PHYSICS DICTIONARY

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Electronegativity

Chemical property which describes tendency of an atom to accept electrons.

Electroplating

 See electro deposition.

Electroscope

An instrument which was first invented by William Gilbert in 16^th century, used to detect electric charge and measure its magnitude.

Electrostatic Precipitation

A process that removes suspended dust particles from a gas by applying electrostatic charge to the particles and collecting them on charged plates.

Electrochemical Cell

Arrangement in which two different metals are placed in conducting liquid called as electrolyte, with electrical potential difference between them. A pair of different metals arranged in this way is an electrochemical cell. This arrangement transfers chemical energy to electrical energy.

Electrodynamics

The quantum mechanical laws combined with laws of electricity came to be known as special branch called electrodynamics.

Electrolysis

The phenomenon which occurs in electrolytic cells involving breaking of electrolytes when electric current is passed through them.

Electrolyte

A solution through which an electric current may be generated by motion of ions.

Electromagnetic Radiation

Radiation which propagates in the form of wave in which electric field and magnetic field lies perpendicular to each other and also to the direction of propagation.

Electromagnet

It is a device consisting of a core of magnetic material such as iron, surrounded by a coil through which an electric current is passed to magnetize the core. Electromagnets are particularly useful where ever controllable magnets are required, as in devices in which the magnetic field is to be varied, reversed, or switched on and off. Other devices that utilize electromagnets include particle accelerators, telephone receivers, loudspeakers, and televisions.

Electromagnetic Induction

Phenomenon in electromagnetics as per which “any electric field that changes over time will produce a magnetic field in the space around it” and “any magnetic field that changes over time will produce an electric field in the space around it “.

 

Electromagnetic Interaction

The type of interaction between two charged particles by exchange of photons is called as electromagnetic interaction. The strength of electromagnetic interaction is given by dimensionless fine structure constant α = 1/137 and is due to photon exchanges. 

Electromagnetic Spectrum

Electromagnetic radiation fall in wide range from gamma rays (emitted by radioactive materials) having wavelengths on the order of 10^-14 meter, through x-rays, UV, Visible, Infrared and radio waves with wavelengths as long as 10⁵ meter.

Electromagnetic Theory

The theory which aims at explaining combined effect of electric & magnetic fields. This theory considers electric & magnetic force as different facets of more fundamental force. The most basic idea in electromagnetic theory is “a changing electric field generates a magnetic field” and “a changing magnetic field generates a electric field.” These principles are quantified by Maxwell’s equations, named for James Clerk Maxwell, the Scottish physicist and mathematician whose work in the 19th century established the discipline by revolutionizing how physicists conceived of light. The success of electromagnetic theory led to the collapse of the rest of Newtonian physics in the 20th century.

Electromagnetism

It is branch of physics which deals with electricity and magnetism, their interaction with each other and with electric charges and currents. Michael Faraday showed in 1831, that a changing magnetic field can induce a current in a circuit, and James Clerk Maxwell predicted that a changing electric field has an associated magnetic field.

Electrometer

High sensitive instrument used to measure electric charge or potential difference.

Electromotive Force

It is the work done on unit positive charge in taking it through low potential end to high potential end.

Electromotive Series

A ranking of metallic elements according to their standard electrochemical cell potentials.

Electron

It is an elementary particle classified as Lepton, having spin ½, negatively charged and which interacts weakly with other particles. 

(or)

It is a subatomic particle which carries a negative charge and is considered as the basic unit of electric charge. The rest mass of the electron is 9.109 × 10⁻³¹ kg. It is the basic unit of electric charge having value of 1.6 x 10^-19 Coulomb.

The Davisson-Germer Experiment

The experiment gave the evidence of wave nature possession by materialistic particle(electron) for the first time.

The arrangement of equipment used for the experiment is as follows

                      Procedure:

An electron gun is used in order to produce electrons. The electrons so produced are accelerated by applying a high potential towards the target crystal, in this case the target crystal is nicker. The accelerated electron beam is made into fine beam by passing it through a Collimator ‘c’.

The crystal is mounted on an arrangement which could be rotated in different directions perpendicular to the plane of diagram.

The electrons are scattered in all directions by atomic planes of crystal.

The intensity of electron beam ( no. Of electrons) scattered in a particular direction is measured by electron collector which can be rotated about the same axis as target crystal.

The collector is connected to a sensitive Galvanometer whose deflection is proportional to intensity of electron beam entering collector . The electron collector is also called Faraday cylinder.

A retarding potential is applied to Faraday cylinder such that only fast electrons can reach it and secondary electrons emitted from crystals are stopped.

A graph is then plotted between galvanometer current against angle ‘θ' between incident beam and diffracted beam i.e, beam entering Faraday cylinder.



In the investigation , the electron beam accelerated by 54V and at an angle of 50 between incident and diffracted beam , a sharp maximum has occurred in electron distribution.

The incident beam and diffracted beam in this experiment make an angle of 65⁰   with Braggs plane. 

For a 54 V electron , the de-broglie wavelength associated with the electron is given by

ƛ = 12.25/√V = 12.25/√54 A⁰ = 1.66 A⁰ 

Now from Bragg’s equation for maxima in diffraction pattern for same energy electrons

2d sinθ' = nƛ; 2*0.91*10^-10*sin 65⁰ = 1*ƛ;

ƛ = 1.65 A⁰ 

Thus, both theoretical and experimental values are in excellent agreement.

Thus Davission - Germer experiment provides a direct verification of de-broglie hypothesis of wave nature of moving particles.

BLOCH THEOREM

Bloch assumed that electrons move in a perfect periodic potential. He considered one dimensional array of lattice. The potential of electron at positive ion site is zero and is maximum in between. So long any line passing through the centers of positive ions, the potential variation must be as shown in below figure.

So Bloch gave a condition which is

𝚿(x+Na)=𝚿(x) .............................................................................................................(1)

It is considered as boundary condition.

Consider Schrodinger wave equation for one dimensional lattice.

(d²𝚿(x)/dx²) + (2m/ħ²)*[E-V(x)]*𝚿(x) = 0  .................................................................(2)

The Schrodinger equation for an electron in the potential at x+a is

[d²𝚿(x+a)/d(x+a)²] + (2m/ħ²)*[E-V(x+a)]*𝚿(x+a) = 0  ..............................................(3)

Because of periodicity,

[d/d(x+a)] = d/dx  ; V(x+a) = V(x)

With  this, eqn (3) reduces to

[d²𝚿(x+a)/dx²] + (2m/ħ²)*[E-V(x+a)]*𝚿(x+a) = 0  ...................................................(4)

This is Schrodinger  equation at x+a.

as 𝚿 at x+a is also obeying Schrodinger wave equation as 𝚿 at x there should exist a relation between 𝚿(x+a) & 𝚿(x).

Let    𝚿(x+a) = A𝚿(x)..................................................................................................(5)

𝚿(x+2a) = A²𝚿(x) [i.e. A𝚿(x+a) = A.A𝚿(x) = A²𝚿(x)]

𝚿(x+na) = Aⁿ𝚿(x)

from eqn(1),  Aⁿ =1 [i.e by using bloch condition]

Aⁿ =exp(2πij) [i.e. exp(2πij) =1 for j=01,2............]

or

A=exp(2πij/n)

Therefore,  𝚿(x+a) = exp(2πij/n)*𝚿(x) --------------------------------------------------(6) [ from eqn 5]

𝚿(x) can be written in terms of other function Uk(x )

𝚿(x) = exp(ikx)*Uk(x) where k=(2πj/n) ..................................................................(7)

From eqns (6) & (7),

exp[ik(x+a)]*Uk(x+a) = exp(2πij/n)*exp(ikx)*Uk(x)

exp[ika]*Uk(x+a) = exp(2πij/n)*Uk(x)

noting that  Ka = 2πj/n,

we can write that  Uk(x+a) = Uk(x) ..........................................................................(8)

Conclusion

Bloch Theorem is a mathematical theorem and it gives us the form of electron wave function in a periodic potential.

 𝚿(x) = exp(ikx)*Uk(x) represents Plane Wave

Thus, electron in a one dimensional lattice behaves a a plane wave.It only gives Wave nature of electron.

Pair Annihilation

Positron and Electron coalesce to produce atleast two photons

e⁺ + e⁻→2𝛾

Annihilation into three or more Photons is possible but less likely. Each extra photon tends to supress the rate of annihilation by a factor of order of magnitude of fine structure constant 1/137.

A Positron moves thru matter and forms ion pairs giving up energy in the process. There is about 2% chance that a Positron will hit an electron and annihilate.

But more likely output is that Positron will stop and become attracted to an electron. The atom formed by these two particles is called Positronium.

The Positron-Electron system drops into successively lower energy states, emitting (low energy) photons, until it arrives in ground state.

Nuclear Changes