Learning Objectives

Learning Objectives At the end of this lesson, students will be able to: state the laws of refraction. list the properties of light. explain the scattering of light and its various kinds. understand the images formed by concave and convex lens. analyze the ray diagram of concave and convex lens. understand the working of human eye and optical instruments solve numerical problems Learning Objectives At the end of this lesson, students will be able to Understand the concept of heat and temperature Know the absolute scale of temperature Understand the thermal energy and the thermal equilibrium Classification of expansion of substances Know the fundamental laws of gases Distinguish between real gas and ideal gas Derive the ideal gas equation Solve the numerical problems Thermal Physics The relation between the different types of scale of temperature: Celsius and Kelvin: K = C + , Fahrenheit and Kelvin: [K] = (F + ) × K = – °C. . . Thermal equilibrium Two or more physical systems or bodies are said to be in thermal equilibrium if there is no net flow of thermal energy between the systems. Heat energy always flows from one body to the other due to a temperature difference between them. Thus, you can define thermal equilibrium in another way. If two bodies are said to be in thermal equilibrium, then, they will be at the same temperature. What will happen if two bodies at different temperatures are brought in contact with one other? There will be a transfer of heat energy from the hot body to the cold body until a thermal equilibrium is established between them. This is depicted in Figure . . Figure . Establishing thermal equilibrium When a cold body is placed in contact with a hot body, some thermal energy is transferred from the hot body to the cold body. As a result, there is some rise in the temperature of the cold body and decrease in the temperature of the hot body. This process will continue until these two bodies attain the same temperature. . THERMAL ENERGY If you leave a cup of hot milk on a table for some time, what happens? The hotness of the milk decreases after some time. Similarly, if you keep a bottle of cold water on a table, the water becomes warmer after some time. What do you infer from these observations? In the case of hot milk, there is a flow of energy from the cup of milk to the environment. In the second case, the energy is transferred from the environment to the water bottle. This energy is termed as “thermal energy”. When a hot object is in contact with another cold object, a form of energy flows from the hot object to the cold object, which is known as thermal energy . Thus, thermal energy is a form of energy which is transferred between any two bodies due to the difference in their temperatures. Thermal energy is also known as 'heat energy' or simply 'heat'. Heat energy is the agent, which produces the sensation of warmth and makes bodies hot. The process in which heat energy flows from a body at a higher temperature to another object at lower temperature is known as heating . This process of transmission of heat may be done in any of the ways like conduction, convection or radiation. Heat is a scalar quantity. The SI unit of heat energy absorbed or evolved is joule (J) . During the process of transferring heat energy, the body at lower temperature is heated while the body at higher temperature is cooled. Thus, sometimes, this process of transfer of heat energy is termed as 'cooling'. But, in most of the cases the term 'heating' is used instead of 'cooling'. When the thermal energy is transferred from one body to another, this results in the rise or lowering of the temperature of either of the bodies. . . Characteristic features of heat energy transfer . Heat always flows from a system at higher temperature to a system at lower temperature. . The mass of a system is not altered when it is heated or cooled. . For any exchange of heat, the heat gained by the cold system is equal to heat lost by the hot system. Heat gained = Heat lost . . Other units of Heat energy Though the SI unit of heat energy is joule, there are some other commonly used units. Calorie: One calorie is defined as the amount of heat energy required to rise the temperature of gram of water through °C. Kilocalorie: One kilocalorie is defined as the amount of heat energy required to rise the temperature of kilogram of water through °C. . EFFECT OF HEAT ENERGY When a certain amount of heat energy is given to a substance, it will undergo one or more of the following changes: Temperature of the substance rises. The substance may change its state from solid to liquid or from liquid to gas. The substance will expand when heated. The rise in temperature is in proportion to the amount of heat energy supplied. It also depends on the nature and mass of the substance. About the rise in temperature and the change of state, you have studied in previous classes. In the following section, we shall discuss about the expansion of substances due to heat. . . Expansion of Substances When heat energy is supplied to a body, there can be an increase in the dimension of the object. This change in the dimension due to rise in temperature is called thermal expansion of the object. The expansion of liquids (e.g. mercury) can be seen when a thermometer is placed in warm water. All forms of matter (solid, liquid and gas) undergo expansion on heating. a) Expansion in solids When a solid is heated, the atoms gain energy and vibrate more vigorously. This results in the expansion of the solid. For a given change in temperature, the extent of expansion is smaller in solids than in liquids and gases. This is due to the rigid nature of solids. The different types of expansion of solid are listed and explained below: . Linear expansion . Superficial expansion . Cubical expansion . Linear expansion: When a body is heated or cooled, the length of the body changes due to change in its temperature. Then the expansion is said to be linear or longitudinal expansion . The ratio of increase in length of the body per degree rise in temperature to its unit length is called as the coefficient of linear expansion . The SI unit of Coefficient of Linear expansion is K - . The value of coefficient of linear expansion is different for different materials. Figure . Linear expansion The equation relating the change in length and the change in temperature of a body is given below: = α L ∆T ∆L L o ∆ L - Change in length (Final length- Original length) L o - Original length ∆T - Change in temperature (Final temperature - Initial temperature) α L - Coefficient of linear expansion. Thermal Physics . Superficial expansion: If there is an increase in the area of a solid object due to heating, then the expansion is called superficial or areal expansion. Superficial expansion is determined in terms of coefficient of superficial expansion. The ratio of increase in area of the body per degree rise in temperature to its unit area is called as coefficient of superficial expansion . Coefficient of superficial expansion is different for different materials. The SI unit of Coefficient of superficial expansion is K - The equation relating to the changein area and the change in temperature is given below: Figure . Superficial expansion Learning Objectives At the end of this lesson, students will be able to: Make an electric circuit. Differentiate between electric potential and potential difference. Infer what electrical resistivity and conductivity mean. Know the effective resistance of a system of resistors connected in series and parallel. Understand the heating effect of the electric current. Define electric power and electric energy and explain domestic electric circuits. Know the modern appliances such as LED bulb and LED television. Learning Objectives By the end of this section, the students will be able to: Understand how sound is produced and transmitted. Relate the speed of sound, its frequency, and its wavelength. Know the speed of sound in various media. Explain the factors affecting the speed of sound in a gaseous medium. Demonstrate the phenomenon of reflection of sound. Determine the speed of sound using the method of echo. Understand Doppler Effect. Solve numerical problems related to the above topics. INTRODUCTION Sound plays a major role in our lives. We communicate with each other mainly through sound. In our daily life, we hear a variety of sounds produced by different sources like humans, animals, vehicle horns, etc. Hence, it becomes inevitable to understand how sound is produced, how it is propagated and how you hear the sound from various sources. It is sometimes misinterpreted that acoustics only deals with musical instruments and design of auditoria and concert halls. But, acoustics is a branch of physics that deals with production, transmission, reception, control, and effects of sound. You have studied about propagation and properties of sound waves in IX standard. In this lesson we will study about reflection of sound waves, Echo and Doppler effect. . SOUND WAVES When you think about sound, the questions that arise in your minds are: How is sound produced? How does sound reach our ears from various sources? What is sound? Is it a force or energy? Let us answer all these questions. By touching a ringing bell or a musical instrument while it is producing music, you can conclude that sound is produced by vibrations. The vibrating bodies produce energy in the form of waves, which are nothing but sound waves (Figure . ). Figure . Production of sound waves Learning Objectives After learning this unit, students will be able to Define radio activity. Distingush between natural and artificial radio activity. Relate the properties of alpha, beta and gamma rays. State Soddy and Fajan’s displacement law of nuclear disintegration. Understand the concept of nuclear fission and nuclear fusion. Identify fissionable materials. Analyze controlled and uncontrolled chain reactions. Explain the principle of atom bomb and hydrogen bomb. List the uses of radio activity. Understand the components of a nuclear reactor. Identify the precautionary measures while handling a radioactive material. Nuclear physics . RADIOACTIVITY . . Discovery of radioactivity In , French physicist Henri Becquerel finished his research for the week and stored a certain amount of uranium compound away in a drawer for the week end. By chance, an unexposed photographic plate was also stored in the same drawer. After a week he returned and noticed that the film had been exposed to some radiation. He discovered that he could reproduce the effect whenever he placed uranium near a photographic film. Apparently, uranium radiated something that could affect a photographic plate. This phenomenon was called as Radioactivity . Uranium was identified to be a radioactive element. Two years later, the Polish physicist Marie Curie and her husband Pierre Curie detected radioactivity in 'Pitchblende', a tiny black substance. They were not surprised at the radioactivity of pitchblende, which is known as an ore of uranium. Later, they discovered that the radiation was more intense from pure uranium. Also, it was found that the pitchblende had less concentration of uranium. They concluded that some other substance was present in pitchblende. After separating this new substance, they discovered that it had unknown chemical properties and it also emitted radiations spontaneously like uranium. They named this new substance as 'Radium' .The radioactive elements emit harmful radioactive radiations like alpha rays or beta rays or gamma rays. . . Definition of radioactivity The nucleus of some elements is unstable. Such nuclei undergo nuclear decay and get converted into more stable nuclei. During this nuclear reaction, these nuclei emit certain harmful radiations and elementary particles. The phenomenon of nuclear decay of certain elements with the emission of radiations like alpha, beta, and gamma rays is called 'radioactivity' and the elements, which undergo this phenomenon are called 'radioactive elements'. . . Natural Radioactivity The elements such as uranium and radium undergo radioactivity and emit the radiations on their own without any human intervention. This phenomenon of spontaneous emission of radiation from certain elements on their own is called 'natural radioactivity'. The elements whose atomic number is more than undergo spontaneous radioactivity. Eg: uranium, radium, etc. There are only two elements, which have been identified as radioactive substances with atomic number less than . They are technetium (Tc) with atomic number and promethium (Pm) with atomic number . There have been radioactive substances discovered so far. Most of them are rare earth metals and transition metals. . . Artificial Radioactivity (or) Induced Radioactivity The phenomenon by which even light elements are made radioactive, by artificial or induced methods, is called 'artificial radioactivity' or 'man-made radioactivity'. This kind of radioactivity was discovered by Irene Curie and F.Joliot in . Artificial radioactivity is induced in certain lighter elements like boron, aluminium etc., by bombarding them with radiations such as 'alpha particles' emitted during the natural radioactivity of uranium. This also results in the emission of invisible radiations and elementary particles. During such a disintegration, the nucleus which undergoes disintegration is called 'parent nucleus' and that which is produced after the disintegration is called a 'daughter nucleus'. The particle, which is used to induce the artificial disintegration is termed as projectile and the particle which is produced after the disintegration is termed as ejected particle. When the projectile hits the parent nucleus, it is converted into an unstable nucleus, which in turn decays spontaneously emitting the daughter nucleus along with an ejected particle. Activity . Using the periodic table, list out the radioac- tive elements. Also identify the name of the groups in which they are present. If you denote the parent and daughter nuclei as X and Y respectively, then the nuclear disintegration is represented as follows: X (P,E) Y. Here, P and E represent the projectile particle and ejected particle respectively. Example: Be + He C * C * C + n In the above nuclear reaction, C * is unstable and is radioactive. This reaction can be represented as Be (α, n) C Be + He C + n . . Units of Radioactivity Curie: It is the traditional unit of radioactivity. It is defined as the quantity of a radioactive substance which undergoes . × disintegrations in one second. This is actually close to the activity of g of radium . curie = . × disintegrations per second. Rutherford (Rd): It is another unit of radioactivity. It is defined as the quantity of a radioactive substance, which produces disintegrations in one second. Rd = disintegrations per second. Becquerel (Bq) : It is The SI unit of radioactivity is becquerel. It is defined as the quantity of one disintegration per second. Roentgen (R): It is The radiation exposure of γ and x-rays is measured by another unit called roentgen. One roentgen is defined as the quantity of radioactive substance which produces a charge of . × - coulomb in kg of air under standard conditions of pressure, temperature and humidity. Table . Comparison between Natural and Artificial Radioactivity S.No. Natural radioactivity Artificial radioactivity Emission of radiation due to self- disintegration of a nucleus. Emission of radiation due to disintegration of a nucleus through induced process. Alpha, beta and gamma radiations are emitted. Mostly elementary particles such as neutron, positron, etc. are emitted. It is a spontaneous process. It is an induced process. Exhibited by elements with atomic number more than . Exhibited by elements with atomic number less than . This cannot be controlled. This can be controlled. Nuclear physics . . Radioactive displacement law In , Soddy and Fajan framed the displacement laws governing the daughter nucleus produced during an alpha and beta decay. They are stated below: (i) When a radioactive element emits an alpha particle, a daughter nucleus is formed whose mass number is less by units and the atomic number is less by units, than the mass number and atomic number of the parent nucleus. (ii) When a radioactive element emits a beta particle, a daughter nucleus is formed whose mass number is the same and the atomic number is more by unit, than the atomic number of the parent nucleus. . ALPHA, BETA AND GAMMA RAYS When a radioactive nucleus undergoes radioactivity, it emits harmful radiations. These radiations are usually comprised of any of the three types of particles. They are alpha(α), beta (β) and gamma(γ) rays. Uranium, named after the planet Uranus, was discovered by Martin Klaproth, a German chemist in a mineral called pitchblende. . . Properties of Alpha, Beta and Gamma rays These three particles possess certain similarities and dissimilarities in their properties as listed below in Table . . Table . Properties of alpha, beta and gamma rays Properties α rays β rays γ rays What are they? Helium nucleus ( He ) consisting of two protons and two neutrons. They are electrons ( – e ), basic elementary particle in all atoms. They are electromagnetic waves consisting of photons. Charge Positively charged particles. Charge of each alpha particle = +2e Negatively charged particles. Charge of each beta particle = –e Neutral particles. Charge of each gamma particle = zero Ionising power time greater than β rays and , times greater than γ rays Comparatively low Very less ionization power Penetrating power Low penetrating power (even stopped by a thick paper) Penetrating power is greater than that of α rays. They can penetrate through a thin metal foil. They have a very high penetrating power greater than that of β rays. They can penetrate through thick metal blocks. Effect of electric and magnetic field Deflected by both the fields. (in accordance with Fleming’s left hand rule) Deflected by both the fields; but the direction of deflection is opposite to that for alpha rays. (in accordance with Fleming’s left hand rule) They are not deflected by both the fields. Speed Their speed ranges from / to / times the speed of light. Their speed can go up to / times the speed of light. They travel with the speed of light. . . Alpha decay A nuclear reaction in which an unstable parent nucleus emits an alpha particle and forms a stable daughter nucleus, is called 'alpha decay'. E.g.: Decay of uranium (U ) to thorium (Th ) with the emission of an alpha particle. U Th + He ( α - decay ) In α - decay, the parent nucleus emits an α particle and so it is clear that for the daughter nucleus, the mass number decreases by four and the atomic number decreases by two as illustrated in Figure . Figure . Alpha decay . . Beta decay A nuclear reaction, in which an unstable parent nucleus emits a beta particle and forms a stable daughter nucleus, is called 'beta decay'. E.g.: Beta decay of phosphorous. P S + - e (β - decay) In β - decay there is no change in the mass number of the daughter nucleus but the atomic number increases by one. Note: In a nuclear reaction, the element formed as the product nucleus is identified by the atomic number of the resulting nucleus and not by its mass number. . . Gamma decay In a γ - decay, only the energy level of the nucleus changes. The atomic number and mass number of the radioactive nucleus remain the same. . NUCLEAR FISSION . . Definition In , German Scientist Otto Hahn and F.Strassman discovered that when a uranium nucleus is bombarded with a neutron, it breaks up into two smaller nuclei of comparable mass along with the emission of a few neutrons and energy. This process of breaking (splitting) up of a heavier nucleus into two smaller nuclei with the release of a large amount of energy and a few neutrons is called 'nuclear fission'. E.g.: Nuclear fission of a uranium nucleus (U ) U + n Ba + Kr + n + Q (energy) The average energy released in each fission process is about . × - J. Nuclear fission is pictorially represented in Figure . . Figure . Nuclear fission . . Fissionable materials A fissionable material is a radioactive element, which undergoes fission in a sustained manner when it absorbs a neutron. It is also termed as 'fissile material'. E.g.: U , plutonium (Pu and Pu ) All isotopes of uranium do not undergo nuclear fission when they absorb a neutron. For example, natural uranium consists of . % of U and . % of U . Of these two, U does not undergo fission Nuclear physics material. This results in the release of a huge amount of energy within a fraction of a second. This kind of chain reaction is used in the atom bomb to produce an explosion. Figure . represents an uncontrolled chain reaction. Kr U Ba n n n Figure . Uncontrolled chain reaction . . Critical Mass During a nuclear fission process, about to neutrons are released. But, all these neutrons may not be available to produce further fission. Some of them may escape from the system, which is termed as 'leakage of neutrons' and some may be absorbed by the non-fissionable materials present in the system. These two factors lead to the loss of neutrons. To sustain the chain reaction, the rate of production of neutrons due to nuclear fission must be more than the rate of its loss. This can be achieved only when the size (i.e., mass) of the fissionable material is equal to a certain optimum value. This is known as 'critical mass'. The minimum mass of a fissile material necessary to sustain the chain reaction is called 'critical mass (m c )'. It depends on the nature, density and the size of the fissile material. If the mass of the fissile material is less than the critical mass, it is termed as 'subcritical'. If the mass of the fissile material is more than the critical mass, it is termed as 'supercritical'. Activity . Using beads make a chain reaction model whereas U undergoes fission. Hence, U is a fissionable material and U is non- fissionable. There are some radioactive elements, which can be converted into fissionable material. They are called as fertile materials . E.g.: Uranium- , Thorium- , Plutonium- . . . Chain Reaction A uranium nucleus (U- ) when bombarded with a neutron undergoes fission producing three neutrons. These three neutrons in turn can cause fission in three other uranium nuclei present in the sample, thus producing nine neutrons. These nine neutrons in turn may produce twenty seven neutrons and so on. This is known as 'chain reaction'. A chain reaction is a self- propagating process in which the number of neutrons goes on multiplying rapidly almost in a geometrical progression. Two kinds of chain reactions are possible. They are: (i) controlled chain reaction and (ii)uncontrolled chain reaction. (a) Controlled chain reaction In the controlled chain reaction the number of neutrons released is maintained to be one. This is achieved by absorbing the extra neutrons with a neutron absorber leaving only one neutron to produce further fission. Thus, the reaction is sustained in a controlled manner. The energy released due to a controlled chain reaction can be utilized for constructive purposes. Controlled chain reaction is used in a nuclear reactor to produce energy in a sustained and controlled manner. (b) Uncontrolled chain reaction In the uncontrolled chain reaction the number of neutrons multiplies indefinitely and causes fission in a large amount of the fissile . . Atom bomb The atom bomb is based on the principle of uncontrolled chain reaction. In an uncontrolled chain reaction, the number of neutrons and the number of fission reactions multiply almost in a geometrical progression. This releases a huge amount of energy in a very small time interval and leads to an explosion. Structure: An atom bomb consists of a piece of fissile material whose mass is subcritical. This piece has a cylindrical void. It has a cylindrical fissile material which can fit into this void and its mass is also subcritical. When the bomb has to be exploded, this cylinder is injected into the void using a conventional explosive. Now, the two pieces of fissile material join to form the supercritical mass, which leads to an explosion. The structure of an atom bomb is shown in Figure . U U U Chemical Substance explosion by remote control method Moving cylinder Less than the critical mass Figure . Atom bomb During this explosion tremendous amount of energy in the form of heat, light and radiation is released. A region of very high temperature and pressure is formed in a fraction of a second along with the emission of hazardous radiation like γ rays, which adversely affect the living creatures. This type of atom bombs were exploded in at Hiroshima and Nagasaki in Japan during the World War II. . NUCLEAR FUSION You have learnt that energy can be produced when a heavy nucleus is split up into two smaller nuclei. Similarly, energy can be produced when two lighter nuclei combine to form a heavier nucleus. This phenomenon is known as nuclear fusion. . . Definition The process in which two lighter nuclei combine to form a heavier nucleus is termed as 'nuclear fusion'. E.g.: H + H He + Q (Energy) Here, H represents an isotope of hydrogen known as 'deuterium'. The average energy released in each fusion reaction is about . × - J. Figure . represents this. Figure . Nuclear fusion Electron Volt (eV) is the unit used in nuclear physics to measure the energy of small particles. It is nothing but the energy of one electron when it is accelerated using an electric potential of one volt. 1eV = . × - joule. million electron volt = MeV = eV (mega electron volt) The energy released in a nuclear fission process is about MeV. Nuclear physics The mass of the daughter nucleus formed during a nuclear reaction (fission and fusion) is lesser than the sum of the masses of the two parent nuclei. This difference in mass is called mass defect. This mass is converted into energy, according to the mass-energy equivalence. This concept of mass-energy equivalence was proposed by Einstein in . It stated that mass can be converted into energy and vice versa. The relation between mass and energy proposed by Einstein is E = mc where c is the velocity of light in vacuum and is equal to × ms – . The nuclear bomb that was dropped in Hiroshima during World War II was called as 'Little boy'. It was a gun-type bomb which used a uranium core. The bomb, which was subsequently dropped over Nagasaki was called as 'Fat man'. It was an explosion type bomb, which used a plutonium core. . . Conditions necessary for nuclear fusion Earth’s atmosphere contains a small trace of hydrogen. If nuclear fusion is a spontaneous process at normal temperature and pressure, then a number of fusion processes would happen in the atmosphere which may lead to explosions. But, we do not encounter any such explosions. Can you explain why? The answer is that nuclear fusion can take place only under certain conditions. Nuclear fusion is possible only at an extremly high temperature of the order of to K and a high pressure to push the hydrogen nuclei closer to fuse with each other. Hence, it is named as 'Thermonuclear reaction'. Nuclear fusion is the combination of two lighter nuclei. The charge of both nuclei is positive. According to electrostatic theory, when they come closer they tend to repel each other. This repulsive force will be overcome by the kinetic energy of the nuclei at higher temperature of the order of to K. . . Stellar Energy The stars like our Sun emit a large amount of energy in the form of light and heat. This energy is termed as the stellar energy. Where does this high energy come from? All stars contain a large amount of hydrogen. The surface temperature of the stars is very high which is sufficient to induce fusion of the hydrogen nuclei. Fusion reaction that takes place in the cores of the Sun and other stars results in an enormous amount of energy, which is called as 'stellar energy. Thus, nuclear fusion or thermonuclear reaction is the source of light and heat energy in the Sun and other stars. . . Hydrogen Bomb Hydrogen bomb is based on the principle of nuclear fusion. A hydrogen bomb is always designed to have an inbuilt atom bomb which creates the high temperature and pressure required for fusion when it explodes. Then, fusion takes place in the hydrogen core and leads to the release of a very large amount of energy in an uncontrolled manner. The energy released in a hydrogen bomb (or fusion bomb) is much higher than that released in an atom bomb (or fission bomb). Sun fuses about million metric tons of hydrogen each second and radiates about . × joule of energy per second. When this energy is radiated towards the Earth, it decreases in its intensity. When it reaches the Earth its value is about . kilo joule per unit area in unit time. . USES OF RADIOACTIVITY Many radio isotopes can be obtained from radioactivity. These radio isotopes have found wide variety of applications in the fields of medicine, agriculture, industry and archeological research. . . Agriculture The radio isotope of phosphorous (P- ) helps to increase the productivity of crops. The radiations from the radio isotopes can be used to kill the insects and parasites and prevent the wastage of agricultural products. Certain perishable cereals exposed to radiations remain fresh beyond their normal life, enhancing the storage time. Very small doses of radiation prevent sprouting and spoilage of onions, potatoes and gram. . . Medicine Medical applications of radio isotopes can be divided into two parts: i) Diagnosis ii) Therapy Radio isotopes are used as tracers to diagnose the nature of circulatory disorders of blood, defects of bone metabolism, to locate tumors, etc. Some of the radio isotopes which are used as tracers are: hydrogen, carbon, nitrogen, sulphur, etc. • Radio sodium (Na ) is used for the effective functioning of heart. • Radio – Iodine (I ) is used to cure goiter. • Radio-iron is (Fe ) is used to diagnose anaemia and also to provide treatment for the same. • Radio phosphorous (P ) is used in the treatment of skin diseases. Table . Features of Nuclear fission and nuclear fusion S.No. NUCLEAR FISSION NUCLEAR FUSION The process of breaking up (splitting) of a heavy nucleus into two smaller nuclei is called 'nuclear fission' . Nuclear fusion is the combination of two lighter nuclei to form a heavier nucleus. Can be performed at room temperature. Extremely high temperature and pressure is needed. Alpha, beta and gamma radiations are emitted. Alpha rays, positrons, and neutrinos are emitted. Fission leads to emission of gamma radiation. This triggers the mutation in the human gene and causes genetic transform diseases. Only light and heat energy is emitted. Nuclear physics • Radio cobalt (Co ) and radio-gold (Au ) are used in the treatment of skin cancer. • Radiations are used to sterilize the surgical devices as they can kill the germs and microbes. . . Industries In industries, radioactive isotopes are used as tracers to detect any manufacturing defects such as cracks and leaks. Packaging faults can also be identified through radio activity. Gauges, which have radioactive sources are used in many industries to check the level of gases, liquids and solids. An isotope of californium (Cf ) is used in the airlines to detect the explosives in the luggage. An isotope of Americium (Am ) is used in many industries as a smoke detector. . . . Archeological research Using the technique of radio carbon dating, the age of the Earth, fossils, old paintings and monuments can be determined. In radio carbon dating, the existing amount of radio carbon is determined and this gives an estimate about the age of these things. . SAFETY MEASURES In day to day life, you do receive some natural radiation from the Sun. The radioactive elements present in the soil and rocks, the house hold appliances like television, microwave ovens, cell phones and the X-rays used in hospitals. These radiations do not produce any severe effects as they are very low in intensity. The second source of radiation exposure is man-made. These are due to nuclear reactors and during the testing of the nuclear devices in the atmosphere or in the ground. Improper and careless handling of radioactive materials release harmful radiations in our environment. These radiations are very harmful to the human body. A person who is exposed to radiations very closely or for a longer duration, is at a greater health risk and can be affected genetically. How old is our mother Earth? Any guess?? It is nearly . × years (around Crore lakh years). Wow!! . . Permitted range The International Commission on Radiological Protection (ICRP) has recommended certain maximum permissible exposure limits to radiation that is believed to be safe without producing any appreciable injury to a person. Safe limit of overall exposure to radiation is given as milli sievert per year. In terms of roentgen, the safe limit of receiving the radiation is about mR per week. If the exposure is R, it may cause fatal diseases like leukemia (death of red blood corpuscle in the blood) or cancer. When the body is exposed to about R, it leads to death. *Dosimeter is a device used to detect the levels of exposure to an ionizing radiation. It is frequently used in the environments where exposure to radiation may occur such as nuclear power plants and medical imaging facilities. Pocket dosimeter is used to provide the wearer with an immediate reading of his/her exposure to X-rays and γ rays. . . Preventive measures Figure. . Lead coated aprons model. Radioactive materials should be kept in a thick walled lead container. Lead coated aprons and lead gloves should be used while working with hazardous radioactive materials. You should avoid eating while handling radioactive materials. The radioactive materials should be handled only by tongs or by a remote control device. Dosimeters should be worn by the users to check the level of radiation. . NUCLEAR REACTOR A Nuclear reactor is a device in which the nuclear fission reaction takes place in a self-sustained and controlled manner to produce electricity. The first nuclear reactor was built in at Chicago, USA. . . Types of nuclear reactors Breeder reactor, fast breeder reactor, pressurized water reactor, pressurized heavy water reactor, boiling water reactor, water- cooled reactor, gas-cooled reactor, fusion reactor and thermal reactor are some types of nuclear reactors, which are used in different places world-wide. . . Components of a nuclear reactors The essential components of a nuclear reactor are (i) fuel, (ii) moderator, (iii) control rod, (iv) coolant and (v) protection wall. i. Fuel: A fissile material is used as the fuel. The commonly used fuel material is uranium. ii. Moderator: A moderator is used to slow down the high energy neutrons to provide slow neutrons. Graphite and heavy water are the commonly used moderators. iii. Control rod: Control rods are used to control the number of neutrons in order to have sustained chain reaction. Mostly boron or cadmium rods are used as control rods. They absorb the neutrons. iv. Coolant: A coolant is used to remove the heat produced in the reactor core, to produce steam. This steam is used to run a turbine in order to produce electricity. Water, air and helium are some of the coolants. v. Protection wall: A thick concrete lead wall is built around the nuclear reactor in order to prevent the harmful radiations from escaping into the environment. Control rod Figure . Schematic diagram of a nuclear reactor . . Uses of a nuclear reactor Nuclear reactors are widely used in power generation. They are also used to produce radio isotopes, which are used in a variety of applications. Some reactors help us to do research in the field of nuclear physics. Breeder reactors are used to convert non- fissionable materials into fissionable materials. . . Nuclear power plants in India Indian Atomic Energy Commission (AEC) was established in August by the Nuclear physics Department of Indian Scientific Research committee at Bombay (now Mumbai) in Maharashtra. It is the nodal agency for all the research done in the field of atomic energy. Dr. Homi Jahangir Bhaba was the first chairman of Indian Atomic Energy Commission. Now, it is known as Bhaba Atomic Research Centre (BARC). Nuclear power is the fifth largest source of power in India. Tarapur Atomic Power Station is India’s first nuclear power station. Now, there are a total of seven power stations, one each in Maharashtra, Rajasthan, Gujarat, Uttar Pradesh and two in Tamilnadu. In Tamilnadu, we have nuclear power stations in Kalpakkam and Kudankulam. Apsara was the first nuclear reactor built in India and Asia. Now, there are nuclear reactors which are operating in India. Some other operating reactors are Cirus Dhuruva Purnima Solved problem . Identify A, B, C, and D from the following nuclear reactions. (i) Al + A --------> P + B (ii) Mg + B --------> Na + C (iii) U + B --------> Np + D Solution: (i) Al + He --------> P + n (ii) Mg + n --------> Na + H (iii) U + n --------> Np + – e A is alpha particle, B is neutron, C is proton, and D is electron. Solved problem . A radon specimen emits radiation of . × GBq per second. Convert this disintegration in terms of curie. (one curie = . × disintegration per second) Bq = one disintegration per second one curie = . × Bq Bq = curie ∴ . × G Bq = . × × × . × = curie Solved problem . U experiences one α - decay and one β - decay. Find number of neutrons in the final daughter nucleus that is formed. Solution: Let X and Y be the resulting nucleus after the emission of the alpha and beta particles respectively. U α decay X + He X β decay Y + - e Number of neutrons = Mass number – Atomic number = – = Solved problem . Calculate the amount of energy released when a radioactive substance undergoes fusion and results in a mass defect of kg. Solution: Mass defect in the reaction (m) = kg Velocity of light (c) = × m s - By Einstein’s equation, Energy released E = mc So E = × ( × ) = . × J Points to Remember This phenomenon of spontaneous emission of radiation from certain elements on its own is called 'natural radioactivity'. Curie is defined as the quantity of a radioactive substance, which undergoes . × disintegrations in one second. This is actually close to the activity of g of radium- . . × Rutherford (Rd) is defined as the quantity of a radioactive substance which produces disintegrations in one second. Rd = disintegrations per second. The SI unit of radioactivity is becquerel. It is defined as the quantity of one disintegration per second. Helium nucleus ( He ) consisting of two protons and two neutrons is known as alpha particle. Beta particles are electrons ( - e ), which are the basic elementary particles present in all atoms. Gamma rays are electromagnetic waves consisting of photons. A nuclear reaction in which an unstable parent nucleus emits an alpha particle and forms a stable daughter nucleus is called as 'alpha decay'. A nuclear reaction in which an unstable parent nucleus emits a beta particle and forms a stable daughter nucleus is called as 'beta decay'. The process of breaking (splitting) up of a heavier nucleus into two smaller nuclei with the release of a large amount of energy is called 'nuclear fission'. The energy released in a nuclear fission process is about MeV. There are some radioactive elements which can be converted into a fissionable material. They are called as ' fertile materials '. e.g. Uranium- , Thorium- , Plutonium- . Controlled chain reaction is used in a nuclear reactor to produce energy in a sustained and controlled manner. The process in which two lighter nuclei combine to form a heavier nucleus is termed as 'nuclear fusion'. Nuclear fusion or thermonuclear reaction is the