MODERN PHYSICS Modern physics deals with the nuclear model of an atom which consists of three particles, the neutron, protons and electrons. An atom is made up of a central part called a nucleus around which a cloud of electrons rotates. The nucleus is made up of two particles namely, protons and neutrons. Properties Name Symbol Charge Electron Negative Proton positive Neutron No charge Atomic Mass number or Mass number is the total number of protons and neutrons in the nucleus and is denoted by A. Atomic number is the number of protons or electrons in an electrically stable atom denoted Z. Neutron number is the number of neutrons in the nucleus denoted N. e.g Lithium with chemical symbol , this implies that the mass number is 7, Atomic number is 3 and Neutron number is 7 – 3 = 4. i.e Neutron number = Mass number – Atomic number A = Z + N Isotopy: This is the existence of the element with the same atomic number but different mass number. Isotopes of an element are atoms of the same element having the same atomic number but different mass numbers. Examples of isotopes. Carbon: and , Oxygen: , and , Uranium: and Isotopes that are radioactive in nature are called radioisotopes. RADIOACTIVITY: This is the spontaneous disintegration of an unstable nuclei with emission of Alpha ( ) - particles, Beta ( )- particles and Gamma ( )-rays. Examples of radioactive nuclides are Uranium, Thorium, Radium, Polonium, e.t.c.. Question: What is a radioactive nuclide? eds@ghsphysics department 2007 PROPERTIES OF RADIATIONS ALPHA ( ) – PARTICLES These are Helium atoms are atoms which have lost their 2 orbital electrons. i.e they have a net positive charge. They have a mass number 4, an atomic number 2. Properties: • They are positively charged since they are Helium atoms which have lost two electrons. • They have a mass number 4 and an atomic number 2. • They are deflected by the electric field towards the negative plate because they are positively charged. • They cause ionisation of gases (most ionizing). • They have a short range of penetration (they penetrate air but are stopped by thin sheets) • They are slightly deflected by magnetic fields. • They cause a heating effect in metals. • They affect a photographic plate (film). • They cause fluorescence in fluorescent materials. When an alpha particle is emitted by a radioactive source, the mass number reduces by 4 and atomic number by 2. i.e 1st Law of radioactive decay: when an element disintegrates by the emission of an - particle, it turns into an element with chemical properties similar to those of an element too places earlier in the periodic table. Example. 1. A radioactive substance undergoes decay and emits an alpha particle to substance Y. Find the mass number and atomic number of Y. Mass number is 234 and atomic number is 90. 2. A radioactive substance X with mass number 238 and atomic number 92 emits two alpha particles and turns to Y. Find the mass number and atomic number of Y. BETA ( ) PARTICLES These are high energy speed electrons. They have a mass number 0 and a negative charge. Properties: • They have a mass number of 0. • They are negatively charged high speed electrons. • They are deflected in an electric field towards the positive plate. • They can penetrate a thin sheet but cannot penetrate a thick sheet. • They are strongly deflected by a magnetic field. • The ionizing effect is much less than that of - particles. • They affect a photographic plate. So when a nucleus emits a beta- particle, its atomic number increases by 1 and mass number remains unchanged. 2nd Law of radioactive decay: when an element disintegrates by emission of a - particle, it turns into an element with properties similar to those of an element one place later in the periodic table. Example: Carbon decays by emitting a beta particle. Find the atomic number and mass number of the new atom. Questions; 1. The following atoms decay and emit beta particles. Write out their decay equations. i) ii) 2. decays by emitting an - particle followed by an - particle. Write down the decay equation. 3. A radio active substance decays by emitting 2 alpha particles and 3 beta particles. Find the mass number and atomic number of daughter nuclide. GAMMA ( ) RAYS These are electromagnetic waves of the shortest wave length. Gamma rays have no charge. Properties: • They are electromagnetic waves of very short wave length. • They cause ionization of gases. • They have the most penetrating power and can only be stopped by lead. • They are not deflected by both electric and magnetic fields. • They have no charge. • They travel at speed of light. • They cause fluorescence. • They have no effect on the mass number and atomic number of emitting atom. Similarity among - particles and rays: • All cause ionization of gases. • All have penetrating power. • All cause fluorescence. N.B: Radioactive disintegration differs from chemical reactions in that its spontaneous, not affected by chemical combinations, temperature and pressure. Assignment: * Briefly outline the major differences among the - particles and rays. * Draw a diagram to show the effect of electric field on - particles and rays. NATURE OF THE RAYS FROM RADIOACTIVE SUBSTANCES. Rutherford placed a little radium at the bottom of a small lead box and subjected the rays that emerged from if to the action of very strong magnetic field at right angles to their direction. It showed that - rays were deflected in the direction opposite to that of - rays while the - rays were not affected by magnetic field. CLOUD CHAMBER When a radioactive source emits particles into an air space saturated with water or vapour inside a vessel with a glass window. As the particles speed through the air, they collide with the air molecules leaving off a trial of positive and negative ions. If the air space is expanded cooling occurs revealing the paths of the particles. The appearance depends on the particles and is used as a means of identification. APPEARANCE OF CLOUD CHAMBER TRACKS The - particles are straight and thick. The - particles are in an irregular pattern as they suffer frequent repulsions from the electrons near by and make ionization less frequent. The - rays do not produce cloud tracks along their own paths as they interact with atoms and give up either part or all the energy. It’s not clear what causes a particular atom to disintegrate at a particular moment. This activity is purely random. Heating or cooling has no effect on the rate of decay but experiments show that every radioactive element has a definite rate of decay expressed by its half-life period. HALF LIFE: is the time taken for a radioactive element to decay to half its original mass. Example: 1. A radioactive sample has a mass of 16 g and its half life is 10 days. What was the mass of the original sample that remains after, i) 10 days ii) 40 days Mass Time(days) 16 g 0 8 g 10 4 g 20 2 g 30 1 g 40 Mass after 10 days = 8 g Mass after 40 days = 1 g 2. A radioactive element has a half life of 4 hrs. If the initial mass of the substance is 9.6 g, calculate the time taken for the material to decay to 0.15 g. Time (hrs) Mass 0 9.6 g 4 4.8 g 8 2.4 g 12 1.2 g 16 0.6 g 20 0.3 g 24 0.15 g The time taken for the mass to decay to 0.15 g is 24 days. 3. A radioactive element takes 160 years for its mass to disintegrate from 4 g to 0.125 g. Find the half life of the radioactive element. Number of half lives Mass left 0 4 g 1 2 g 2 1 g 3 0.5 g 4 0.25 g 5 0.125 g 5 = 160 thus = = 32 years Question: Find the number of half lives above. Assignment: 1. The half life of a certain radioactive element is 40 days. Find the time taken for its mass to disintegrate from 80 g to 5 g. 2. If the half life of a radioactive gas is 2 min, after 8 min, what fraction of the initial value will the activity be? 3. A radioactive material has a half life of 4 hours. If the initial mass is 9.6 g what mass has decayed after 24 hours? 4. A radioactive material has a half life of 4 hours. If after 24 hours 0.15 g remains. Calculate the initial mass of the material. 5. 9.6 kg of a radioactive substance undergoes decay and after 24 years 0.15 kg remains. Calculate the half life of the substance. FINDING HALF LIFE FROM A GRAPH. • Identify the initial sample • Divide the initial sample by 2 i.e , and mark a horizontal line to intercept the curve and make it to meet the x- axis. • The value is the half life. Assignment: 1. What is meant by radioactivity? 2. A radioactive material takes 50 hours for 93.75% of its mass to decay. Find its half life. 3. State any 3 differences between alpha particles and beta particles. 4. The table shows the count rates of a certain radioactive material. Plot a graph and use it to find the half life of the material. Count rate/s 6400 5380 3810 2700 1910 1350 Time/min 0 1 3 5 7 9 USES OF RADIOACTIVITY 1. Medical application: • Radioactive substances are used as tracers in medicine, detect brain tumours, blood clots e.t.c. • Radio therapy in the treatment of cancer. Gamma rays from cobalt are used in treatment of cancer. • Radiations are used to sterilize surgical instruments. (kill germs) • Can be used to investigate lung and heart conditions. 2. Industrial use: • Atomic energy research centres have developed a mechanism of converting radioactivity to electricity. Nuclear used as energy. • Radioactive isotopes can be for hardening polythene, petroleum and metal sheets during manufacture. • As tracers in industries to measure fluid flow in pipes. • Is used in determining the thickness of paper, plastics in factories. 3. Biological and Agriculture: • Science of genetics depends mainly on radioactivity. • Radioactivity is used in modification of plant which are more resistant to diseases. • Radiation can be used to sterilize insects hence eliminate pests. • Radiations can be used fro preserving food. 4. Radioactivity can be used by archeologists in determining the date of archeological remains. DANGERS OF RADIATION (HEALTH HAZARDS) The need for extreme care when dealing with radioactive substances should be extremely emphasized. People soon learn to keep away from fire because the pain is immediate and obvious yet one can receive a sever dose of radiation without being aware and this may lead to leukaemia or cancer. • The alpha particles can be stopped by the outer layer of the skin, however, they can damage the eye sight, skin tissues, e.t.c. • The gamma and beta radiations are more penetrating thus can destroy body cells. • Produce undesirable genetic changes which can be inherited. • Can cause radiation burns i.e redness and sores on the skin. • Destroy the natural chemical reactions which may lead to injury or death of living organisms. • Can lead to cancer. PRECAUTIONS TAKEN WHEN HANDLING RADIOACTIVE SUBSTANCES. • Radioactive elements should be handled using long tongs. • Machines should be operated by remote controls behind thick walls of lead, concrete or any other suitable material that can absorb the radiations. • They should be transported in thick lead containers. • In laboratories, radioactive samples are carefully shielded in lead castles built of lead bricks. • There should be radiation dose detectors to avoid un-necessary exposure. • Minimise the exposure to radiation for the eyes and any body cut. Nuclear Fission: This is the splitting up of heavy nuclei into lighter nuclides with the release of energy. Nuclear fission is the source of energy for the generation of electricity. Nuclear Fusion: This is a nuclear reaction where two smaller nuclei unite (fuse) to form a single heavy nucleus. This also leads to loss of mass and results in release of energy. Ionisation: This is the energy gained by an electron of an atom to escape. When an electron escapes from the atom, it becomes an ion. PRODUCTION OF ELECTRONS: THERMIONIC EMISSION: This is the process by which electrons are produced on a hot metal for example a filament made of tungsten of a bulb. The electrons remain around the hot metal to form an “electron cloud.” CATHODE RAYS. These are a stream of fast moving electrons emitted by metal surfaces when heated to very high temperatures. CATHODE RAY OSCILLOSCOPE(CR0). This is an instrument used for studying the current and voltage wave forms in various electric circuits. The chief feature of an oscilloscope is the vacuum tube containing three main parts; 1. Electron gun: this consists of a heater, a cathode, a control grid and cylindrical (ring-like) anodes. The heater (filament) heats the cathode to produce electrons by thermionic emission. The control grid is usually negative with respect to the cathode. It controls the number of electrons passing through its hole. i.e the number of electrons reaching the screen determines the brightness of the spot on the screen, so the grid controls the brightness or brilliance. The set of anodes at high positive potentials accelerate the electrons and focus them into a fine beam. 2. The Deflecting system: this deflects the electron beam either vertically or horizontally. It consists of two pair of plates; a horizontal pair called Y-plates and a vertical pair called X-plates. The Y-plates deflects the beam of electrons vertically. The X-plates deflects the beam horizontally. When a potential difference is applied across either of plates, the electric field is set up which deflects the beam. 3. Fluorescent screen: is at the end of the tube to which the electron beam is focused to form a bright spot. Its coated with a material which glows when hit by electrons. N.B The brightness of the spot depends on the rate at which electrons strike the screen. The number of electrons will increase if there is increase in the heater current and the grid is less negative. USES OF C.R.O 1. To study wave forms of alternating current: When a potential difference is applied to X-plates, using a time-base circuit or sweep generator, it sets up an electric and electrons move horizontally. This results in the spot to move horizontally across the screen and back to zero. Thus, at low sweep frequencies, the trace appears as a moving spot but owing to persistence of vision, it becomes a continuous line at higher frequencies. When the time base is switched off, the spot moves vertically and returns to zero. A vertical line is observed on the screen. When both the time-base (X-plates) and signal (Y-plates) are connected and switched on, the resultant pattern is the wave form as shown below. 2. To measure d.c and a.c voltages. 3. To measure time intervals (as clocks) 4. To measure and compare frequencies. X-RAYS X-rays are radiations of electromagnetic wave that are produced when fast moving electrons are stopped by matter. i.e when high energy electric beams (cathode rays) are incident on metals. Production of X-rays. In an X-ray tube, the filament is heated by a low voltage, and electrons are emitted by thermionic emission. The electrons are accelerated across the vacuum by applying a high p.d between the cathode and anode. The anode is a copper voltage block with a target where electrons are focused by a concave cathode to produce X–rays. On reaching the anode, the electrons hit a target of tungsten filament which decelerates them resulting in the production of X-rays. The target should be of high melting point because a lot of heat is created. The X-ray tube is evacuated to prevent the fast moving electrons to be hindered by friction due to air resistance. Only part of the energy conveyed by the electrons is transferred to X-radiation. The rest is transferred to internal energy which is conducted away as heat through the copper anode to the cooling fins. The X-ray tube is covered by a lead shield with a small window for the X-rays to prevent leakage of X-rays. N.B The intensity of the X-rays is governed by the filament current, which controls the temperature of the tungsten filament and hence the rate of emission of electrons. TYPES OF X-RAYS: High penetrating X-rays are called hard X-rays, produced by high p.d’s and low penetrating X-rays are called soft X-rays, produced by low p.d’s. Hard X-rays have a very short wave length, high penetration power and high energy and are produced by high p.d. These are used to destroy cancer cells. Soft X-rays have a very long wave length, low penetrating power, low energy and are produced by low p.d. These are used in X-ray photography for human body. PROPERTIES OF X-RAYS: • They readily penetrate matter. • They are not deflected by electric and magnetic fields. • They ionize a gas making it a conductor. • They have no charge. • They affect a photographic film just as light does. • They travel in straight lines. • They posses kinetic energy. • They travel at a speed of light. • They are electromagnetic waves of very short wave length. USES OF X-RAYS: a) In medicine: • Used to investigate bone fractures (soft X-rays used for photography). • To detect lung tuberculosis. • To treat cancer by radiotherapy. Destruction of cancer cells using hard X-rays. b) In industry, X-rays are used to reveal flows (cracks) in metal castings and welded joints. c) They are used to study changes in works of art for variation and types in pigments. This is used to analyse the genuinity of the pieces. d) To study the structure of crystals. HEALTH HAZARDS CAUSED BY X-RAYS: • Destroy living cells in our bodies especially hard X-rays. • Cause gene mutation (genetic changes). • Cause damage of eye sight and blood. • Produce deep skin burns. Its highly important to remember that each time you are exposed to X-rays your health is also at risk yet we cannot live without it. SAFETY PRECAUTIONS: • Avoid unnecessary exposure to X-rays. • When exposure is necessary, keep the time as short as possible. • X-ray beam should be ONLY restricted to the body part being investigated. • A worker should wear a shielding jacket with a layer of lead. • Exposure should be avoided for unborn babies and very young children. QUESTIONS: 1. a) What are cathode rays? b) Distinguish between cathode rays and X-rays. 2. a) Draw a well labeled diagram of a cathode ray oscilloscope and state the function of the various parts. b) State any 2 uses of a CRO. 3. a) State the properties of X-rays and what safety precautions must be observed when dealing with X-rays? 4. a) How is penetrating power of X-rays affected by reduction in their wave length? b) How does the penetrating power of –rays depend on the voltage? c) What causes change of the following in X-rays? i) Penetrating power. ii) Intensity of X-rays (radiations). iii) Wave length of X-rays. 5. a) What is an electron and hoe does its mass compare with that of a hydrogen atom? b) State any 4 properties of electrons.
Posted on: Fri, 29 Aug 2014 12:21:39 +0000
Trending Topics
Recently Viewed Topics
© 2015