Nature of Radioactive radiations for std 8 to 10 science

NATURE OF RADIOACTIVE RADIATIONS
Rutherford, a British physicist and others studied the nature of radioactive radiations, in 1898. They found that the radiations can be classified into three distinct types.
They named these three types as

alpha (a) rays, 

bata (b ) rays and 
gamms (g) rays.

Experiments show that
a-rays consist of positively charged particles,
B-rays consist of negatively charged particles (steam of electrons) and
y-rays (energy) are electrically neutral.
All radioactive atoms emit either a-rays or B-rays, never both,
and this may be accompanied by y-rays.
Alpha rays( or a-particles) are nothing but helium nuclei, i.e.
an a-particle consists of 2 protons and 2 neutrons bound to together.
Beta rays are electrons, identical to those that orbit the nucleus. But they are created within the nucleus. Gamma rays are high energy photons whose energy is higher then that of X-rays.

TRANSMUTATION 

When a radioactive nucleus emits an a-particle, the remaining nucleus will be different from the original because there is loss of two protons and two neutrons. For example, radium 226 (A=226, Z=88) is an a. emitter. It changes (decays) to a nucleus with z=(88-2)=86 and A=(226-4)=222. The new nucleus is radon (Rn).

Note :

To specify a given nucleus (or nuclide), we need to give only A and Z, where A represents the mass number, sum of the numbers of protons and neutrons and Z represents the atomic number the proton number, (A-Z) being neutron number.
A nucleus X with mass number A and atomic number, Z is represented by the symbol zXA 
Radium 226 is represented as 88Ra225..

The original nucleus is called parent nucleus and the new nucleus is Called daughter nucleus. The changing of one element into another is known as transmutation.
The transmutation of radium 226 into radon 222 can be written as
 88Ra226 --- > 86Rn222
Transmutation also occurs when a nucleus decays with the emission of a b particle. For example, Carbon-14 decays with the emission of a B particle; this can be written as
 6C14 --- > 7N14
During b decay it is assumed that a neutron in a nucleus changes to a proton and an electron. Newly formed electron is emitted and proton remains in the nucleus. This is how Z increases by 1.

HALF-LIFE 

Theoretically, the decay process of a radioactive element in never complete and there is always some residual radioactivity. In other words, time taken for all the atoms to disintegrate, is infinite. For this reason, the half-life of a radioactive element is considered. Half-life of a radioactive element is defined as the time taken by a radioactive sample of that element, to get reduced to half its initial amount. Half-life is useful in comparing the activities of different radioactive elements. Half-life varies from micro seconds to billions of years.
Half –life is found to be a characteristic feature of a radioactive element. Half-life is represented by T/2. If a sample of radioactive element contains N nuclei at a time (taken as Zero), then after a time equal to half –life, the number reduces to N/2; during the next half life that is after two half-lives N/4 and after three half-lives N/8 and so on. This is represented in a graph. 

Half-Life of a radioactive element

For example half-life of radium-226 is about 1600 years. Suppose we start with1 mg of radium, In 1600 years it reduces to 0.5 mg, 0.25 mg in 3200 yrs. 0.125 mg in 4800 years and so on.

Half life of Polonium 214 is only 164 m s (m s = micro second = 10-6S). Half-life of uranium 238 is 4.5 billion years which accounts for its presence in nature. The half life of radium is 1622 yrs and the 200 mg sample of Radium separated by the Curies in 1898 now contains about 190 mg. 

more details:

Radioactivity elements radiations isotopes for std 8 to 10

http://simplewayoflearnphysics.blogspot.in/2012/05/radioactivity-elements-radiations.html

Radioactivity elements radiations isotopes for std 8 to 10

REVIEW
You have learnt the following earlier, recall them.
An atom is made up of three fundamental particles namely proton, neutron and electron.
Protons and neutrons are bound together is the central part of an atom, called the nucleus.
Electrons revolve round the nucleus in different imaginary orbits.
The number of protons in an atom is called its atomic number, represented by Z.
Protons and neutrons is a nucleus have a common name ‘nucleons’ the total number of nucleons in an atom is known as the atomic mass number represented by A. Mass number is very close to the atomic mass.
Electron (-1eo) is a negatively charged particle with negligible mass.
Proton (+1p1) is a positively charged particle with unit mass considering the mass of hydrogen atom to be ‘one unit’.
Neutron (0N1) is an electrically neutral particle with unit mass.

INTRODUCTION 

In 1896, Henri Becquerel (1852-1908) a French scientist, accidentally discovered radioactivity. He found that a photographic plate, wrapped well to protect is from visible light, was blackened by a mineral containing uranium. He concluded that the mineral emitted a new kind of radiations which occurred without any external cause. The emission of these radiations is not affected by external factors like temperature, pressure, electric field and magnetic field. The radiations were called Becquerel rays.

Radioactivity 

All elements which emit Becquerel rays are said to be radioactive and the elements are called radioactive elements.
All the elements from atomic number 81 to 92 are found to be radioactive. Experiments show that atoms of radioactive elements disintegrate into atoms of other elements. It became clear that the radiations must emanate from the nucleus and radioactivity is a nucler phenomenon. We can conclude that the phenomenon is due to disintegration of an ustable nucleus accompanied by some type of radiation.
The property of spontaneous disintegration of certain unstable. This is referred to as natural radioactivity. Radioactivity produced artificially or induced, is called artificial or induced radioactivity. The radiations emitted by radioactive elements are called radioactive radiations.
In 1898, Marie Curie (1867-1934) and her husband Pierre Cune (1959-1906) discovered a new element named Polonium and later on, another element named Radium, In 1903 Nobel prize in physics was shared by H. Becquerel, M. Curie and P. Curie, for their discovery of radioactivity. In 1911, M. Curie was awarded Nobel prize in chemistry for the extraction of radium in its purest form.

Radio for std to 10 C B S E Course

3.6  RADIO

Introduction : In a basic sense, communication means transmission of information from one person to another. (Anything having some meaning or sense is called ‘information’. A written message, a spoken word, a picture are all examples of information). In a technical sense, communication refers to the sending, receiving and processing of information by ‘electrical means’.

The different types of communication systems are telegraph, telephone, radio, television, radar and so on, have been developed depending upon the information to be sent and received.

Know more about transistors

The transistor described in the above section is a

‘bipolar junction transistor’.

Field Effect Transistor(FET),

Metal Oxide Semiconductor Field Effect Transistor(MOSFET) and Junction Effect Transistor are some of the other types.

Radio communication : Radio communication is the process of sending information or intelligence (music or speech in the form of words) from one place and receiving it at another place, Radio waves are spherical and spread in all directions without using connecting wires.

There are two major parts in radio broadcasting system namely a transmitter and a receiver. Transmission of speech or music over long distances, using radio waves is called radio broadcasting. Radio signals sent from the transmitter travel through space and are received by the receiver.

Note

Guglielmo Marconi (1874-1937), an Italian engineer, invented and developed telegraph in 1890’s. In 1901, he transmitted a message across the Atlantic.

On Christmas Eve in 1906, Reginald Fessendeu transmitted both speech and music over several hundred miles.

Dr. S K Mitra(1890-1963), an Indian physicist had constructed a radio transmitted in 1923, about four years before radio broadcasting started in our country.

Radio transmitter:

                        The block diagram of a radio transmitter is shown in fig. The audio(sound) information is changed into corresponding electrical signal using a transducer. Transducer is a device that converts information energy to be transmitted into electrical signal. For sound transmission, the transducer is microphone (1). This signal called audio frequency (AF) signal, being weak, is amplified by a suitable amplifier called AF amplifier(2). 

Radio transmitter

As AF signals, have low energy, they cannot be transmitted over long distances. Therefore radio waves of suitable frequency are generated in an RF oscillator (3). These radio waves are mixed with the amplified AF signal in the mixer stage(4). In this stage, the AF signal is superposed on the RF wave. The  RF wave is said to be modulated. The RF wave that carries the AF signal, is called carrier frequency. The modulated signal is amplified using an RF amplifier (5) and then sent to the antenna(6). The antenna radiates the RF signal into space, in the form of spherical radio waves.

Know this 

Radio transmission in AM and FM.

You have read in dailies, about radio programmes given in AM and FM. What is the about? The mixing of audio and carrier frequencies tha is modulation can be done in two ways. If the amplitude of the carrier is varied in accordance with the AF signal, then the modulation is called amplitude modulation(AM). If the frequency of the carrier is varied in accordance with the AF signal, then the modulation is called frequency modulation(FM). AM radio stations have carrier frequencies from about 530  to 1600 kHz, FM radio stations have much higher carrier frequencies, between 88 and 108 MHz. Markings on radio dial corresponds to carrier frequencies. 

Radio Receiver :

                        Block diagram of a simple radio receiver is shown in fig . \radio waves sent by all stations, are received by the antenna(1) of the receiver. A particular station is ‘tuned in’ and the RF signal amplified (2). The signal goes to the detector or demodulator(3). Here the AF signal is separated from the carrier. The audio signal is then amplified by AF amplifier (4)and sent to a loud speaker or head phones (5). 

Radio receiver

3.7  TELEVISION

                        Television is one of the marvels of electronics. The word television, the combination of two words ‘tele’ and ‘vision’ literally means seeing at a distance. Television broadcasting is the process of transmission of transient visual image of an actual or recorded scene, along with audio signal to a distance using radio waves.

                        A television transmitter works similar to that of a radio transmitter. At the TV station, a pick up instrument (TV camera) explores (scans) different portions of the picture or scene to be transmitted, in a particular order. The camera converts the variations in brightness in the various portions into electrical impulses (photoelectric device is used as transducer). These electrical impulses constitute the video signal (picture signal). This video signal is amplified and is used to amplitude modulate an RF  carrier.

                        The sound picked up be a microphone is amplified and is used to frequency modulate and RF carrier (The frequency of this carrier is 5.5MHz higher that that of the video carrier). Carrier frequencies in TV for various channels lie in the range from about 40 MHz to 600 MHz.

                        The two modulated carriers(Video and Audio signals) are amplified and transmitted through the same transmitting antenna.

                        As the receiving end, RF signals are collected by the receiving antenna and the desired station is selected. The output corresponding to the video signal is used to control the intensity of light in a cathode ray tube; to obtain the picture transmitted. An F.M. receiver is used to separate the audio signal and reproduce sound.

Activity

Make a list of a some AM and FM radio stations to which you listen.

How are their wavelengths and frequencies related?

Do you know?

The first demonstration of television, was given during 1927 by J.L. Barid (UK) and C.F. Jenkins (USA). The actual TV came into existence in 1930, with the invention of cathode ray tube and TV camera.

POINTS TO REMEMBER

Electronics is a field of pure and applied science dealing with the design and application of electronic devices.

Substances whose conductivity lies in between that of conductors and insulators are called semiconductors.

Ex: Silicon and Germanium

When an electron is removed from a covalent bond, it leaves a vacancy, equivalent to positive charge. An electron from a neighbouring atom can drop into this vacancy, leaving the neighbour with a vacancy. Such a vacancy is usually called a hole and it is considered as an apparent charge carrier.

Doping is the process of adding minute traces of impurities to enhance the conductivity of semiconductors.

The traces of impurities added fordoping are called dopants.

Pentavalent impurity is a donor impurity. On addition of a donor impurity the pure semiconductor becomes n-type.

Trivalent impurity is a acceptor impurity. On addition of an acceptor impurity the pure semiconductor becomes p-type.

Diode is a p-n junction. It acts as a conductor when it is forward biased and acts as a resistor when it is reversed biased.

Transistor is a specially constructed three terminal semiconductor device.

In an npn transistor p-type  semiconductor is sandwiched between two n-type  semiconductors.

In a pnp transistor n-type semiconductor is sandwiched between two p-type semiconductors.

Three regions of a transistor are base, emitter and collector.

In a radio transmitter microphone acts as a transducer. RF oscillator produces carrier waves and transmitting antenna transmits the radio waves in the form of spherical waves.

The mixer in a radio transmitter modulates audio frequencies and radio frequencies. Audio frequencies and modulated frequencies are amplified by respective amplifiers.

A radio receiver (radio) receives the signals of a tuned radio station by receiver antenna. Audio signals and carrier waves are separated by demodulator. Audio frequencies are amplified and the loud speaker reproduces the sound.

Television broadcasting is the process of transmission of transient visual image of an actual or recorded scene along with audio signal to a distance using radio waves.

Camera acts as a transducer in the television transmission.

Transistor for std to 10 C B S E Course

3.5  TRANSISTORS

The term ‘transistor’ is a shortened form for transfer resistor. Transistor is a specially constructed three terminal semiconductor device. It is a device which consists of either one p-type region sandwiched between two n-type regions (npn transistor) or one n-type region sandwiched between two p-type regions (pnp transistor). The middle region is called the base and the two end regions are called the emitter and the collector. Figure shows npn transistor with its circuit symbol.

Transistor

Transistor was first fabricated in 1947 at the Bell Telephone Laboratories, by three American physicists John Bardeen, Walter H. Brattain and William B. Shockley. They were awarded Nobel prize for physics in 1956.

npn Transistor

A pnp transistor with its circuit symbol is shown in Fig. The   base is very thin compared to the emitter and the collector. In a transistor, the emitter is more doped then the collector region and the base is lightly doped. The function of the emitter is to emit electrons (in the case of npn transistor) or holes (in the case of pnp transistor) into the base. The base controls the flow of charge carriers into the collector which collects them from the base. Emitter of a transistor is always forward biased and the collector is always reverse biased.

pnp Transistor

Action : in the circuit shown in fig. using a n-p-n transistor the voltage from a source is applied to the base of the transistor. The emitter n layer has a small negative voltage with respect to the base p layer to permit the forward flow of current across the n-o junction. The collector n layer has a large positive voltage with respect to base p layer to prevent the reverse flow of current. Electrons from the emitter are attracted to the collector and a current flows through the outer circuit.

Circuits of npn transistor

To understand the practical application of the circuit, in Fig  3.6, circuit shown in fig 3.7 is considered. A variable weak voltage is applied across a resistor in the emitter base circuit(This is called input voltage) . The output voltage is taken across a resistor (called load) in the collector emitter circuit. Small changes in the input voltage result in large changes in output voltage; in other words, there is amplification.

Amplication of npn transistor

Applications of transistors :

            Transistor is used in

                     (i)        Amplifier

                    (ii)        Oscillator

                   (iii)        Switching circuits.

            Oscillator is a device for producing electric oscillations of a desired frequency.

Activity

Draw an appropriate circuit diagram, using p-n-p transistor.

For more see on LABLE list on right side.

Types of Semiconductors Std 8 to 10

3.3  TYPES OF S SEMICONDUCTORS

In a pure semiconductor, holes and electrons are always present in equal numbers and the resulting conductivity is called intrinsic conductivity. Pure semiconductors are called intrinsic semiconductors.

            The conductivity of semiconductors can be enhanced by the addition of minute traces of impurities (other elements) called dopants. The process of adding dopants is called doping and the resulting semiconductor is called doped semiconductor or extrinsic semiconductor. The conductivity in this case, is called extrinsic conductivity.

            There are two types of extrinsic semiconductors depending upon the types of impurities. They are classified as

(A) n-type semiconductors and

(B) p-type semiconductors.

These are used in the manufacture of diodes and transistors.

(A)   n-type semiconductors : if a very small amount of pentavalent impurity for example antimony (or arsenic or phosphorous), is introduced into the crystal of germanium, four out of five electrons of each impurity atom enters into bonds with the nearest germanium atoms, to form covalent bonds. The fifth electron is set free. Such free electrons act as current carries.

The added pentavalent impurity donates electrons to the semiconductor and the semiconductor becomes n-type semiconductor. The name n-type indicates that the majority charge carries are electrons. There will be free-electrons and holes in equal numbers due to breaking of some covalent bonds.

n-type Semiconductors

(B)   p-type semiconductors : When a trivalent impurity like boron, indium, gallium or aluminum is introduced into a germanium crystal, each impurity atom takes away one electron from the neighboring germanium atoms, to form covalent bonds. This results in the creation of  holes. Electrons from neighboring atoms can fill up these holes and there will be apparent motion of holes. Thus holes act as current carries. The added trivalent impurity accepts electrons from the semiconductor and the semiconductor becomes a p-type indicates that the majority charge carries are holes equivalent to positive charge. There will be free electrons and holes in equal numbers due to breaking of some covalent bonds.

p-type semiconductors