PHYSICS DICTIONARY

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Electrostatics

Branch of physics which deals with charges permanently fixed in space or in steady flow in a circuit.

Electrostriction

It is phenomenon of elastic deformation of dielectrics in an electric field. Electrostrictive strain is proportional to square of electric field strength and is independent of reversal of direction of electric field. Electrostriction is caused by die electric polarization in an electric field and occurs in all dielectrics, whether solid, liquid or gaseous. In solid dielectrics, electrostriction is very small and is of no practical importance.  

Element

Substance consisting of only one type of atoms that have same atomic number.

Elementary Charge

Electric charge carried by a single proton or electron. It is fundamental physical constant. This charge has a measured value of approximately  

1.602176565x10^-19 coulombs. In CGS, system it is 4.80320425x10^-10 stat coulomb.

Elementary Particle

Particles which are considered as not having sub structure i.e. irreducible constituent of matter.

Elliptically Polarized Light

Due to the superposition of two plane polarized light waves with a definite phase difference, the resultant light vector revolves with a varying amplitude periodically in a plane perpendicular to direction of propagation of light and the tip of light vector describes an ellipse. Light is called elliptically polarized light. 

EMF

 It is the influence which maintains permanent potential difference between the terminals of source, ensuring continuous supply of current. EMF is electrical energy of non electro static origin.

Emissivity

The ability of a surface to emit radiant energy compared to that of a black body at the same temperature and with same area. It is defined as the total amount of radiant energy emitted per second per unit area by a surface is called total emissivity of that surface.

Emission Spectrum

It is of 3 types i) Continuous spectrum   ii) line spectrum & iii) Band spectrum

Empirical

Scientific information acquired by means of experience or observation or experimentation without using scientific method or theory.

Electromagnetic Unit (emu)

Any unit that belongs to system of CGS units for electricity and magnetism based on system of equations in which permeability of free space is taken as unity.  

Endoergic

 See Endothermic reaction.

Endothermic Process

Chemical reactions for which energy absorption is must. They cannot occur spontaneously. Work must be done in order to get these reactions to occur. When endothermic reactions absorb energy, a temperature drop is measured during the reaction. Endothermic reactions are characterized by positive heat flow and an increase in enthalpy.

Energy Band

In solids, when atoms come within close proximity of one another, electrons are acted upon, or perturbed, by the electrons and nuclei of adjacent atoms. This influence is such that each distinct atomic state may split into a series of closely spaced electron states in the solid, to form what is termed electron energy band. The extent of splitting depends on inter atomic separation and begins with outermost electron shells since they are first to be perturbed as atoms coalesce. Within each band, energy states are discrete.  

Energy Density

It is the amount of energy stored in a given system or region of space per unit volume or mass.

Energy Level

Quantized states for particles in atom or nuclei.

Energy Minimum Principle

For an isolated system in state of equilibrium, energy reaches a minimum value and after that remains constant.

Energy of Dissociation

It is the energy required to dissociate two atoms of molecule in to an infinite separation.

Energy

It is ability to do work. Energy is an attribute of a physical object or of whatever is contained in a specific region of space.

Enrichment

A process of isotopic separation by which relative abundance of isotopes of a given element is altered, thus producing a form of element that has been enriched in one or more isotopes and depleted in others.

Ensemble

An ensemble is defined as a collection of large number of microscopically identical but essentially independent systems. 

Brief Description of Maxwell's Equations

Electromagnetism - Detailed Explanation

Why do we call the subject electromagnetism?

 For stationary charges Coulomb’s law holds. But it is not true, when charges are moving the electrical forces depends also on motion of charges in a complicated way. One part of the force between moving charges we call the magnetic force. It is really one aspect of electric effect. This is why we call the subject Electromagnetism.

What is Field?

          A “FIELD” is any physical quantity which takes on different values at different points in spaces.

 

Example: -     Temperature --------  a scalar field T(x,y,z).

                         Velocity of a flowing fluid is vector field.

       The relationship between the values of field at one point and the values at a nearby point are very simple With only a few relationship in form of differential equations we can describe fields completely. It is in terms of such equations that laws of electrodynamics are most simply written.

Characteristics of vector field

There are two mathematically important properties of a vector field which we use in description of laws of electricity from field point of view.

·       The flux of vector field.

·       The circulation of vector field.

For an arbitrary closed surface, the next outward flow or flux is the average outward normal component of vector field times the area of surface.

Flux = (average normal component)*(surface area)

The second property of vector field has to do with a line rather than a surface. If there is a net rational motion around some loop then vector field is said to be circulating.

For any vector field the circulation around any imaginary closed curve is defined as average tangential component of vector multiplied by circumference of loop.

    Circulation = (average tangential component)*(distance around)

The properties flux and circulation can describe as laws of electricity and magnetism at once.

The laws of electromagnetism

The first law of electromagnetism describes the “flux” of electric field.

 

 If we have an arbitrary stationary curve in space and measure circulation of electric field around curve, we will find that it is not, in general, zero (although it is for coulomb field).         

For electricity there is a second law that states for any surface ‘S’ (not closed) whose edge is curve C,

We can complete laws of electromagnetic field by writing two corresponding equations for magnetic field B.

 

           Flux of B through any closed surface = 0 -----------(3)

For and surface bounded by curve ‘C’    

 

Constant  'c' appears in above equation because (“magnetism is in reality a relativistic effect of electricity”).

The force on charge in general in electromagnetism is given as follows –

 The second part of above equation can be demonstrated by passing a current through a wire which hangs above bar magnet

The wire will move when a current is turned on because of force

“F = qv*b”. When a current exists, the charges inside wire are moving, so they have a velocity ‘V’ and magnetic field from magnet exerts a force on them, which results in pushing wire sideways.

When the wire is pushed to left, we would expect that magnet must fell push to right to conserve momentum. Although the force is too small to make movement of bar magnet visible, a more sensitively supported magnet, like a compass needle, we will show movement.

How does the wire push on magnet?

The current in wire produces a magnetic field of its own that exerts a force on magnet. According to last term in equation (4), a current results in circulation of ‘B’ in this case lines of B are loops around wire as shown in figure. This ‘B’ is responsible for force on magnet.

Equation (4) tells us that for a fixed current through wire the circulation of ‘B’ is same for any curve that surrounds the wire.

For curves say circles that are farther away from wire, the circumference is larger, so tangential component of ‘B’ must decrease.

We have said that a current through a wire produces a magnet field, and that when there is a magnetic field present there is a force on a wire carrying a current.

Then we should also expect that if we make a magnetic field with a current in one wire, it should exert a force on another wire which also carries a current. When current are in same direction they attract each other.Electrical currents as well as magnets make magnetic fields.

But what is a magnet anyway

If magnetic fields are produced by moving charges, is it is not possible that magnetic fields from a piece of iron is really the result of current.

We can replace bar magnet of our experiment with a coil of wire carrying current. So a piece of iron acts as though it contains a perpetual circulating current. We can in fact understand magnets in terms of permanent currents in atoms of iron.

Where do these currents come from?

One possibility would be from motion of electrons in atomic orbits. Actually that is not case for iron, although it is for some materials. It is due to spin of electron and it is this current that gives magnetic field in iron.

 All the electromagnetism is contained in Maxwell equations.

Maxwell Equations

 

 

Now consider the ‘static’ case. All charges are permanently fixed in space or if they do not move, they move as a steady flow in a circuit.

In these circumstances, all of the terms in the Maxwell equations which are time derivatives of field are zero. So Maxwell equations becomes

Electrostatics

Magnetostatics

This means that

The interdependence of E & B does not appear until there are changes in charges or currents, as when a condenser is charged or a magnet moved.

Only when there are sufficiently rapid changes, so that the time derivatives in Maxwell equations become significant, will E & B depend on each other.

Electrostatics is a clean example of vector field with zero curl and a given divergence.

Magnetostatics is a neat example of a field with zero divergence and a given curl.

How to find electric field at a point ‘P’ due to a charge distribution?

The electric field ‘E’ at point , from a charge distribution, is obtained from an integral over the distribution (shown in following figure).

 

 

The work done against the electric forces in carrying a charge along some path in the negative of component of electric force in direction of motion, integrated along the path. If we carry a charge from point ‘a’ to point ‘b’ then 

 

 F  = Electric force on charge at each point.

Ds = Differential vector displacement along path