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Tuesday, 16 May 2017

Superconductor

Superconductor
Defination-
When some material cold below the certain temperature  (called critical temp.) then the resistance dropped to zero.this type of material are called superconductor and this phenomenona is called superconductivity

What is a superconductor?
Superconductors are materials that conduct electricity with no resistance. This means that, unlike the more familiar conductors such as copper or steel, a superconductor can carry a current indefinitely without losing any energy. They also have several other very important properties, such as the fact that no magnetic field can exist within a superconductor.

Persistant current
When a superconductor placed in a magnetic field and the field is removed/switched off a current is induced in the superconductor.The magnitude of current remains constant even though there is no sources of emf as the resistance of the s.c. is zero.such steady current flowing into the s.c. are called persistent current. 

A superconductor is a prefect Di magnetic
The magnetic induction inside the material in normal state 
B= meu(H+M)
When temp less than tc than magnetic field inside the s.c.is zero so,
B=0
0=meu(H+M)
M=-H
Susecptibility=-1.               
Where Susecptibility=M/H
So, it show that superconductor is a perfect diamagnetic
Meisneer effect
The exclusion of magnetic line of forces from a superconductor materials when it is cooled below the critical temperature(t) in a magneticfield is called meisneer effect.

Type 1 & type 2 susuperconductor
Type I 
Type 1 super show complete meisneer effect .They have low critical temperatures, typically between 0 and 10 K (-273°C and -263°C respectively). As discussed above, this type experiences a sudden decrease in resistance as well as the complete expulsion of magnetic fields (perfectly diamagnetic) at critical temperature.They are also called soft superconductor.

Type I metals achieve superconductivity through slowing down molecular activity via low temperatures. According to BCS theory, this creates an environment conducive to Cooper pairing so that electron pairs are able to overcome molecular obstacles, leading to free electron flow without applied voltage.
Copper, silver, and gold are shown the type 1 superconductivity
 Type II
 Type 2 superconductor doesn't show complete meisneer effect.They are capable of superconductivity at much higher critical temperatures.Type II materials have critical temperatures within the 10-130 K range.
Type II materials also take on a mixed state, which contrasts with plunging resistance at Tc for Type I materials, when approaching their critical temperature. Mixed states are caused by the fact that Type II superconductors never completely expel magnetic fields.
Uses of type 1 and type 2 superconductor
1) it is used for produzction of high magnetic field in nuclear imagine system for medical dygnosis and high speed trains
2) they are used in particle acceleration
BCS theory
This theory is discovered by John cooper ,John berdean &John Rschiffer
In 1957.
This theory is based on two experimental results,the isotropic effect & variations of electronic specific heat with temperature
                  This theory is based on the interaction between two electrons through the intermediatory phonon.
When a free election approaches a positive ion in the lattice, there is a columb attraction force between two electrons and the lattice ion.this produces a lattice distortion which causes an increase in the density of ions in the region of distortion.the distorted region attract another free electron.thus a free electron exercts a small attractive force on another electron through phonons  which is  quanta if lattice vibration.
Cooper pair-----
A pair of Free electrons couple together through phonons is called cooper pair.
At normal temperature the columb repulsion between two electron is very large as compare to the electron phonon interaction force but at low temperature the electron-phonon interaction in a.c. is very large compared to columb repulsion between electron.
Formation of cooper pair
The probability of formation of cooper pair is very large when two electron have equal opposite menenta with opposite spin at T <critical temp.,the Cooper pair of electrons will have a particular property of smooth riding over the lattice inperfection without ever exchanging energy with them I.e. The Cooper pair are not scattered by the lattice point. 

Applications of superconductor
Superconductors are used in many fields: electricity, medical applications, electronics and even trains. They are used in laboratories, especially in particle accelerators, in astrophysics with the use of bolometers, in ultrasensitive magnetic detectors called SQUIDs, and in superconducting coils to produce very strong magnetic fields.

Magnetic Resonance Imaging (MRI)


Efficient Electricity Transportation
MRI is a technique developed in the 1940s that allows doctors to see what is happening inside the body without directly performing surgery. The development of superconductors has improved the field of MRI as the superconducting magnet can be smaller and more efficient than an equivalent conventional magnet
Superconductors have many uses - the most obvious being as very efficient conductors; if the national grid were made of superconductors rather than aluminium, then the savings would be enormous - there would be no need to transform the electricity to a higher voltage (this lowers the current, which reduces energy loss to heat) and then back down again.
Superconducting magnets are also more efficient in generating electricity than conventional copper wire generators - in fact, a superconducting generator about half the size of a copper wire generator is about 99% efficient; typical generators are around 50% efficient.

Superconducting Maglev Trains

While it is not practical to lay down superconducting rails, it is possible to construct a superconducting system onboard a train to repel conventional rails below it. The train would have to be moving to create the repulsion, but once moving would be supported with very little friction. There would be resistive loss of energy in the currents in the rails. Ohanian reports an engineering assessment that such superconducting trains would be much safer than conventional rail systems at 200 km/h.
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Friday, 5 May 2017

Thermodynamics

Thermodynamics

Thermodynamic

definations of thermodynamics

Thermodynamics is a branch of science concerned with heat and temperature and their relation to energy and work. The behavior of these quantities is governed by the four laws of thermodynamics, irrespective of the composition or specific properties of the material or system in question.
                                                                    or
thermodynamics is the science of energy equilibrium and entropy.
                                                                     or
Thermodynamics is the branch of physics that deals with the relationships between heat and other forms of energy. In particular, it describes how thermal energy is converted to and from other forms of energy and how it affects matter.
                                                                    or
some basic definations
macroscopic approach       -the approach for determining the behavier of systemin which our attention is foccused on certain quantity of matter without taking into account the event occuring at                                                                    moleculer level
microscopic approach-the approach for determining the behavier of the system in which matter is consider to be comprised of large number 
                                        of tiny particles or molecules which move randamly and then the overall behavier is predicted by averaging the                                                     behavier of individual particles or molecule is known as microscopic approach
system-   the part of the universe on which our consideration is focused for analysis of problem is called system
surrounding-the part of the universe excluding the systm is called surrounding
system boundry-  the boundry which seperate the system and surrounding is called system boundry .it may be real or imagenary
                                                                     type of system
open system -a system which can exchange energy and mass with surrouning ,is called open system. e.g.-boiler turbine
closed system-the system which can exchange only energy but not mass with surrounding is called closed system. e.g-refergerator,battery
adiabatic system- the system which can neighter exchange energy nor mass with surrounding is called adiabatic system
                               it is also known isolated system.e.g-thermoflax                  
                                                                       concept  of continum
In macroscopic approach of study ,the matter is considered contiinious ,negleting the voids between molecule and the behavier of molecule 
 this is called continum
                                                                       thermodynamic equilibrium
if the system is simultaneously in a state of mechanical equlibrium,thermal equilibrium,chemical equilibrium ,then the system is said to be in equilibrium
                                                                         thermodynamic properties
1) intensive properties- the properties which are independent of massof the system is callled intensive properties. e.g.-press.,temp.,density
2)extensive properties- the properties which are inependent of massnof the system is called extensive properties. e.g.-                                                                                volume,entropy,enthalpy
path- the locus of series of state through which the system process in going initial state to final state is called path
process- the specified path on which the state is change is called path
                                                                          type of process
1 )isobaric process(pressure  constant)
2) isochoric process(volume constant)
3) isothermal process(temp. const)
4)adiabatic process(all constant)
5polytropic process(pv^n=const)
Quansi-statis process
the departure of state of system from thermodynamics equilibrium state will be inifinitisinially small ,so the every state will be an equilibrium  state. the locus osf series of such equilibrium state is calle quasi-static state.
*it is represented graphically continious line 
*Infinitly slowness is characteristic of quasi-static process
*a quasi-static process is succection process of equilibrium state
reversible process
A thermodynamics process is reversesable process if the system pass through continious series of equilibrium state 
assumptions
*there should not be friction (solid or liquid)
*heat exchange to or from the system ,if it should be only due to infinitly small temperature difference
*the process should be quasi-static
ex- frictionless adiabatic process
irriversible process 
the process is irriversible if the system is passes through a sequence of non  equilibrium state.when irriversible peocess is maid to proceeed in a backword direction the original state of the system is not restored and irrivesible process can"t be represented by a continious line or digram
Thermal energy is the energy a substance or system has due to its temperature, i.e., the energy of moving or vibrating molecules,Thermodynamics involves measuring this energy, which can be "exceedingly complicated," according to David McKee, a professor of physics at Missouri Southern State University. "The systems that we study in thermodynamics … consist of very large numbers of atoms or molecules interacting in complicated ways. But, if these systems meet the right criteria, which we call equilibrium, they can be described with a very small number of measurements or numbers. Often this is idealized as the mass of the system, the pressure of the system, and the volume of the system, or some other equivalent set of numbers. Three numbers describe 1026 or 1030 nominal independent variables." 

Heat

Thermodynamics, then, is concerned with several properties of matter; foremost among these is heat. Heat is energy transferred between substances or systems due to a temperature difference between them, according to Energy Education. As a form of energy, heat is conserved, i.e., it cannot be created or destroyed. It can, however, be transferred from one place to another. Heat can also be converted to and from other forms of energy. For example, a steam turbine can convert heat to kinetic energy to run a generator that converts kinetic energy to electrical energy. A light bulb can convert this electrical energy to electromagnetic radiation (light), which, when absorbed by a surface, is converted back into heat. 

Temperature

The amount of heat transferred by a substance depends on the speed and number of atoms or molecules in motion, according to Energy Education. The faster the atoms or molecules move, the higher the temperature, and the more atoms or molecules that are in motion, the greater the quantity of heat they transfer.
Temperature is "a measure of the average kinetic energy of the particles in a sample of matter, expressed in terms of units or degrees designated on a standard scale. The most commonly used temperature scale is Celsius, which is based on the freezing and boiling points of water, assigning respective values of 0 degrees C and 100 degrees C. The Fahrenheit scale is also based on the freezing and boiling points of water which have assigned values of 32 F and 212 F, respectively.
Scientists worldwide, however, use the Kelvin (K with no degree sign) scale, named after William Thomson, 1st Baron Kelvin, because it works in calculations. This scale uses the same increment as the Celsius scale, i.e., a temperature change of 1 C is equal to 1 K. However, the Kelvin scale starts at absolute zero, the temperature at which there is a total absence of heat energy and all molecular motion stops. A temperature of 0 K is equal to minus 459.67 F or minus 273.15 C. 

Specific heat

The amount of heat required to increase the temperature of a certain mass of a substance by a certain amount is called specific heat, or specific heat capacity, according to Wolfram Research. The conventional unit for this is calories per gram per kelvin. The calorie is defined as the amount of heat energy required to raise the temperature of 1 gram of water at 4 C by 1 degree. 
The specific heat of a metal depends almost entirely on the number of atoms in the sample, not its mass.  For instance, a kilogram of aluminum can absorb about seven times more heat than a kilogram of lead. However, lead atoms can absorb only about 8 percent more heat than an equal number of aluminum atoms. A given mass of water, however, can absorb nearly five times as much heat as an equal mass of aluminum. The specific heat of a gas is more complex and depends on whether it is measured at constant pressure or constant volume.

Thermal conductivity

Thermal conductivity (k) is “the rate at which heat passes through a specified material, expressed as the amount of heat that flows per unit time through a unit area with a temperature gradient of one degree per unit distance,” according to the Oxford Dictionary. The unit for k is watts (W) per meter (m) per kelvin (K). Values of k for metals such as copper and silver are relatively high at 401 and 428 W/m·K, respectively. This property makes these materials useful for automobile radiators and cooling fins for computer chips because they can carry away heat quickly and exchange it with the environment. The highest value of k for any natural substance is diamond at 2,200 W/m·K.
Other materials are useful because they are extremely poor conductors of heat; this property is referred to as thermal resistance, or R-value, which describes the rate at which heat is transmitted through the material. These materials, such as rock wool, goose down and Styrofoam, are used for insulation in exterior building walls, winter coats and thermal coffee mugs. R-value is given in units of square feet times degrees Fahrenheit times hours per British thermal unit  (ft2·°F·h/Btu) for a 1-inch-thick slab.

Newton's Law of Cooling

In 1701, Sir Isaac Newton first stated his Law of Cooling in a short article titled "Scala graduum Caloris" ("A Scale of the Degrees of Heat") in the Philosophical Transactions of the Royal Society. Newton's statement of the law translates from the original Latin as, "the excess of the degrees of the heat ... were in geometrical progression when the times are in an arithmetical progression." Worcester Polytechnic Institute gives a more modern version of the law as "the rate of change of temperature is proportional to the difference between the temperature of the object and that of the surrounding environment." 
This results in an exponential decay in the temperature difference. For example, if a warm object is placed in a cold bath, within a certain length of time, the difference in their temperatures will decrease by half. Then in that same length of time, the remaining difference will again decrease by half. This repeated halving of the temperature difference will continue at equal time intervals until it becomes too small to measure.

Heat transfer

Heat can be transferred from one body to another or between a body and the environment by three different means: conduction, convection and radiation. Conduction is the transfer of energy through a solid material. Conduction between bodies occurs when they are in direct contact, and molecules transfer their energy across the interface. 
Convection is the transfer of heat to or from a fluid medium. Molecules in a gas or liquid in contact with a solid body transmit or absorb heat to or from that body and then move away, allowing other molecules to move into place and repeat the process. Efficiency can be improved by increasing the surface area to be heated or cooled, as with a radiator, and by forcing the fluid to move over the surface, as with a fan.
Radiation is the emission of electromagnetic (EM) energy, particularly infrared photons that carry heat energy. All matter emits and absorbs some EM radiation, the net amount of which determines whether this causes a loss or gain in heat. 

The Carnot cycle

In 1824, Nicolas Léonard Sadi Carnot proposed a model for a heat engine based on what has come to be known as the Carnot cycle. The cycle exploits the relationships among pressure, volume and temperature of gasses and how an input of energy can change form and do work outside the system.
Compressing a gas increases its temperature so it becomes hotter than its environment. Heat can then be removed from the hot gas using a heat exchanger. Then, allowing it to expand causes it to cool. This is the basic principle behind heat pumps used for heating, air conditioning and refrigeration.
Conversely, heating a gas increases its pressure, causing it to expand. The expansive pressure can then be used to drive a piston, thus converting heat energy into kinetic energy. This is the basic principle behind heat engines. 

Entropy

All thermodynamic systems generate waste heat. This waste results in an increase in entropy, which for a closed system is "a quantitative measure of the amount of thermal energy not available to do work," . Entropy in any closed system always increases; it never decreases. Additionally, moving parts produce waste heat due to friction, and radiative heat inevitably leaks from the system. 
This makes so-called perpetual motion machines impossible. Siabal Mitra, a professor of physics at Missouri State University, explains, "You cannot build an engine that is 100 percent efficient, which means you cannot build a perpetual motion machine. However, there are a lot of folks out there who still don't believe it, and there are people who are still trying to build perpetual motion machines."
Entropy is also defined as "a measure of the disorder or randomness in a closed system," which also inexorably increases. You can mix hot and cold water, but because a large cup of warm water is more disordered than two smaller cups containing hot and cold water, you can never separate it back into hot and cold without adding energy to the system. Put another way, you can’t unscramble an egg or remove cream from your coffee. While some processes appear to be completely reversible, in practice, none actually are. Entropy, therefore, provides us with an arrow of time: forward is the direction of increasing entropy.

The four laws of thermodynamics

The fundamental principles of thermodynamics were originally expressed in three laws. Later, it was determined that a more fundamental law had been neglected, apparently because it had seemed so obvious that it did not need to be stated explicitly. To form a complete set of rules, scientists decided this most fundamental law needed to be included. The problem, though, was that the first three laws had already been established and were well known by their assigned numbers. When faced with the prospect of renumbering the existing laws, which would cause considerable confusion, or placing the pre-eminent law at the end of the list, which would make no logical sense, a British physicist, Ralph H. Fowler, came up with an alternative that solved the dilemma: he called the new law the “Zeroth Law.” In brief, these laws are: 
The zeroth law -states that if two bodies are in thermal equilibrium with some third body, then they are also in equilibrium with each other. This establishes temperature as a fundamental and measurable property of matter. 
The first law  states that the total increase in the energy of a system is equal to the increase in thermal energy plus the work done on the system. This states that heat is a form of energy and is therefore subject to the principle of conservation.or we can say that it is a principle of conservation of energy.
the second law- states that heat energy cannot be transferred from a body at a lower temperature to a body at a higher temperature without the addition of energy. This is why it costs money to run an air conditioner.
The third law-  states that the entropy of a pure crystal at absolute zero is zero. As explained above, entropy is sometimes called "waste energy," i.e., energy that is unable to do work, and since there is no heat energy whatsoever at absolute zero, there can be no waste energy. Entropy is also a measure of the disorder in a system, and while a perfect crystal is by definition perfectly ordered, any positive value of temperature means there is motion within the crystal, which causes disorder. For these reasons, there can be no physical system with lower entropy, so entropy always has a positive value.the entropy of universe is always increases
The science of thermodynamics has been developed over centuries, and its principles apply to nearly every device ever invented. Its importance in modern technology cannot be overstated.
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Wednesday, 3 May 2017

Local field or internal fields

Local field or internal fields
DEFINATION
When a liquid or solid dielectric placed in an external electric field , it's​ atoms becomes electric dipole which provides field which is different than applied field.the total field at the atomic sites is called the internal field or local field.it is represented by Ei or EL
       E(local field)=E+p/3€
Expression for local field or internal field
Determination of local field or internal field
In this article we derive the proper expression for the local field  in the book for
the case of a uniform dielectric in a plane-plate capacitor. Consider the situation in
where we have a plane plate capacitor filled with a dielectric. We know
the average electric field in the dielectric, the field we have called E in the book.
E has two contributions
E = E0 + E1, (1)
where E0 = σ/.0 is the contribution from the charges on the plate of the capacitor
and E1 = −P/.0 is the contribution from the charges on the surface of the dielec-
tric. As discussed in the book, E1 opposes E0 und thus leads to a reduction of E.
In order to calculate the local field in the middle of the dielectric, we now “artifi-
cially” introduce two additional fields. Image a little sphere in the dielectric that is
small on the scale of the capacitor but large on an atomic scale. The sphere shall
be filled with dielectric as well, it is only an artificial construction to help with the
calculation because we can calculate the field from everywhere except the inside
of the sphere macroscopically and then the field from the inside of the sphere mi-
croscopically. We can write the total field at the point in the centre of the sphere
as
Eloc = E0 + E1 + E2 + E3 = E + E2 + E3, (2)
where E2 and E3 are the fields created by the charges on the surface of the sphere
and by the microscopic inside of the sphere, respectively.
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Tuesday, 2 May 2017

Nanoparticles and application

Nanoparticles and application

Nano particles

DEFINATION
particles that have at least one dimension that measures 100 nanometers or less are called nano particles.
The properties
of many conventional materials change when formed from nanoparticles. This is typically because nanoparticles have a greater surface area per weight than larger particles which causes them to be more reactive to some other molecules.
Nanoparticles are used, or being evaluated for use, in many fields. The list below introduces several of the uses under development.
Ex.- carbon nanotubes,fullerens etc
Properties of nano particles
1. Nano particles have size of 10^-9 m 
2. Tools of nanotechnology permit observation and meniculation to form new structures
3. The developed nano particles have mechanical, electrical, magnetic &other properties are not not possible in normal materials

Nanoparticle Applications

1. In modern times the nanotechnology​ is a very important and applicable tool of science and technology
2. Nanotechnology is used in food science in agriculture,in food sefty,and bio security.
3. It is used in energy resources like storeage conservation manufacturing.
4. Nano particles are used in paint and casting materials
5. Carbon nanotubes are used for febricating field emitter for flat panel,full colour display like t.v. and computers
Transparent nano particles of ZnO asbsgos ultraviolet light and it is used in modern transparent and highly effective sun glasses
6. Nano particles Titanium dioxide (Ti2O3) are key ingredients in solar panels.

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Monday, 1 May 2017

Working of two stroke engine

Working of two stroke engine

 2 stroke engine

Theory

In this engine cycle is completed in two stroke of the piston or in one revolution of crankshaft . The preparatory stroke. In two stroke engine,insted of valve port are used for sunction and exhaust purpose. As the moving parts are less,mechanical efficiency is high but thermal efficiency is low due to some changes are escape withoutburning through the exhaust port. In two stroke engine as the power is avaliable once in every revolution of crankshaft.
Working---

Intake

The fuel/air mixture is first drawn into the crankcase by the vacuum that is created during the upward stroke of the piston. The illustrated engine features a poppet intake valve; however, many engines use a rotary value incorporated into the crankshaft.
Compression
During the downward stroke, the poppet valve is forced closed by the increased crankcase pressure. The fuel mixture is then compressed in the crankcase during the remainder of the stroke.

Transfer or exhaust

Toward the end of the stroke, the piston exposes the intake port, allowing the compressed fuel/air mixture in the crankcase to escape around the piston into the main cylinder. This expels the exhaust gasses out the exhaust port, usually located on the opposite side of the cylinder. Unfortunately, some of the fresh fuel mixture is usually expelled as well.

Compression

The piston then rises, driven by flywheel momentum, and compresses the fuel mixture. (At the same time, another intake stroke is happening beneath the piston).
Power 
At the top of the stroke, the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete the cycle. (At the same time, another crankcase compression stroke is happening beneath the piston.

Since the two stroke engine fires on every revolution of the crankshaft, a two stroke engine is usually more powerful than a four stroke engine of equivalent size. This, coupled with their lighter, simpler construction, makes the two stroke engine popular in chainsaws, line trimmers, outboard motors, snowmobiles, jet-skis, light motorcycles, and model airplanes.
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Four stroke engine

Four stroke engine

Four stroke Engine

Some important definition in engine
1)clearence volume - the volume between upper part of cylinder and tdc is called clearence volume
2)Stroke volume - the volume swept by the piston from BDC to TDC is called stroke volume.it is also known as swept volume.
3)Break power- the power supply/transfer to the crankshaft by the piston is known asbreak power.
4)Indicated power- the power developed due to the expension of gas into the cylinder is known as indicated power. 
5) compression ratio-it is defineasy as the ratio of the total volume to the Clearence volume
         C.P=total vol./clearence vol.
here total volume.  Is clearence vol+stroke volume.
6) Mechanical efficiency- it is defined as the ratio between break power to indicated power
       M.e.=B.P./I.P.
7) Thermal efficiency- it is defined as the ratio between intigrated power to heat supply to the system
         T.e=I.p./heat supply
 ☆Workin of four stroke engine
1) Intake stroke
The intake event is when the air-fuel mixture is introduced to fill the combustion chamber. The intake event occurs when the piston moves from TDC to BDC and the intake valve is open. The movement of the piston toward BDC creates a low pressure in the cylinder. Ambient atmospheric pressure forces the air-fuel mixture through the open intake valve into the cylinder to fill the low pressure area created by the piston movement. The cylinder continues to fill slightly past BDC as the air-fuel mixture continues to flow by its own inertia while the piston begins to change direction. The intake valve remains open a few degrees of crankshaft rotation after BDC. Depending on engine design. The intake valve then closes and the air-fuel mixture is sealed inside the cylinder.
2) Compression stroke
The compression stroke is when the trapped air-fuel mixture is compressed inside the cylinder. The combustion chamber is sealed to form the charge. The charge is the volume of compressed air-fuel mixture trapped inside the combustion chamber ready for ignition. Compressing the air-fuel mixture allows more energy to be released when the charge is ignited. Intake and exhaust valves must be closed to ensure that the cylinder is sealed to provide compression.                                         When the piston of an engine compresses the charge, an increase in compressive force supplied by work being done by the piston causes heat to be generated. The compression and heating of the air-fuel vapor in the charge results in an increase in charge temperature and an increase in fuel vaporization. The increase in charge temperature occurs uniformly throughout the combustion chamber to produce faster combustion (fuel oxidation) after ignition.                                       The increase in fuel vaporization occurs as small droplets of fuel become vaporized more completely from the heat generated. The increased droplet surface area exposed to the ignition flame allows more complete burning of the charge in the combustion chamber. Only gasoline vapor ignites. An increase in droplet surface area allows gasoline to release more vapor rather than remaining a liquid.                                               The more the charge vapor molecules are compressed, the more energy obtained from the combustion process. The energy needed to compress the charge is substantially less than the gain in force produced during the combustion process. For example, in a typical small engine, energy required to compress the charge is only one-fourth the amount of energy produced during combustion.                     Ignition Event                      The ignition (combustion) event occurs when the charge is ignited and rapidly oxidized through a chemical reaction to release heat energy. Combustion is the rapid, oxidizing chemical reaction in which a fuel chemically combines with oxygen in the atmosphere and releases energy in the form of heat.                                               Proper combustion involves a short but finite time to spread a flame throughout the combustion chamber. The spark at the spark plug initiates combustion at approximately 20° of crankshaft rotation before TDC (BTDC). The atmospheric oxygen and fuel vapor are consumed by a progressing flame front. A flame front is the boundary wall that separates the charge from the combustion by-products. The flame front progresses across the combustion chamber until the entire charge has burned.
  1. 3) power stroke or working stroke
    The power stroke is an engine operation Stroke in which hot expanding gases force the piston head away from the cylinder head. Piston force and subsequent motion are transferred through the connecting rod to apply torque to the crankshaft. The torque applied initiates crankshaft rotation. The amount of torque produced is determined by the pressure on the piston, the size of the piston, and the throw of the engine. During the power Stroke, both valves are closed.
    4) Exhaust stroke
    The exhaust stroke occurs whenspent gases are expelled from the combustion chamber and released to the atmosphere. The exhaust stroke is the final stroke and occurs when the exhaust valve is open and the intake valve is closed. Piston movement evacuates exhaust gases to the atmosphere.
    As the piston reaches BDC during the power stroke combustion is complete and the cylinder is filled with exhaust gases. The exhaust valve opens, and inertia of the flywheel and other moving parts push the piston back to TDC, forcing the exhaust gases out through the open exhaust valve. At the end of the exhaust stroke, the piston is at TDC and one operating cycle has been completed.                           This area, combined with the design and style of combustion chamber, determines the compression ratio. Gasoline engines commonly have a compression ratio ranging from 6:1 - 10:1. The higher the compression ratio, the more fuel-efficient the engine. A higher compression ratio normally provides a substantial gain in combustion pressure or force on the piston. However, higher compression ratios increase operator effort required to start the engine. Some small engines feature a system to relieve pressure during the compression stroke to reduce operator effort required when starting the engine.                                              ☆working of two stroke engine                                     
    Working of two stroke engine                            

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Saturday, 29 April 2017

Solid state

Solid state

Definition

the state of matter in which materials are retain their boundaries without support, the atoms or molecules occupying fixed positions with respect to each other and unable to move freely.
Types of solid:-
1.      Crystalline Solid :- Eg:- Metals And Non metals
2.      Amorphous Solid :- Eg:- Rubber ,Glass ,Plastic

Difference and Comparison between of characteristics

Crystalline Solid: - 

                           Contain regular arrangement having short range & as well as long range order.
Definite geometric shape, Sharpe melting point, they have definite heat fusion, they undergo clean cleavage, and they are true solid.

Amorphous Solid: - 

                         Containing irregular arrangement having short range order only, irregular shape, melting over range of temp, they do not have definite heat of fusion they undergo an irregular cut they are pseudo solid or super cooled solid.

Classification of crystalline solid:-

   1. Ionic Solid 
2. Molecular Solid
3. Covalent of network Solid
4. Metallic Solid

1. Ionic solid: - 

                     In these crystalline solids, constituent particle are positive or negative ion held together by columbic or electrostatic forces of attraction.
Eg:-NaCl, Mgo,Cao, kcl

2.      Molecular solids:- 

                                     These are of three types
1     Non polar molecular solid: - in these the constituent particles are atoms of noble gas or non polar molecules are held by London dispersion forces.
Eg: - Ar, H2.

2- polar molecular solids: - In these solids constituent particles are held together by dipole-dipole attraction forces.
 Eg: - Hcl, SO2.

3.      Hydrogen bonded molecular solids: - in these types of molecular solids constituent particles are molecule containing hydrogen linked to F, O and N held by hydrogen bonding.
Eg: - H2O (ice), NH3.

3. Covalent or Network Solids: - 

                                                 Constituent particles are atoms held together by covalent bonds
Eg: - C (diamond), Sio2 (quartz)

4. Metallic solids: - 

                          Constituent particles are positive ions in a sea of mobile electrons held together by metallic bonds.
Eg: - All metals and alloys.

Crystal Lattice: - 

                          Regular arrangement of constituent particles in 3-D is called crystal lattice or space lattice.

Unit cell: -

           The smallest three dimensional portion of a complete space lattice which when repeated over and again in different direction produces the complete space lattice is called unit cell.

Bravais lattice: - 

                        The fourteen lattices corresponding to seven crystal system are known as Bravais Lattice.

Coordination number: - 

                                   Tthe number of spheres touching to a particular sphere.

No. of atoms per unit cell:-

(a)    No. of atoms in simple cubic lattice:- 8 atoms are at corner
       8*1/8=1  

Simple Cubic Latice
                            
(b)   No. of atoms in BCC:
8 atoms are at corner and one in the body
         8*1/8+1+2    
                                  
BCC physical chemistry
BCC
(c)    No. of atoms in FCC:-
8 atoms are at corner and six atoms are at faces (one on each face)
8*1/8+6*1/2=4
FCC solid state
FCC

Close packing in crystals:-

(a)    Close packing in one dimension : - sphere are touching each other in a row. In this arrangement coordination no. is 2
Close packing in one dimension
Close packing in one dimension

(b)    Close packing in two dimension:- it is done by two ways-
1.      Square close packing: - in this arrangement spheres of second row are exactly above the first row.
                                           
Square Close Packing
Square Close Packing

2.      Hexagonal close packing: - in this arrangement the spheres of second row may be placed in the depression of the first row.
Hexagonal Close Packing
Hexagonal Close Packing
                                 
(c)     Close packing in three dimension
1.      Three dimensional close packing from two dimensional closed packed layer: - starting from the square close packed layer the second layer and all further layers will be built up such that they are horizontally as well as vertically aligned with each other.
Simple cubic lattice formed by A A A......... arrengment
Simple cubic lattice formed by A A A......... arrengment
2.      Three dimensional packing from two dimensional hexagonal close packing:- when spheres are arranged in hexagonal close packing in 2-Dthe two types of voids ‘a’& ‘b’ are formed.
‘a’ & ‘b’ voids are triangular when second layer is placed over the void of first layer (only one void is full filled) in hexagonal close packing and these layer form two voids
C – Triangular and D – Octahedral.
A stack of two layers of close packed spheres and voids generated in them. T = Tetrahedral void; O = Octahedral void
A stack of two layers of close packed spheres and voids generated in them. T = Tetrahedral void; O = Octahedral void

 Packing efficiency: -

                            The percentage of the total space filled by the particles is called packing efficiency.

Packing fraction: -

                           The fraction of the total space filled is called packing fraction.

=>>  Calculation of spaces occupied

1.      in a simple cubic unit cell:-

Suppose radius of sphere is = r
Edge length of unit cell       = a
                      As sphere are touching each other
                       a=2r
              No. of sphere per unit cell= 8*1/8=1
unit cell

             Volume of sphere = 4/3Ï€r3
              Volume of cube =a3= (2r)3 = 8r3
        Packing fraction

% occupied = 52.4%

2.      in face centered cubic structure (Cubic close packing):- spheres of the face center touching the sphere of the corner,
Then, AC = 4r
in ∆ ABC
chemistry

% occupied = 74%

3.      In body centred cubic structure: - Sphere at the body centred touching the sphere of the corner.
    Body diagonal, AD = 4r
Further, face diagonal,
% occupied = 68%

=>  Radius (r) of the octahedral void =0.414R
=>  Radius (r) of the tetrahedral void = 0.225R
           In CCP or HCC. If there are N spheres in the packing.
                        No. of octahedral voids = N
                        No. of Tetrahedral voids = 2N

Imperfection and defects in solids

Types of Defects:-

1.      Stoichiometric defects
2.      Non-Stoichiometric defects
3.      Impurity defects

1.      Stoichiometric defects:- 

                                             In these defects the ratio of anion and cation remains same as the Stoichiometry of the substance not changed.
             These are also classified as:
a.       Vacancy defects: - These arise when some sites of the crystal lattice are vacant, it decrease density.
b.      Interstitial Defects: - When some constituent particles present in the interstitial side, it increase the density of the crystal.
c.       Schottky Defects:- If in a ionic crystal +ve or –ve ions missing their lattice sites so that electric neutrality is maintained
                 Ex: NaCl, KCl, CsCl, AgBr.
d.      Frenkel Defect: - If an ion missing their lattice site (causing vacancy or hole their) and comes in the interstitial side, so maintain electric neutrality or Stoichiometry of the crystal is called Frenkel defect.
                 Ex: ZnS, AgCl, AgBr, AgI.

2.      Non-Stoichiometric Defect: - 

                                                      In this defect ratio of cation & anion became different from the original ratio.
It is of two types:
a.       Metal excess: - this may occur in two way-
                                                                              I.            By anion vacancies: A –ve ion may missing their lattice site and leaving a hole which occupied by an electron there by maintaining electrical balance.
                                                                           II         By the presence of extra cation in the interstitial site: -  it is extra cation (+ve ion) is occupying the interstitial site.
Anion = -ve ion
Cation = +ve ion
And electric neutrality is maintained by an electron is present in another interstitial site.
b.      Metal deficiency:- This defect shows variable vacancy and occurs usually due to the missing of a cation from its lattice site and two monovalent cation is replaced one divalent cation to maintain electric neutrality.
    
                                                                        III.           Impurity defects :- These defects arises when foreign atoms are present at the lattice site in place of host atoms or in the interstitial sites.

Doping: - 

         The process of adding impurities to a crystalline substance so as to change its properties is called doping.

Electrical properties of solids

1.      Conductors:- 

                             The solids which have conductivity in the range of 104 to 107 ohm-1m-1 are called conductors.

2.      Insulators :-  

                           The solids which have extremely low conductivity ranging between 10-20 to 10-10ohm-1m-1 are called insulators.

3.      Semiconductors:- 

                                      The solids which have conductivity between the conductors and insulators, ranging from 10-6 to 104 ohm-1m-1 are called semiconductors.

Band Theory

=>  In case of metals (conductors), the atomic orbital’s of the metal atom are so close in energy that they overlap to form a large no. of molecular orbital’s very close in energy. This set of molecular  orbital’s is called a band. Two types of band formed then electron can easily flow by gaining energy and conduct electricity.
=>  If gap between band is large electron can not jump from valance band to conduction band, so these behave as insulator.
=>  If the gap is small then electron may jump and show some conductivity and act as semiconductor.
Band Theory
Band Theory

Magnetic properties of solids: -

1.      Diamagnetic Substances: - 

                                                  Substances which are weakly repelled by the external magnetic field are called diamagnetic substances.
Ex: TiO2, H2O, NaCl

2.      Paramagnetic Substances: - 

                                                   Substances which are attracted by the external magnetic field are called paramagnetic substances.
Ex: O2, Cu2+, Fe3+

3.      Ferromagnetic Substances: - 

                                                     Substances which show permanent magnetism even in the absence of magnetic field are called ferromagnetic substances.
Ex: Fe, Ni

4.      Anti- Ferromagnetic Substances: - 

                                                               Substances which are expected to posses’ paramagnetism and ferromagnetism on the basis of magnetic moments of the domains but actually they posses Zero magnetic moment are called anti- ferromagnetic substances.
Ex: MnO

5.      Ferromagnetic Substances: - 

                                                      Substances which are expected to posses large magnetism on the basis of magnetic moments of the domains but actually have small net magnetic moment are called ferromagnetic substances.
Ex: Fe3O4
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