

For every action, there is equal and opposite reaction.

 

Paulis Hypothesis of Beta Decay

Pauli introduced concept of third particle, a neutral particle which gets emitted in β⁻decay. This particle has the generic name of Neutrino.

The total energy is shared by 3 particles

The recoil nucleus
The Electron
The Neutrino

Because of its comparatively great mass, the recoil energy of Nucleus is very small and nearly all Kinetic Energy is shared between the Beta particle and the Neutrino.

In addition to laws of conservation of charge and energy, we must also apply the laws of conservation Linear and angular momentum to every nuclear process. Taking our reference system as the parent nucleus at rest, the vector sum of Linear Momenta of the recoil nucleus, the beta particle and neutrino must be zero.

To conserve angular momentum in β⁻decay, we note that parent and daughter nuclei are isobars; i.e. they have equal number of nucleons. Hence, the total change in nuclear angular momenta will be either zero or an integral multiple of ℏ.

The beta particle has an intrinsic spin angular momenta of 1/2ℏ.The vector sum of angular momenta of Neutrino and beta particle will be either zero or one in units of ℏ.

The present accepted theory, which is supported by experimental evidence shows that there are two types of neutrino or two components of Neutrino. It has been found that the axis of spin of neutrino is parallel to its direction of motion; one type spins according to the left hand rule with respect to its direction of motion as its axis, the other component spins according to right hand rule.

The first type is usually called neutrino represented by symbol 𝜈, the second type is called antineutrino.
 
The spin vector of neutrino points opposite to direction of its motion.

The spin vector of anti neutrino points in the direction of its motion.

Another way of saying this is that the helicity of neutrino is negative and that of anti neutrino is positive or one has right handed helicity and other has left handed helicity.
 

PAULI's NEUTRINO HYPOTHESIS

Pauli postulated the existence of new particle, “Neutrino” as early as in 1930. According to Pauli, an additional particle called a neutrino denoted by ‘ν’ is emitted in process of β-decay.

This particle according to Pauli, carries away an amount of energy equal to difference between the observed energy for a β-particle and maximum energy of continuous beta spectrum. The principle of conservation of energy is thus satisfied.

To satisfy Principle of Conservation of Angular Momentum, Neutrino must be assigned following properties

i) It must have zero charge; because in a β-decay process the charge is conserved without neutrino. Also if Neutrino is charged, it would produce ionization which certainly could have been detected. Zero charge in turn implies negligible magnetic moment.

ii) It must have zero or almost zero mass: the mass-energy balance of β-decay processes shows that neutrino rest mass is negligible.

iii) It must have a spin of ½: This will satisfy Law of Conservation of Angular Momentum in β-decay process. Further Neutrino must be a Fermion, so that nuclear statistical requirements are fulfilled.

iv) A neutrino has an antiparticle called anti-neutrino which has zero rest mass, zero charge and spin-1/2.