What is Bonds Ionic and Covalent Bonds simple Explanation

 Bonds 

The atoms form molecules primarily due to the electrostatic interaction between the electrons and the nuclei. These interactions are described in terms of different kinds of bonds. We shall briefly discuss two important bonds that frequently occur in materials.

Ionic Bond

In an ionic bond two atoms come close to each other and an electron is completely transferred from one atom to the other. This leaves the first atom positively charge and the other one negatively charged. There is an electrostatic attraction between the ions which keeps them bound. For example, when a sodium atom comes close to a chlorine atom, an electron of the sodium atom is completely transferred to the chlorine atom. The positively charge sodium ion and the negatively charged chlorine ion attract each other to form an ionic bond resulting in sodium chloride molecule.

Covalent Bond 

In many of the cases a complete transfer of electron from one atom to another does not take place to forma bond. Rather, electrons from neighbouring atoms are made available for sharing between the atoms. Such bonds are called covalent bond. When two hydrogen atoms come close to each other, both the electrons are available to both the nuclei. In other words, each electron moves through the total space occupied by the two atoms. Each  electrons is pulled by both the nuclei. Chlorine molecule is also formed by this mechanism. Two chlorine atoms share a pair of electrons to form the bond. Another example of covalent bond is hydrogen chloride (HCL) Molecule.

Three States Of Matter Examples In Physics

Three States Of Matter 

If two molecules are kept at a separation r = r。,  they will stay in equilibrium. If they are slightly pulled apart so that r > r。 , an attractive force will operate between them. If they are slightly pushed so that r < r。, a repulsive force will operate. Thus, if a molecule is slightly displaced from its equilibrium position, it will oscillate about its mean position. This is the situation in a solid. The molecules are close to each other, very nearly at the equilibrium separations. The amplitude of vibration is very small and the molecules remain almost fixed at their positions. This explains why a solid has a fixed shape if no external force act to deform it.

In liquids, the average separation between the molecules is somewhat large. The attractive force is weak and the molecules are more free to move inside the whole mass of the liquid. In gases, the separation is much larger and the molecular force is very weak.

(1). Solid State

In solids, the intermolecular forces are so strong that the molecules or ions remain  almost fixed at their equilibrium positions. Quite often these equilibrium positions have a very regular three-dimensional arrangement which we call crystal. The positions occupied by the molecules or the ions are called lattice points. Because of this long range ordering, the molecules or ions combine to form large rigid solids.

The crystalline solids are divided into four categories depending on the nature of nature of the bonding between the basic units.

(2). Molecular Solid 

In a molecular solid, the molecules are formed due to covalent bonds between the atoms. The bonding between the molecules depends on whether the molecules are polar or nonpolar as discussed below. If the centre of negative charge in a molecule coincides with the centre of the positive charge, the molecule is called nonpolar. Molecules of hydrogen, oxygen, chlorine, etc., are of this type. Otherwise, the molecule is called a polar molecule. Water molecule is polar. The bond between polar molecules is called a dipole-dipole bond. The bond between nonpolar molecules is called a van der Waals bond. Molecular solids are usually soft and have low melting point. They are poor conductors of electricity.

(a). Ionic Solid

In an ionic solid, the lattice points are occupied by positive and negative ions. The electrostatic attraction between these ions binds the solid. These attraction force are quite strong so that the material is usually hard and has fairly high melting point. they are poor conductors of electricity.

(b).Covalent Solid 

In a covalent solid, atoms are arranged in the crystalline form. The neighbouring atoms are bound by shared electrons. Such covalent bonds extend in space so as to form a large solid structure. Diamond, silicon, etc., are examples of covalent solids. Each carbon atom is bonded to four neighbouring carbon atoms in a diamond structure. They are quite hard, have high melting point and are poor conductors of electricity.

(c). Metallic Solid

In a metallic solid, positive ions are situated at the lattice points. These ions are formed by detaching one or more electrons from the constituent atoms. These electrons are highly mobile and move throughout the solid just like a gas. They are very good conductors of electricity.

(3). Amorphous or Glassy State

There are several solids which do not exhibit a long range ordering. However, they still show a local ordering so that some molecules (say 4-5) are bonded together to form a structure. Such independent units are randomly arranged to form the extended solid. In this respect the amorphous solid is similar to a liquid which also lacks any long range ordering. However, the intermolecular force in amorphous solids are much stronger than those in liquids. This prevents the amorphous solids to flow like a fluid. A typical example is glass made of silicon and oxygen together with some other elements like calcium and sodium. The structure contains strong Si-O-Si bonds, but the structure does not extend too far in space.

The amorphous solids do not have a well-defined melting point. Different bonds have different strengths and as the material is heated the weaker bonds break earlier starting the melting process. The stronger bonds break at higher temperatures to complete the melting process.

DEFINITION OF BASE UNITS, Metre, Kilogram, Second, Ampere, Kelvin, Mole, Candela

 UNITS

Physics describes the law of nature. This description is quantitative and involves measurement and comparison of physical quantities. To measure a physical quantity we need some standard unit of that quantity.

Who Decides the Units ?

How is a standard unit chosen for a physical quantity ? The first thing is that it should have international acceptance. Otherwise, everyone will choose his or her own unit for the quantity and it will be difficult to communicate freely among the persons distributed over the world. A body named Conference Generale des poids et Mesures or CGPM also known as General Conference on Weight and Measures  English has been given the authority to decide the units by international agreement. It holds its meetings and any changes in standard units are communicated through the publications of the conference.

Definitions Of Base Units

Any standard unit should have the following two properties :

(a) Invariability : The standard unit must be invariable. Thus, defining distance between the tip of the middle finger and the elbow as a unit of length is not invariable.

(b) Availability : The standard unit should be easily made available for comparing with other quantities.

The procedures to define a standard value as a unit are quite often not very simple and use modern equipments. Thus, a complete understanding of these procedures cannot be given in the first chapter. We briefly mention the definitions of the base units which may serve as a reference if needed.

Metre

It is the unit of length. the distance travelled by light in vacuum in 1/299,792,458 second is called 1 m. 

Kilogram 

The mass of a cylinder made of platinum-iridium alloy kept at International Bureau of Weights and Measures is defined as 1 kg.

Second

Cesium-133 atom emits electromagnetic radiation of several wavelengths. A particular radiation is selected which corresponds to the transition between the two hyperfine levels of the ground state of Cs-133. Each radiation has a time period of repetition of certain characteristics. The time duration in 9,192,631,770 time periods of the selected transition is defined as 1 s.

Ampere

Suppose two long straight wires with negligible cross-section are placed parallel to each other in vacuum at a separation of 1 m and electric currents are established in the two in same direction. The wires attract each other. If equal currents are maintained in the two wires so that the force between them is 2 ｘ 10^-7  newton per metre of the wires, the current in any of the wires is called 1 A. Here, newton is the SI unit of force.

Kelvin

The fraction 1/273.16 of the thermodynamic temperature of triple point of water is called 1K.

Mole

The amount of a substance that contains as many elementary entities (molecules or atoms if the substance is monatomic) as there are number of atoms in 0.012 kg of carbon-12 is called a mole. This number (number of atoms in 0.012 kg of carbon-12) is called Avogadro constant and its best value available is 6.022045 ｘ 10 ^23 with an uncertainty of about 0.000031 ｘ 10^23 .

Candela

The SI unit of luminous intensity is 1 cd which is the luminous intensity of a blackbody of surface area 1/600,000 m^2 placed at the temperature of freezing platinum and at a pressure of 101,325 N/m^2 , in the direction perpendicular to its surface.

TWIN PARADOX REAL LIFE SIMPLE EXPLANATION

 Twin Paradox Real Life Simple Explanation 

As the postulates of special relativity lead to result which contradict 'common sense' a number of interesting paradoxes have been floated. We shall describe one of the most famous paradoxes of relativity- the twin paradox. consider the twins Ram and Balram living happily on the earth. Ram decides to make a trip to a distant planet p, which is at rest with respect to the earth, and come back. He boards a spaceship S1, going toward the planet with a uniform velocity. When he reaches the planet, he jumps from the spaceship S1 to another spaceship S1 which is going toward the earth. When he reaches the earth, he jump out and meets his brother Balram. 

As Ram returns from his trip and stands next to Balram, do they have equal age ? Or is Ram younger than Balram or is he older than Balram ?

To keep the calculation simple, let us assume the following date:

Distance between the earth and the planet = 8 light-years,

speed of S1 with respect to earth = 0.8c, and speed of S2 with respect to earth = 0.8c.

When we said that the distance between the earth and the planet P is 8 light-years, was it clear to you that this length is the length as measured from the earth frame ?

First, let us analyse the events from the point of view of Balram who is on the earth. For him, both the spaceships move at a speed 0.8c. So,

γ = 1/√1-v^2/c^2 = 1/0.6 . 

When Ram is on S1, he is moving and all his clocks run slower because of time dilation. His heartbeat, pulse beat, etc., represent clocks in themselves and they all run slower. Balram calculates that Ram will take 8 light-year/0.8c = 10 years to reach the planet P. But during all these 10 years, time is passing slowly on S1 and the clock will read only 10 yearsｘ 0.6 = 6 years in this period. The number of breaths taken by Ram corresponds to 6 years only.

Ram jumped into S2 for the return journey. This spaceship is also moving at 0.8c and for Balram , time passes slowly on S2 as well. Although 10 years passed on the earth during Ram's return journey, on the spaceship the journey was clocked at 6 years. Thus, Ram has aged only 12 years whereas Balram has aged 20 years during this expedition. Ram has become younger than Balram by 8 years. This difference in aging is real in the sense that Ram shows lesser signs of aging like he has lesser white hairs than his brother.

The observation of Balram is quite consistent with the special theory of relativity. Such experiment are indeed performed in laboratories with radioactive particles. particles are accelerated to large speed and are kept at these speed for quite some time by magnetic fields. These particles with large speeds have longer lives than their counterparts kept at rest in the laboratory.

The paradox arises when we analyse the events from the point of veiw of Ram. When he is in the spaceship S1, to him the distance between the earth and the planet is not 8 light-years. The earth and the planet P  are moving with respect to Ram and hence he is measuring contracted length. The separation is, therefore, 8 light-years ｘ0.6 = 4.8 light-years. As the planet is approaching Ram at 0.8c, the taken by the planet to reach Ram is 4.8 light-year/0.8c = 6 years. So according to Ram's clock, he jumped from S1 to S2 6 years after getting into S1. Once he is on S2, the earth and the planet are again moving with the same speed 0.8 Again, the earth is 4.8 light-years from the planet and is approaching at 0.8c. It takes 6 years for the earth to reach Ram. Thus, according to Ram's clock, he was out for 12 years from the earth, the same result as Balram had expected.

But how about Ram's calculation of Balram's age ? When Ram is on S1, the earth is going away from him with a speed 0.8c. Ram will find that the time on the earth is passing slower by a factor of 0.6 so that Balram is again slower than he is. The same is true when he is on S2. During this period also,  Balram is moving (toward Ram) with a speed 0.8c and hence time is passing slowly for Balram. As 12 years passes on Ram's clock, he calculates that Balram's clocks have advanced only by 12 years ｘ0.6 = 7.2 years in this period, According to this analysis, Ram should find that Balram is 12 - 7.2 = 4.8 years younger than him. 

This is the paradox. According to Ram, Balram's clocks are running slow and according to Balram, Ram's clock are running slow. Each thinks the other is younger. Where lies the fallacy ?

The fallacy lies in the fact that Ram has changed frames whereas Balram has stayed in an inertial frame. Thus, the roles of the twins are not symmetrical. The ordering of events are different in different frames and Ram must take that into account when he changes frames. Suppose Ram gets into the spaceship S1 when his clock reads zero. So does Balram's clock. What is the reading of the planet's clock at this instant ? According to Balram, it is zero because both the earth and the planet are at rest and the clock are synchronized in his frame. But that is not so in S1. As Ram gets into S1, he may have the following conversation with the captain of the ship. 

Captain: Welcome aboard S1. I saw you on the earth, coming toward us. Your jump to board this ship was perfect. Where are you going ?

Ram: Thank you. I am going to the planet P . How far is it from here and how long will it take for the planet to come to us ?

Captain: Planet P is 4.8 light-years from  us at the moment. It is coming toward us at a speed of 0.8c so it will take 4.8 light-years/0.8c= 6 years for the planet P to reach us.

Ram: Well the clock on the earth and the planet are running a bit slower than ours. I have been taught that moving clock run slow by a factor of γ. This factor is 1/0.6 for these clocks. So they will advance by 6 years ｘ0.6 = 3.6 years by the time the planet reaches us.

Captain: Yes, both the clock will advance by 3.6 years by the time you jump on the planet P.

Ram: The earth-clock was reading t = 0 as we passed the earth. This means when I jump on the planet P the clocks on the earth and the planet will be reading 3.6 years.

Captain: Here you are mistaken. Don't you remember that the planet's clock is not synchronized with the earth's clock ? The planet's clock is at the rear end, and hence is running 6.4 years ahead of the earth's clock. At the instant the earth's clock was reading zero, the planet's clock was reading 6.4 years. As the planet reaches us, both the clocks will advance by 3.6 years. So when you jump out of S1, the earth's clock will be reading 3.6 years but the planet's clock will be reading 10 years.

Ram understands the logic. In the earth's frame the two clocks read zero simultaneously. But in S1-frame, the event ''planet's clock reading zero'' occurred several years before '' earth's clock reading zero''. Six years pass in S1 and Ram finds that the planet P has reached him. He finds another spaceship S2 which is heading towards the earth. Ram jumps onto S2. In the process he looks at the planet's clock and finds that it is reading 10 years as calculated by him on S1. On S2, he starts talking to the commander of the ship.

Commander: Welcome to S2. How long will you be with us ? 

Ram: Thank you. I am going to Earth. Earth is at present 4.8 light-years from here and is coming toward us with a speed of 0.8c. So I will be with you for 6 years. The captain of S1 told me that the earth's clock is reading 3.6 years at this moment whereas the planet's clock reads 10 years. There is a difference of 6.4 years in the reading because the two clock are not synchronized. Also....

Commander: Sorry for interrupting you, but you are mistaken. It is true that the earth's clock and the planet's clock are not synchronized as they are moving past us. Also the difference in the reading of the two clocks is 6.4 years. But the planet's clock is at the front and the earth's clock is at the rear. It is the earth's clock that is leading by 6.4 years. At the moment the planet's clock reads 10 years and hence the earth's clock must be reading 16.4 years.

Ram: hmm... you are right. In S1, the earth was at the front and its clock lagged behind the planet's clock. But in S2 it is the other way round. Indeed the earth's clock reads 16.4 years whereas the planet's clock reads 10 years.

Commander: That's right. The earth's clock is reading 16.4 years at present. It will advance by another by another 3.6 years during the 6 years you will be with us. So it will be reading 20 years when the earth reaches you.

We see that the paradox is resolved.

ATMOSPHERIC ELECTRICITY IN PHYSICS FULL INFORMATION

 Atmospheric electricity in Physics Full Information 

The earth and the atmosphere surrounding it show very interesting electric phenomena. The earth has a negative charge spread with approximately uniform density over its surface. The average surface charge density on the earth is little less than one nanocoulomb per square metre. There is a corresponding electric field of about 100Vm^-1 in the atmosphere above the earth. This field is in the vertically downward direction. This means, if you look at a flat desert, the electric potential increases by about 100 V as you move up by 1 m. The potential keeps on increasing as one goes higher in atmosphere but the magnitude of the electric field gradually decreases. At about 50 km from the earth's surface, the field is negligible. The total potential difference between the earth's surface and the top of the atmosphere is about 400 kV.

The atmosphere contains a number of ions, both positively charge and negatively charged. The main source of these ions is cosmic rays which come from outside the earth, even from outside the solar system. These rays come down to the earth and ionize molecules in the air. Air contains dust particles which become charged by friction as they move through the air. This is another source of the presence of charged ions in air. Because of the electric field in the atmosphere, positive ions come down and negative ions go up. Thus, there is an electric current in the atmosphere. This current is about 3.5 ｘ10^-12 A over a square metre area parallel to the earth's surface. When the total surface area of the earth is considered, 1800 A of current reaches the earth.

The density of ions increases with height over the earth;s surface. Also, the density of air decreases and the ions can travel larger distance between collisions. Both these factors contribute to the fact that ''conductivity of air'' increases with altitude. At about 50 km above the earth's surface, the become highly conducting. We can draw an equivalent picture by assuming that at about this height there is a perfectly conducting surface having a potential of 400 kV and current comes down from this surface to the earth.

If 1800 A of current flows towards the earth, the entire negative charge of the earth should get neutralised in about half an hour and the electric field in the atmosphere should reduce to zero. But it is not so. So, there must be some mechanism which brings negative charge back to the earth, so that the 400 kV potential difference is maintained. This situation is like that of a battery. The current provided by a battery discharges it. There is a source of emf which maintains the potential difference across the battery's terminals. So, what is the source that charge our atmospheric battery. The answer is thunderstorms and lighting. 

Because of  the difference in temperature and pressure between different parts of the atmosphere, air packets keep on moving in a rather systematic fashion. As the upper atmosphere is cool (temperature is around -10 ℃ at a height of 3-4 km and -20℃ at a height of 6-7 km ), water vapour condenses to form small water droplets and tiny ice particles. A parcel of air with these droplets and ice particles forms a thunderstorm. A typical thunderstorm may have an average horizontal extension of about 7-8 km and a vertical extension of about 3 km. A matured thunderstorm is formed with its lower end at a height of about 3-4 km above the earth's surface and the upper end at about 6-7 km above the earth's surface. 

The upper part of a thunderstorm contains excess positive charge and the lower part contains excess negative charge. The density of negative charge in the clouds in the lower part of the storm is very high. This negative charge created a potential difference of 20 to 100 MV between these clouds and earth. Note that this potential difference is much larger than the 400 kV between the earth and the top of atmosphere and is opposite in sing. 

 

The Human Eye Parts Construction And Mechanism

THE HUMAN EYE PARTS CONSTRUCTION AND MECHANISM

THE EYE

Optical instrument are used primarily to assist the eye in viewing an object. Let us first discuss in brief the construction of a human eye and the mechanism by which we see, the most common but most important experiment we do from the day we open our eyes.

Figure shows schematically the basic components of diameter about at inch. The front portion is more sharply curved and is covered by a transparent protective membrane called the cornea. It is this portion which is visible from outside. Behind the cornea, we have a space filled with a liquid called the aqueous humor and behind that a crystalline lens.

PARTS

Iris And Pupil

Between the aqueous humor and the lens, we have a muscular diaphragm called iris, which has a small hole in it called pupil. Iris is the coloured part that we see in an eye. The pupil appears black because any light falling on it goes into the eye and there is almost no chance of light coming back to the outside. The amount of light entering the eye, may be controlled by varying the aperture of the pupil with the help of the iris. In low-light condition, the iris expands the pupil to allow more light to go in. In good light conditions, it contracts the pupil.

Lens 

The lens is hard in the middle and gradually becomes soft towards the outer edge. The curvature of the lens may be altered by the ciliary muscles to which it is attached. The light entering the eye forms an image on the retina which covers the inside of the rear part of the eyeball. The retina contains about 125 million receptors called rods and  cones which receive the light signal and about one million optic-nerve fibres which transmit the information to the brain. The space between the lens and the retina is filled with another liquid called the vitreous humor.

Aqueous Humor

The aqueous humor and the vitreous humor have almost same refractive index 1.336. The refractive index of the material of the lens is different in different portions but on the average it is about 1.396. When light enters the eye from air, most of the behind occurs at the cornea itself because there is a sharp change in the refractive index. Some additional bending is done by the lens which is surrounded by a fluid of somewhat lower refractive index. In normal conditions, the light should be focussed on the retina.

Cornea Lens Fluid

The cornea-lens-fluid system is equivalent to a single converging lens whose focal length may be adjusted by the ciliary muscles. Now onwards, we shall use the word eye-lens to mean this equivalent lens.

When the eye is focussed on a distant object, the ciliary muscles are relaxed so that the focal length of the eye-lens has its maximum value which is equal to its distance from the retina. The parallel rays coming into the eye are then focussed on the retina and we see the object clearly.

Ciliary Muscles

When the eye is focussed on a closer object, the ciliary muscles are strained and the focal lenght of the eye-lens decreases. The ciliary muscles adjust the focal length in such a way that the image is again formed on the retina and we see the object clearly. This process of adjusting focal length is called is called accommodation. However, the muscles cannot be strained beyond a limit and hence, if the object is brought too close to the eye, the focal length cannot be adjusted to form the image on the retina. Thus, there is a minimum distance for the clear vision of an object.

The nearest point for which the image can be focussed on the retina, is called the near point  of the eye. The distance of the near point from the eye is called the least distance for clear vision. This varies from person to person and with age. At a young age (say below 10 years), the muscles are strong and flexible and can bear more strain. The near point may be as close as 7-8 cm at this age. In old age, the muscles cannot sustain a large strain and the near point shifts to large values, say 1 to 2 m or even more. We shall discuss about these defects of vision and use of glasses in a later section. The average value of the least distance for clear vision for a normal eye is generally taken to be 25cm. 

Photoelectric Effect, Threshold Wavelength and Experimental Arrangement.

 Photoelectric Effect.

When light of sufficiently small wavelength is incident on a metal surface, electrons are ejected from the metal. This phenomenon is called the photoelectric effect. The electrons ejected from the metal are called photoelectrons. Let us try to understand photoelectric effect on the basis of the photon theory of light.

We know that there are large number of free electrons in a metal. However, these electrons are not free to leave the surface of the metal. As they try to come out of the metal. the metal attracts them back. A minimum energy, equal to the work function φ, must be given to an electron so as to bring it out of the metal.

When light is incident on a metal surface, the photons collide with the free electrons. In a particular collision, the photon may give all of its energy is more then the work function φ, the electron may come out of the metal. It is not necessary that if the energy supplied to an electron is more then φ, it will come out. The electron after receiving the energy, may lose energy to the metal in course of collision with the atoms of the metal. Only if an electron near the surface gets the extra energy and heads towards the outside, it is able to come out. If it is given an energy E which is greater than φ, and it makes the most economical use of it, it will have a kinetic energy ( E - φ ) after coming out. If it make some collision before coming out, the kinetic energy will be less than ( E - φ ). The actual kinetic energy of such an electron will depend on the total energy lost in collisions. It is also possible that the electron makes several collisions inside the metal and loses so much energy that it fails to come out. So, the kinetic energy of the photoelectron coming out may be kinetic energy of the photoelectron coming out may be anything between zero and ( E - φ ) where E is the energy supplied to the individual electrons. We can, therefore, write 

Kmax = E - φ.

Work function of some photosensitive metals 

Metal                                   Work function (ev)

Cesium                                         1.9                

Potassium                                           2.2                      

Sodium                                          2.3                  

Copper                                            4.5                  

Silver                                            4.7                

Let monochromatic light of wavelength λ be incident on the metal surface. In the particle picture, photons of energy hc/λ fall on the surface. suppose, a particular photon collide with a free electron and supplies all its energy to the electron. the electron gets an extra energy E = hc/λ and may come out of metal. The maximum kinetic energy of this electron is, therefore,

Kmax = hc/λ-φ = hv - φ. .......(2.1)

As all the photons have the same energy hc/λ, equation (2.1) given the maximum kinetic energy of any of the ejected electrons.

Equation (2.1) is called Einstein;s photoelectric equation. Einstein, after an average academic career, put forward this theory in 1905 while working as a grade III technical officer in a patent office. He was awarded the Nobel prize in physics for 1921 for this work.

Threshold wavelength 

Equation (2.1) tells that if the wavelength λ is equal to 

λ༚= hc/φ,

the maximum kinetic energy is zero. An electron may just come out in this case. If λ > λ༚ , the energy hc/λ supplied to the electron is smaller than the work function φ and no electron will come out. Thus, photoelectric effect takes place only if λ ≤ λ༚ . This wavelength λ。is called the threshold wavelength for the metal. The corresponding frequency

v。  =  c/λ。  = φ/h

is called the threshold frequency for the metal. Threshold wavelength and threshold frequency depend on the metal used.

Writing φ = hv。 , equation (2.1) becomes

Kmax = h(v - v。 ).  ....(2.2)

Experimental Arrangement

A systematic study of photoelectric effect can be made in the laboratory with the apparatus shown in figure (2.3) 

Two metal plates C and A are sealed in a vacuum chamber. Light of reasonably short wavelength passes through a transparent window in the wall of the chamber and falls on the plate C which is called the cathode or the emitter. The electrons are emitted by C and collected by the plate A called the anode or the collector. The potential difference between the cathode and the anode can be changed with the help of the batteries, rheostat and the commutator. The anode potential can be made positive or negative with respect to the cathode. The electrons collected by the anode A flow through the ammeter, batteries, etc., and are back to the cathode C  and hence an electric current is established in the circuit. Such a current is called a photocurrent. 

As photoelectrons are emitted from the cathode C, they move toward the anode A. At any time, the space between the cathode and the anode contains a number of electrons making up the space charge. This negative charge repels the fresh electrons coming from the cathode. However, some electrons are able to reach the anode and there is a a photocurrent. When the anode is given a positive potential with respect to the cathode, electrons are attracted toward the anode and the photocurrent increases. The current thus depend on the potential to the anode. figure (2.4) shows the variation in current with potential. If the potential of the anode is increased gradually, a situation arrives when the effect of the space charge become negligible and any electron that is emitted from the cathode is able to reach the anode. 

Figure (2.4)

The current then become constant and is known as the saturation current. This is shown by the part bc in figure (2.4). Further increase in the anode potential does not change the magnitude of the photocurrent.

If the potential of the anode is made negative with respect to the cathode, the electrons are repelled by the anode. Some electrons go back to the cathode so that the current decreases. At a certain value of this negative potential, the current is completely stopped. The smallest magnitude of the anode potential which just stops the photocurrent, is called the stopping potential. 

The stopping potential is related to the maximum kinetic energy of the ejected electrons. To stop the current, we must ensure that even the fastest electron fails to reach the anode. Suppose, the anode is kept at a negative potential of magnitude V。with respect to the cathode. As a photoelectron travels from the cathode to the anode, the potential energy increases by eV。.This is equal to the decrease in the kinetic energy of the photoelectron. as it reaches the anode, is Kmax - eV。. If  the fastest electron just fails to reach the anode, we should have

eV  = Kmax = hc/λ-  φ

or,       V。= hc/e(1/λ) - φ/e .      ...(2.5)

We see that the stopping potential V。depends on the wavelength of the light and the work function of the metal. It does not depend on the intensity of light. Thus, if an anode potential of -2.0 V stops the photocurrent from a metal when a 1 W source of light is used, the same potential of -2.0 V will stop the photocurrent when a  100 W source of light of the same wavelength is udes.

The saturation current increases as the intensity of light increases. This is because, a large number of photons now fall on the meta surface and hence a larger number of electrons interact with photons. The number of electrons emitted increases and hence the current increases.

Figure (2.6a) shows plots of photocurrent versus anode potential for three difference intensities of light.

Note that the stopping potential V。is independent of the intensity of light.

The variation in stopping potential V。with 1/λ is shows in figure(2.6b) for cathodes of two different metals. From equation (2.6), the slope of each curve is 

tanθ = hc/e

which is the same for all metals. Also, the curves intersect the 1/λ axis where V。is zero. Using equation (2.6), this corresponds to 

hc/λ。= φ

or,      1/λ。= φ/hc

Which is inverse of the threshold wavelength.

Let us summaries the results obtained from the experiments described above.

1. When light of sufficiently small wavelength falls on a metal surface, the metal emits electrons. The emission is almost instantaneous.

2. There is a threshold wavelength λ。for a given metal such that if the wavelength of light is more than λ。, no photoelectric effect takes place.

3. The kinetic energies of the photoelectrons vary from zero to a maximum of Kmax = hc/λ - φ

with usual meaning of the symbols.

4. The photocurrent may be stopped by applying a negative potential to the anode with respect to the cathode. The minimum magnitude of the potential needs to stop the photocurrent is called the stopping potential. It is proportional to the maximum kinetic energy of the photoelectrons.

5. The stopping potential does not depend on the intensity of the incident light. This means that the kinetic energy of the photoelectrons is independent of intensity of light.

6. The stopping potential depends on the wavelength of the incident light.

7. The photocurrent increases if the intensity of the incident light is increased.

What is difference of conductors insulators and semiconductors.

CONDUCTORS, INSULATORS AND SEMICONDUCTORS.

Any piece of matter of moderate size contains millions and millions of atoms or molecules. Each atom contain a positively charged nucleus and several electrons going round it.

In gases, the atoms or molecules almost do not interact with each other. In solids and liquids, the interaction is comparatively stronger. It turns out that the material may be broadly divided into three categories according to their behaviour when they are placed in an electric field.

(a) Conductors 

In some materials, the outer electrons of each atom or molecule are only weakly bound to it. These electrons are almost free to move throughout the body of the material and are called free electrons. They are also known as conduction electrons. When such a material is placed in an electric field, the free electrons move in a direction opposite to the field . Such materials are called conductors.

(b) Insulators

Another class of materials is called insulators in which all the electrons are tightly bound to their respective atoms or molecules. Effectively, there are no free electrons. When such a material is placed in an electric field, the electrons may slightly shift opposite to the field but they can't leave their parent atoms or molecules and hence can't move through long distances. Such materials are also called dielectrics. 

(c) semiconductors

In semiconductors, the behaviour is like an insulator at the temperature 0 K. But at higher temperatures, a small number of electrons are able to free themselves and they respond to the applied electric field. As the number of free electrons in a semiconductor is much smaller than that in a conductor, its behaviour is in between a conductor and an insulator and hence, the name semiconductor. A freed electron in a semiconductors leaves a vacancy in its normal bound position. These vacancies also help in conduction.

What is force and how many types of force in nature ,(gravitation, electromagnetic, nuclear, weak..)

WHAT IS FORCE

 INTRODUCTION

Force is a word which we have all heard about. When you push or pull some object you exert a force on it. If you push a body you exert a force away from yourself; when you pull, you exert a force toward yourself. When you hold a heavy block in your hand you exert a large force; when you hold a light block, you exert a small force.

can nonliving bodies exert a force ? Yes, they can. If we stand in a great storm, we feel that the wind is exerting a force on us. When we suspend a heavy block from a rope, the rope holds the block just as a man can hold it in air. When we comb our dry hair and bring the comb close to small pieces of paper, the pieces jump to the comb. The comb has attracted the paper pieces i.e. the comb has exerted force on the pieces. When a cork is dipped in water it comes to the surface; if we want to keep it inside water, we have to push it downward. We say that water exerts a force on the cork in the upward direction.

Force is an interaction between two objects. Force is exerted by an object A on another object B, for any force you may ask two questions, (i) who exerted this force and (ii) on which object was this force exerted ? Thus, when a block is kept on a table, the table exerts a force on the block to hold it.

Force is a vector quantity and if more than one force act on a particle we can find the resultant force using the laws of vector addition.

 

The various types of forces in nature can be grouped in four categories :

(a) Gravitational,         (b) Electromagnetic,

(c) Nuclear          and   (d) Weak. 

 (a) GRAVITATIONAL FORCE

Any two bodies attract each other by virtue of their masses. The force of attraction between two point masses is F = Gm1m2/r^2, where m1 and m2 are masses of the particles and r is the distance between them.G is a universal constant having the value 6.67 ｘ10 ^-11 n-m^2/kg^2 . To find the gravitational force on an extended body by another such body, we have to write the force on each particle of the 1st body by all the particles of the second body and then we have to sum up vectorially all the forces acting on the first body.

(b) ELECTROMAGNETIC (EM) FORCE

Over and above the gravitational force Gm1m2/r^2, the particles may exert upon each other electromagnetic forces. If two particles having charges q1 and q2 are at rest with respect to the observer, the force between them has a magnitude 

F = 1/4πદ q1q2/r^2

where દ = 8.85419 ｘ10^-12  c^2/N-m^2 is a constant. The quantity 1/4πદ is 9.0 ｘ10^9 N-m^2/c^2

Where is called coulomb force and it acts along the line joining the particles. If q1 and q2 and of same nature (both positive or both negative), the force is repulsive otherwise it is attractive. It is this force which is responsible for the attraction of small paper pieces when brought near a recently used comb. The electromagnetic force between moving charge particles is comparatively more complicated and contains term other than the coulomb force.

Ordinary matters. Each electron has 1.6 ｘ 10^-19 coulomb of negative charge and each proton has an equal amount of positive charge. In atoms, the electron are bound by the electromagnetic force acting on them due to the protons. The atoms combine to form molecules due to the electromagnetic forces. A lot of atomic and molecular phenomena result from electromagnetic forces between the subatomic particles (electrons, protons, charged mesons,etc.).

(c) NUCLEAR FORCES

Each atom contains a certain number of protons and neutrons in its nucleus. The nucleus occupies a volume of about 10^-44 m^3 whereas the atom itself has a volume of about 10^-23m^3. Thus, the nucleus occupies only 1/10^21 of  the volume of the atom. Yet it contains about 99.98% of the mass of the atom. The atomic nucleus of a non-radioactive electrons are removed from a helium atom, we get the bare nucleus of helium which is called an alpha particle. The alpha particle is a stable object and once created it can remain intact until it is not made to interact with other objects.

An alpha particle contains two protons and two neutrons. The protons will repel each other due to the Coulomb force and will try to break the nucleus. Neutrons will be silent spectators in this electro-magnetic drama (Remember, neutron is an uncharged particle). Then, why does the coulomb force fail to break the nucleus ? Can it be the gravitational attractive force which keeps the nucleus bound ? All the protons and the neutrons will take part in this attraction, but if calculated, the gravitational attraction will trun out to be totally negligible as compared to the coulomb repulsion.

In fact, a a third kind of force, altogether different and over and above the gravitational and electromagnetic force, is operating here. These forces are called Nuclear force and are exerted only if the interacting particles are protons or neutrons or both.

(d) WEAK FORCES

Yet another kind of force is encountered when reactions involving protons,electrons and neutrons take place. A neutron can change itself into a proton and simultaneously emit an electron and a particle called antinutrino. This is called β(negative) decay. Never think that a neutron is made up of a proton, an electron and an antineutrino. A proton can also change into neutron and simultaneously emit a positron ( and a neutrino ). This is called β (positive) decay. The force responsible for these changes are different from gravitational, electro-magnetic or nuclear force. Such force are called weak force. The range of weak force is very small, in fact much smaller than size of a proton or a neutron. Thus its effect is experienced inside such particles only.

What is Scattering of Light in physics

WHAT IS SCATTERING OF LIGHT

 When a parallel beam of light passes through a gas, a part of it appears in directions other than the incident direction.

This phenomenon is called scattering of light. The basic process in scattering is absorption of light by the molecules followed by its re-radiation in different directions. The strength of scattering can be measured by the loss of energy in the light beam as it passes through the gas. It should be distinguished from the absorption of light as it passes through a medium. In absorption, the light energy is converted into internal energy of the medium whereas in scattering, the light energy is radiated in other direction. The strength of scattering depends on the wavelength of the light beside the size of the particles which cause scattering.If these particles are smaller than the wavelength, the scattering is proportional to (1/λ^ 4). This is known as Rayleigh's law of scattering. Thus, red light is scattered the least and violet is scattered the most. This is why, red signals are used to indicate dangers. Such a signal goes to large distances without an appreciable loss due to scattering.

The blue appearance of sky is due to scattering of sunlight from the atmosphere. When you look at the sky, it is the scattered light that enters the eyes. Among the shorter wavelengths, the colour blue is present in larger proportion in sunlight Light of short wavelengths are strongly scattered by the air molecules and reach the observer. This explains the blue colour of sky. Another natural phenomenon related to the scattering of light is the red appearance of sun at the sunset and at the sunrise. At these times, the sunlight has to travel a large distance through the atmosphere. The blue and neighbouring colours are scattered away in the path and the light reaching the observer is predominantly red.

If the earth had atmosphere, the sky would appear black and stars could be seen during day hours. In fact if you go about 20 km up, where the atmosphere becomes quite thin, the sky does appear black and stars are visible during day hours as astronauts have found.

Besides air molecules, water particles, dust, etc., also scatter light. The appearance of sky is affected by the presence of these scattering centres. On a humid day before rains, the sky appears light blue whereas, on a clear day it appears deep blue. The change in the quality of colour of sky results the fact that the water droplets and the dust particles may have size greater than the wavelength of light. Rayleigh's law of scattering does not operate in this case and colours other than blue may be scattered in larger proportion. The appearance of sky in large industrial cities is also different from villages. An automobile engine typically ejects about 10^11 particles per second, similarly for other machines. Such particles remain suspended in air for quite long time unless rain or wind clears them. Often the sky looks hazy with a greyish tinge in such areas.

What is Significant Digit in physics simple explanation

WHAT IS SIGNIFICANT DIGITS

When a measurement is made, a numerical value is read generally from some calibrated scale. To measure the length of a body we can place a metre scale  in contact with the body. One end  of the body may be made to coincide with the zero of the metre scale and the reading just in front of the other end is noted from the scale. When an electric current is measured with an ammeter the reading of the pointer on the graduation of the ammeter is noted. The value noted down includes all the digit that can be directly read from the scale and one doubtful digit at the end. The doubtful digit corresponds to the eye estimation within the smallest subdivision of the scale. This smallest subdivision is known as the least count of the instrument. In a metre scale, the major graduations are at an interval of one centimetre and ten subdivisions are made between two consecutive major graduations. Thus, the smallest subdivision measures a millimetre. If one end of the object coincides with the zero of the metre scale, the other end may fall between 10.4 cm and 10.5 cm mark of the scale (figure 4.1). We can estimate the distance between the 10.4 cm mark and the edge of the body as follows.

Figure 4.1

We mentally divide the 1mm division in 10 equal parts and guess on which part is the edge falling. We may note down the reading as 10.46 cm. The digits 1, 0 and 4 are certain but 6 is doubtful. All these digits are called significant digits. We say that the length is measured up to four significant digits. The rightmost or the doubtful digit is called the least significant digit and the leftmost digit is called the most significant digit.

There may be some confusion if there are zeroes at the right end of the number. For example, if a measurement is quoted as 600 mm and we know nothing about the least count of the scale we cannot.    be sure whether the last zeros are significant or not. If the scale had marking only at each metre then the edge must be between the marks 0 m and 1m and the digit 6 is obtained only through the eye estimation. Thus, 6 is the doubtful digit and the zeros after that are insignificant. But if the scale had markings at centimetres, the number read is 60 and these two digit are significant, the last zero is insignificant. If the scale used had markings at millimetres, all the three digits 6, 0, 0 are significant To avoid confusion one may report only the significant digit and the magnitude may be correctly described by proper powers of 10. For example, if only 6 is significant in 600mm we may write it as 6ｘ10^2 mm. If 6 and the first zero are significant we may write it as 6ｘ10^2 mm and if all the three digits are significant we may write it as 6.00 ☓ 10^2 mm.

If the integer part is zero any number of continuous zeros just after the decimal part is insignificant. Thus, the number of significant digits in 0.0023 is two and in 1.0023 is five.

SIGNIFICANT DIGITS IN CALCULATIONS 

When two or more numbers are added, subtracted, multiplied or divided, how to decide about the number of significant digits in the answer? For example, suppose the mass of A is measured to be 12.0 kg and of another body B to be 7.0 kg. What is the ratio of the mass of A to the mass of B ? Arithmetic will give this as 

12.0 / 7.0 = 1.714285.....

However, all the digits of this answer cannot be significant. The zero of 12.0 is a doubtful digit and the zero of 7.0 is also doubtful. The quotient cannot have so many reliable digits. The rules for deciding the number of significant digit in a arithmetic calculation are listed below.

1. In  a multiplication or division of two or more quantities, the number of significant digits in the answer is equal to the number of significant digits in the quantity which has the minimum number of significant digits. Thus, 12.0 / 7.0 will have two significant digits only.

The insignificant digits are dropped from the result if they appear after the decimal point. They are replaced by zeros if they appear to the left of the decimal point. The least significant digit is rounded according to the rules given below. If the digit next to the one rounded is more than 5, the digit to be rounded is increased by 1. If the digit next to the one rounded is less than 5, the digit to be rounded is left unchanged. If the digit next to the one rounded is 5, then the digit to be rounded is increased by 1 if it is odd and is left unchanged if it is even.

2. For addition or subtraction write the numbers one below the other with all the decimal point in one line. Now locate the first column from left that has a doubtful digit. All digits right to this column are dropped from all the numbers and rounding is done to this column. The addition or subtraction is now performed to get the answer.