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Grade: 9
        
can someone say me about piezoelectricty in few words 
and also about structure of atom
4 years ago

Answers : (4)

deapan
131 Points
							
IS MATTER AROUND US PURE
CLASSIFICATION OF MATTER
 
MATTER
Matter can be classified into
 
 
MIXTURES
PURE SUBSTANCES
HOMOGENEOUS MIXTURE
HETEROGENEOUS MIXTURE
ELEMENT
COMPOUND
 
 
 
 
 
 
 
 
 
 
Pure, it means that all the constituent particles of that substance are the same in their chemical nature. A pure substance consists of a single type of particles. Most of the matter around us exist as mixtures of different substances and hence not pure. For example, milk is actually a mixture of water, fat, proteins etc.
MIXTURES
Mixtures are constituted by more than one kind of pure form of matter, known as a substance. A substance cannot be separated into other kinds of matter by any physical process. Example: sugar is a substance because it contains only one kind of pure matter and its composition is the same throughout.
TYPES OF MIXTURES
Depending upon the nature of the components that form a mixture, we can have different types of mixtures.
Mixture which has a uniform composition throughout is called homogeneous mixtures or solutions. A homogeneous mixture can have a variable composition.
Mixtures, which contain physically distinct parts and have non-uniform compositions are called heterogeneous mixtures. Example: Mixtures of sodium chloride and iron filings.
 
SOLUTION
          A solution is a homogeneous mixture of two or more substances. Examples: Lemonade, soda water etc. We can also have solid solutions (alloys) and gaseous solutions (air). In a solution there is homogeneity at the particle level. Example: particles of sugar or salt in lemonade are evenly distributed in the solution.
A solution has a solvent and a solute as its components. The component of the solution that dissolves the other component in it is called the solvent. The component of the solution that is dissolved in the solvent is called the solute.
Example: 
  • A solution of iodine in alcohol known as ‘tincture of iodine’, has iodine (solid) as the solute and alcohol (liquid) as the solvent.
  • Air is a mixture of gas in gas. Air is a homogeneous mixture of a number of gases. Its two main constituents are: oxygen (21%) and nitrogen (78%). The other gases are present in very small quantities.
PROPERTIES OF A SOLUTION
  • A solution is a homogeneous mixture.
  • The particles of a solution are smaller than 1 nm (10-9 metre) in diameter. So, they cannot be seen by naked eyes.
  • Because of very small particle size, they do not scatter a beam of light passing through the solution. So, the path of light is not visible in a solution.
  • The solute particles cannot be separated from the mixture by the process of filtration. The solute particles do not settle down when left undisturbed, that is, a solution is stable.
CONCENTRATION OF A SOLUTION
In a solution the relative proportion of the solute and solvent can be varied. Depending upon the amount of solute present in a solution, it can be called a dilute, concentrated or a saturated solution.
When no more solute can be dissolved in a solution at a given temperature, it is called a saturated solution. The amount of the solute present in the saturated solution at this temperature is called its solubility. Different substances in a given solvent have different solubilities at the same temperature.  If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution.
The concentration of a solution is the amount of solute present per unit volume or per unit mass of the solution.
 
Concentration of solution = mass of solute / mass of solution ×100 (mass by mass % solution) or
                                                      Mass of solute/ volume of solute × 100(mass by volume % solution)
Problems
  1. A solution contains 40 grams of common salt in 320 grams of water. Calculate the concentration in terms of mass by mass % of a solution
  2. To make a saturated solution, 36 g of sodium chloride is dissolved in 100 g of water at 293 K . find its concentration at this temperature
Solution
  1. Mass of solute (salt)            =                                 40 g
Mass of solvent (water)      =                              320 g
Mass of solution                   =                                 mass of solute + mass of solvent
                                                    =                                 40 g + 320 g  =   360 g
Mass percentage by solution              =             mass of solute / mass of solution × 100
                                                                        =             40 / 360  × 100   =  11.11 %
ALLOYS
Alloys are homogeneous mixtures of metals and cannot be separated into their components by physical methods. But still, an alloy is considered as a mixture because it shows the properties of its constituents and can have variable composition. For example, brass is a mixture of approximately 30% zinc and 70% copper.
SUSPENSIONS
Solids that are insoluble in a solvent and have particles that are visible to naked eyes form a suspension. A suspension is a heterogeneous mixture where solute remains suspended throughout the medium.
PROPERTIES OF A SUSPENSION
  • Suspension is a heterogeneous mixture.
  • The particles of a suspension can be seen by the naked eye.
  • The particles of a suspension scatter a beam of light passing through it and make its path visible.
  • The solute particles settle down when a suspension is left undisturbed, that is, a suspension is unstable. They can be separated from the mixture by the process of filtration.
COLLOIDAL SOLUTION
Colloids are heterogeneous mixtures in which the particle size is too small to be seen with the naked eye, but is big enough to scatter light. This scattering of a beam of light is called the Tyndall effect after the name of the scientist who discovered this effect. The particles are called the dispersed phase and the medium in which they are distributed is called the dispersion medium. The particles of a colloid are uniformly spread throughout the solution. Tyndall effect can also be observed when a fine beam of light enters a room through a small hole. This happens due to the scattering of light by the particles of dust and smoke in the air.
PROPERTIES OF A COLLOID
  • A colloid is a heterogeneous mixture.
  • The size of particles of a colloid is too small to be individually seen by naked eyes.
  • Colloids are big enough to scatter a beam of light passing through it and make its path visible.
  • They do not settle down when left undisturbed, that is, a colloid is quite stable. They cannot be separated from the mixture by the process of filtration. But, a special technique of separation known as centrifugation (perform activity 2.5), can be used to separate the colloidal particles.
The components of a colloidal solution are the dispersed phase and the dispersion medium. The solute-like component or the dispersed particles in a colloid form the dispersed phase, and the component in which the dispersed phase is suspended is known as the dispersing medium. Colloids are classified according to the state (solid, liquid or gas) of the dispersing medium and the dispersed phase.
 
DISPERSED PHASE
DISPERSED MEDIUM
TYPE
EXAMPLE
LIQUID
GAS
AEROSOL
Fog, clouds, mist
SOLID
GAS
AEROSOL
Smoke, automobile exhaust
GAS
LIQUID 
FOAM
Shaving cream
LIQUID
LIQUID
EMULSION
Milk, face cream
SOLID
LIQUID
SOL
Milk of magnesia, mud
GAS
SOLID
FOAM  
Foam, rubber, sponge, pumice
LIQUID
SOLID
GEL
Jelly, cheese, butter
SOLID
SOLID
SOLID SOL
Coloured gemstone, milky glass
4 years ago
deapan
131 Points
							
SEPARATING THE COMPONENTS OF A MIXTURE
Different methods of separation are used to get individual components from a mixture. Heterogeneous mixtures can be separated into their respective constituents by simple physical methods like handpicking, sieving, filtration that we use in our day-to-day life. Sometimes special techniques have to be used for the separation of the components of a mixture
 
 
 
 
 
METHODS OF SEPARATION
METHOD
PROPERTY  EXPLOITED / DESCRIPTION
APPLICATION
SUBSTANCES
Evaporation
 
Ability of a solvent to evaporate. Separation of the             volatile solvent (evaporating substance) from a non-volatile solute is by the method of evaporation
 
  • Separation of salt and sand
---------
Centrifugation
The principle is that the denser particles are forced to the bottom and the lighter particles stay at the top when spun rapidly. Used when solid particles in liquid are very small and can pass through a filter paper.
  • Used in diagnostic laboratories for blood and urine tests.
  • Used to separate butter from cream.
  • Used to separate cream from milk
  • Used to make cheese     ( paneer) from milk
  • Used in washing machines to squeeze out water from wet clothes.
     ----------
Separation of two immiscible liquids  (separation of liquid – liquid mixture)
The principle is that immiscible liquids separate out in layers depending on their densities.
  • To separate mixture of oil and water.
  • In the extraction of iron from its ore, the lighter slag is removed from the top by this method to leave the molten iron at the bottom in the furnace.
   -------------
Sublimation (separation of solid – solid mixtures)
Ability of one component to sublime. To separate mixtures that contain a sublimable  component from a non-sublimable impurity, the sublimation process is used
To separate
  • NH4Cl from salt
  • Iodine from sand
Sublimable substances
  • Ammonium chloride
  • Camphor
  • Naphthalene
  • Iodine
Chromatography
This process is based on the difference in absorption by a surface of an appropriate absorbent material or solid medium(stationary phase). The rate of a absorption of a particular constituent depends upon its solubility in the solvent (moving phase) and its affinity for the absorbing material. It is used for separation and identification of dissolved constituents of a mixture.
  • Separation of coloured constituents in mixture of ink
  • Separation of drugs from blood
  • Separation of pigments from natural colours
-------------------
Distillation
(solid – liquid – gas mixture)  
Heating a mixture without decomposition containing substances of different boiling points in absence of air followed by condensation of vapours.
  • separation of different gases from air
  • separation of different fractions from petroleum to obtain different petroleum products
  • to separate acetone and water
  • separation of potassium chloride from a solution of potassium chloride and water
  • separation of NaCl from a solution of NaCl and water
--------------------
Solvent extraction
(solid – solid mixture)
Solubility of one component in a solvent
  • A mixture of sulphur and sand. Sulphur is soluble in carbon disulphide (CS2) and sand is insoluble.
---------------------
 
Magnetic separation
(solid – solid
Mixture)
Magnetic property of one component
  • Mixture of iron ore and sand. Iron ore is attracted by magnet and sand is left behind
Magnetic substances
  • Iron
  • Iron ore
Gravity method
(solid – solid
Mixture)          
Differences in densities of component
  • Mixture of sand and chalk powder. Sand being heavier than chalk powder sinks in water whereas chalk floats.
------------------
 
 
  •  
 
 
SEPARATION OF DIFFERENT GASES FROM AIR
Air is a homogeneous mixture and can be separated into its components by fractional distillation.
 
Air
 
 
 
 
                                Compress and cooled by increasing pressure and decreasing temperature
 
Liquid Air
 
 
 
 
                                Allowed to warm up slowly in fractional distillation column          
                                                               
                                                Gases get separated at different heights
 
 
Oxygen
Argon     
Nitrogen
Boiling point (0C)
-183
-186
-193
% Air by volume
20.9
0.9
78.1
 
                          The air is compressed by increasing the pressure and is then cooled by decreasing the temperature to get liquid air. This liquid air is allowed to warm-up slowly in a fractional distillation column, where gases get separated at different heights depending upon their boiling points.
Problems
  1. Arrange the gases present in air in increasing order of their boiling points.
  2. Which gas forms the liquid first as the air is cooled?
 
 
CRYSTALLISATION
The crystallisation method is used to purify solids. For example, the salt we get from sea water can have many impurities in it. To remove these impurities, the process of crystallisation is used. Crystallisation is a process that separates a pure solid in the form of its crystals from a solution. Crystallisation technique is better than simple evaporation technique as –
  • Some solids decompose or some, like sugar, may get charred on heating to dryness.
  • Some impurities may remain dissolved in the solution even after filtration. On evaporation these contaminate the solid.
APPLICATIONS
  • Purification of salt that we get from sea water.
  • Separation of crystals of alum (phitkari) from impure samples.
Thus, by choosing one of the above methods according to the nature of the components of a mixture, we get a pure substance. With advancements in technology many more methods of separation techniques have been devised.
CRYSTALLISATION EXAMPLES
  • Copper sulphate
Problems
Write  the separation techniques will you apply for the separation of the following
  1. Sodium chloride from its solution in water.
  2. Sugar from sugar solution
  3. Ammonium chloride from a mixture containing sodium chloride and ammonium chloride.
  4. Small pieces of metal in the engine oil of a car.
  5. Different pigments from an extract of flower petals.
  6. Butter from curd.
  7. Oil from water
  8. Ammonium chloride from sand
  9.  
 
 
PHYSICAL AND CHEMICAL CHANGES
The physical properties of matter that can be observed and specified like colour, hardness, rigidity, fluidity, density, melting point, boiling point etc. are the physical properties. The interconversion of states is a physical change because these changes occur without a change in composition and no change in the chemical nature of the substance. Although ice, water and water vapour all look different and display different physical properties, they are chemically the same.
We know that oil burns in air whereas water extinguishes fire but both are liquid. It is this chemical property of oil that makes it different from water. Burning is a chemical change. During this process one substance reacts with another to undergo a change in chemical composition. Chemical change brings change in the chemical properties of matter and we get new substances. A chemical change is also called a chemical reaction. During burning of a candle, both physical and chemical changes take place.
TYPES OF PURE SUBSTANCES
On the basis of their chemical composition, substances can be classified either as elements or compounds.
ELEMENTS
Robert Boyle was the first scientist to use the term element in 1661. Lavoisier, a French chemist, defined an element as a basic form of matter that cannot be broken down into simpler substances by chemical reactions.
Elements can be normally divided into metals, non-metals and metalloids. There are more than 100 elements known. Ninety-two elements are a occurring and the rest are manmade. Majority of the elements are solid.
Elements
Colour
Lustre
Malleability
Sonorous
Conduct heat and electricity
Ductile
Metals
Silver- grey or golden-yellow
 
 
 
 
 
Non metals
Variety of colours
×
×
×
×
×
Metalloids
intermediate
Properties
between
metals
And
Non metals
 
Eleven elements are in gaseous state at room temperature. Two elements are liquid at room temperature–mercury and bromine. Elements, gallium and cesium become liquid at a temperature slightly above room temperature (303 K).
COMPOUNDS
A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.
MIXTURES
COMPOUNDS
Elements or compounds just mix together to form a mixture and no new compound is formed.
Elements react to form new compounds.
 
A mixture has a variable composition.
The composition of each new substance is always fixed.
A mixture shows the properties of the constituent substances.
The new substance has totally different properties.
The constituents can be separated fairly easily by physical methods.
The constituents can be separated only by chemical or electrochemical reactions.
Involves a physical change
Involves a chemical change
4 years ago
Abishek arun
153 Points
							
STRUCTURE OF ATOM
Atoms and molecules are the fundamental building blocks of matter. Dalton’s atomic theory explained various laws of chemical combination but Dalton’s idea that the atom is indivisible particle has been disproved by the discovery of various subatomic particles. Atoms are found to be mainly composed of three fundamental particles (electrons, protons and neutrons). A major challenge before the scientists at the end of the 19th century was to reveal the structure of the atom.
DISCOVERY OF FUNDAMENTAL PARTICLES
ELECTRON
The electron was the first fundamental particle which was discovered by J.J Thomson based on the experiments carried out in a discharged tube.
                Sir William Crookes was the first scientist who designed the discharged tube which was called Crooke’s discharged tube or cathode ray tube. It is a long glass tube having two metal plates connected to the opposite charged poles of the battery. The pressure inside the discharge tube can be adjusted by means of an exhaust pump.
This discharge tube was slightly modified by J.J Thomson. When high voltage was applied between cathode and anode with a small hole at the centre of the partially evacuated tube at a pressure of 0.01 mm of Hg, a bright spot of light was formed on a zinc sulphide screen kept at the opposite end of the discharge tube. This was caused by the rays which originated from the cathode called cathode rays. J.J Thomson conducted some experiments with a discharge tube for studying the properties of the cathode rays.
PROPERTIES OF CATHODE RAYS
  • Cathode rays travel in straight lines
  • Are small particles having mass and kinetic energy
  • Bends towards the positive plate because cathode particles are negatively charged
The properties do not depend on the nature of gas taken in discharge tube. Specific charge(e/m value) remains same
MILLIKAN’S OIL DROP EXPERIMENT
Some fine oil droplets were allowed to be sprayed into the chamber by the atomizer. The air in chamber is subjected to ionization by X-rays. The electrons produced by the ionization of air attach themselves to the oil drops. When sufficient amount of electric field is applied which can just balance the gravitational force acting on the oil drop, the oil drop remains suspended in air. From this experiment Millikan observed that the electrons produce 1.59 × 10—19 coulombs and the charge on each drop was always an integral multiple of that value. On the basis of the observation, he concluded that the 1.59 × 10—19 coulombs is the smallest possible charge and considered that value as the charge of electron  
DISCOVERY OF PROTON
The presence of positively charged particles has been predicted by E. Goldstein based on the electrical neutrality of the atom. The discovery of proton by Goldstein was done on the basis of cathode ray experiment conducted by using a perforated cathode.
Just like cathode rays, some rays were found to be emanate from an anode. These are called anode rays or canal rays. These rays were positively charged radiations which ultimately led to the discovery of another sub-atomic particle which had a charge, equal in magnitude but opposite in sign to that of the electron. Its mass was approximately 2000 times as that of the electron. It was given the name of proton.
PROPERTIES OF ANODE RAYS
  • Anode rays travel in straight line
  • Anode rays possess positive charge since they were found to deflect towards cathode
  • The properties of anode depends upon nature of glass taken in discharge tube
  • The mass of particles was same as the atomic mass of the gas inside the discharge tube 
In general, an electron is represented as ‘e’ and a proton as ‘p+’. The mass of a proton is taken as one unit and its charge as plus one. The mass of an electron is considered to be negligible and its charge is minus one. It seemed highly likely that an atom was composed of protons and electrons, mutually balancing their charges.
For explaining this, many scientists proposed various atomic models. J.J. Thomson was the first one to propose a model for the structure of an atom.
PARTICLE
MASS IN KG
ABSOLUTE CHARGE
RELATIVE CHARGE
SYMBOL
Electron
9.109 × 10-31
-1.6026 × 10-19 C
-1
-1e0
Proton
1.67266 × 10-27
+1.6026 × 10-19 C
+1
1p1
Neutron
1.6748 × 10-27
0
0
0n1
 
THOMSON’S ATOMIC MODEL
 
Thomson proposed the model of an atom is popularly known as apple pic or plum pudding model or watermelon model.
Thomson proposed that:
  • An atom contains electrons embedded (seeds) uniformly throughout the positively charged mass (red edible part).
  • The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.
Although Thomson’s model explained that atoms are electrically neutral, but it failed to explain how the protons are shielded from the electrons without getting neutralized.
RUTHERFORD’S MODEL OF AN ATOM
 
Rutherford designed an experiment to find the model of the atom called α - particle scattering experiment in 1911.
 
In this experiment, a stream of high energy alpha particles emanating from radium was directed at a thin foil of gold metal. The thin gold metal foil had a circular fluorescent zinc sulphide screen around it. Whenever alpha particles struck the screen, a tiny flash of light was produced at that point.
The gold foil was about 1000 atoms thick approximately thickness of 100 nanometre. Alpha particles are doubly-charged helium ions and highly energetic particles. Since they have a mass of 4 u, the fast-moving α-particles have a considerable amount of energy. It was expected that alpha particles would be deflected by the sub-atomic particles in the gold atoms. But, the particle scattering experiment gave totally unexpected results. The following observations were made:
  • Most of the fast moving particles passed straight through the gold foil.
  • Some of the particles were deflected by the foil by small angles.
  • One out of every 10000 particles reflected about 1800.
Rutherford concluded from the a particle scattering experiment that
 
The atom is mostly composed of empty space. Very few alpha particles were deflected by 1800. The deflection must be due to enormous repulsive force showing that positive charge of the atom is not spread throughout the atom. The entire positive charge and most mass of the atom has to be densely concentrated in a very small volume known as nucleus that repelled and deflected the positively charged alpha particles.
The diameter of the nucleus is estimated by Rutherford as 10-15 cm in contrast to that of an atom to be 10-10 cm. The nucleus is surrounded by electrons that move around the nucleus with very high speed in circular paths called orbits. Electrons and nucleus are held together by the electrostatic forces of attraction
DRAWBACKS OF RUTHERFORD’S ATOMIC MODEL
Rutherford’s atomic model is like a small scale solar system with the nucleus playing the role of sun and electrons to planets. When classical mechanics is applied to the solar system it shows that planets describe well defined orbits around the sun.
The similarity between the solar system and the nuclear model suggest that electrons should move around the nucleus in well defined orbits. However when a body is moving in an orbit, it undergoes acceleration. So an electron moving in an orbit undergoes acceleration. According to electromagnetic theory of Maxwell, charged particles when in acceleration emit electromagnetic radiation. Therefore an electron in the orbit emit radiation, the electronic radiation comes from electronic motion. Thus the orbit continue to shrink. Calculations show that it would take an electron only 10-8 s to spiral into nucleus. But this does not happen. Thus the Rutherford’s model of atom cannot explain the stability of the atom.
          If the electrons were stationary electrostatic attraction between the dense nucleus and electrons would pull the electrons toward the nucleus to form a miniature version of Thomson’s atomic model. This model could not explain how the electrons are distributed around the nucleus and what are the energies of these electrons.          
BOHR’S MODEL OF ATOM
 
To overcome the above defects of Rutherford’s model, Niels Bohr in 1913 gave a modification based on Quantum theory of radiation. The important postulates are:
  • The electrons revolve round the nucleus only in certain fixed energy levels  called orbits. These orbits are associated with definite energies and are called energy shells or energy levels or quantum levels. These are numbered as 1, 2, 3, 4 ….. etc. (starting from the nucleus) are designated as K, L, M, N ….etc. (Fig. 3.2).
  • As long as an electron remains in a particular orbit, it does not lose or gain energy. This means that energy of an electron in a particular path remains constant. Therefore, these orbits are also called stationary states.
  • If an electron jumps from one stationary state to another, it will absorb or emit radiation of a definite frequency giving a spectral line of that frequency which depends upon the initial and final levels. When an electron jumps back to the lower energy level, it radiates same amount of energy in the form of radiation.
`The exchange of energy is possible only when electron jumps from one energy level to another energy level.
  E = E2 – E1 = hv = hc/ʎ  joule
Where h is planck’s constant having fixed value
                h = 6.63 × 10-34 J / second  and ʎ is wavelength.
                When an electron falls from an orbit of high energy level to lower energy level, the difference in energy is radiated in the form of electromagnetic radiation of particular wavelength. Since each atom has its specific energy levels, it can energy levels, it can emit radiations of specific wavelength. Ground state means the lowest energy state.  When the electrons absorb energy and jump to outer orbits, this state is called excited state.
HOW ARE ELECTRONS DISTRIBUTED IN DIFFERENT SHELLS
 
              The distribution of electrons into different orbits of an atom was suggested by Bohr and Bury. The following rules are followed for writing the number of electrons in different energy levels or shells:
  • The maximum number of electrons present in a shell is given by the formula 2n2, where ‘n’ is the orbit number or energy level index, 1,2,3,…. Hence the maximum number of electrons in different shells are as follows:  
first orbit or K-shell will be = 2 × 12 = 2, second orbit or L-shell will be = 2 × 22 = 8, third orbit or M-shell will be = 2 × 32 = 18, fourth orbit or N-shell will be  2 × 42= 32, and so on.
  • The maximum number of electrons that can be accommodated in the outermost orbit is 8.
  • Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step-wise manner.
 
LIMITATIONS OF BOHR’S THEORY
 
  • According to Bohr, the radiation results when an electron jumps from one energy orbit to another energy orbit, but how this radiation occurs is not explained by Bohr.
  • This theory was applicable only for monoelectronic system that is H, He+, Li++ and H2+
  • Bohr Theory had explained the existence of various lines in H spectrum, but it predicted that only a series of lines exist. At that time this was exactly what had been observed. However, as better instruments and techniques were developed, it was realized that the spectral line that had been thought to be a single line was actually a collection of several lines very close together (known as fine spectrum). Thus for example, the single H¥-spectral line of Balmer series consists of many lines very close to each other.
  • It fails to explain why spectrum of hydrogen atom is discontinuous.
  • Thus the appearance of the several lines implies that there are several energy levels, which are close together for each quantum number n. This would require the existence of new quantum numbers.
  • Bohr’s theory has successfully explained the observed spectra for hydrogen atom and hydrogen like ions (e.g. He+, Li2+, Be3+ etc.), it cannot explain the spectral series for the atoms having a large number of electrons.
  • Bohr assumes that an electron in an atom is located at a definite distance from the nucleus and is revolving round it with definite velocity, i.e. it is associated with a fixed value of momentum. This is against the Heisenberg’s Uncertainty Principle according to which it is impossible to determine simultaneously with certainty the position and the momentum of a particle.
  • No explanation for Zeeman effect: If a substance which gives a line emission spectrum, is placed in a magnetic field, the lines of the spectrum get split up into a number of closely spaced lines. This phenomenon is known as Zeeman effect. Bohr’s theory has no explanation for this effect.
  • No explanation of the Stark effect: If a substance which gives a line emission spectrum is placed in an external electric field, its lines get spilt into a number of closely spaced lines. This phenomenon is known as Stark effect. Bohr’s theory is not able to explain this observation as well. 
4 years ago
Abishek arun
153 Points
							
Piezoelectricity is the electric charge that accumulates in certain solid materials in response to applied mechanical 
In practice, the crystal becomes a kind of tiny battery with a positive charge on one face and a negative charge on the opposite face; current flows if we connect the two faces together to make a circuit. Now, by crystals, scientists don't mean intriguing bits of rock we find in gift shops: a crystal is the regular, repeating arrangement of atoms in a solid in a three dimensional manner. The atoms are essentially fixed in place but can vibrate slightly.
In most crystals such as metals the atoms are symmetrical; in piezoelectric crystals, it isn't. Normally, piezoelectric crystals are electrically neutral: the atoms inside them may not be symmetrically arranged, but their electrical charges are perfectly balanced: a positive charge in one place cancels out a negative charge nearby. However, if you squeeze or stretch a piezoelectric crystal, you deform the structure, pushing some of the atoms closer together or further apart, upsetting the balance of positive and negative, and causing net electrical charges to appear. This effect carries through the whole structure so net positive and negative charges appear on opposite, outer faces of the crystal. The reverse-piezoelectric effect occurs in the opposite way.
In what way piezoelectricity is used for?
There are all kinds of situations where we need to convert mechanical energy  into electrical signals or vice-versa. Often we can do that with a piezoelectric transducer. A transducer is simply a device that converts small amounts of energy from one kind into another.
Piezoelectricity is found in many useful applications such as the production and detection of sound, generation of high voltages, electronic frequency generation. 
4 years ago
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