Physics Chapter 2.pdf

. Chapter 2 Physics Activity of science is building a structure of theories and models to help us predict how nature behaves. For prediction, theories, and models have to be built on observation of natural phenomena. When making observations you should present interpretations and build models to he...

. Chapter 2 Physics Activity of science is building a structure of theories and models to help us predict how nature behaves. For prediction, theories, and models have to be built on observation of natural phenomena. When making observations you should present interpretations and build models to help us predict how things will behave. Electricity is a naturally occurring force that exists all around us. Greek philosophers noticed that a piece of amber rubbed with cloth would attract bits of straw, recorded references to static electricity and lightning. Lightning and the small annoying shock you sometimes feel are the same thing. Electrostatics are a branch of physics dealing with electric charges at rest. (2.1) 1. Interaction of glass rods rubbed with silk (A) Place a glass rod horizontally in a stirrup hung on a silk thread (B) Bring a second glass rod near the first without touching it. Nothing happens! The rods neither attract nor repel each other. (C) Rub one of the glass rods with a piece of silk and the place it in the stirrup. (D) Rub the second glass rod with silk and bring in near the first. The two rods repel each other. 2. Interaction of plastic (polythene) rods rubbed with paper Repeat the same experiment using plastic rods rubbed with paper or wool. The plastic rods repel each other. 3. Interaction of rubbed plastic and glass Repeat the same experiment using a plastic rod rubbed with paper and a glass rod rubbed with silk. The two rods attract each other. In experiment 3, if the piece of paper and the piece of silk are brought close to each other, they will attract each other. In each of the above experiments, where the attractive or repulsive forces are detected, rubbing has changed the properties of the object involved we say the objects have been electrified or charged. Rubbing objects of the same material against each other results in neither object being charged. For objects to be charged by rubbing, they must be rubbed against objects of a different material. (2.2) Only two electric states exist, one similar to charged glass and one similar to charged polythene as shown in the experiments above. In order to differentiate between these TWO states, we classify charged objects in two categories: 1- objects that behave like polythene in the above experiments are said to be negatively charged, 2- objects that behave like glass are said to be positively charged. The terms negative and positive ((-) and (+)) associated to charged bodies are arbitrary chosen. They also have a mathematical significance. Like charges repel each other while unlike charges attract each other. Extra note: When rubbed with cloth, polythene becomes negatively charged and Perspex acetate becomes positively charged; these particular cases are useful to remember for examination purposes. Experiment – Charging by Contact Get a rod that is negatively charged by friction and let it touch an uncharged pith ball suspended by a light string. You will observe that the ball suddenly repels after contact. Interpretation Being repelled by the rod, the ball has acquired the same charge as the rod. Therefore we can say that the ball is charged by contact. The above experiment can be repeated by taking a positively charged rod we get the same result. (2.3) Experimental Observation 1. Figure 1 shows a metal rod placed on a glass beaker such the it is in contact with a suspended metal-coated ball. 2. Electrify a glass rod and rub (not merely touch) it against the metal rod. 3. Observation: the metal ball is repelled away and hangs away from the metal rod. 4. If the experiment is repeated with a plastic rod on the glass beaker as shown in Figure 2, the metal-coated ball will not be repelled. Theoretical Definitions Substances that behave like the metal rod in this experiment are called conductors, and substances that behave like the plastic rod are called insulators. Interpretation In the first case of the above experiment, positive charge passed from the glass to the metal rod spread immediately through it and the metal-coated ball. Hence, the resulting positively charged rod and the positively charged ball repelled each other. In the second case, no charge spread through the plastic rod, and the ball was not repelled. Charge does not spread through insulators. Since the glass beaker did not permit the charge to spread through it to other bodies, glass is an insulator and so is plastic. Another generalization! 2.4 After charging both the metallic rod and the ball in the last experiment, a large metal sphere on an insulating handle touches the metal rod. The metal ball comes closer to the metal rod but does not touch it. Part of the charge on the metal rod is transferred to the large metal sphere, so the repulsion between the rod and the small ball decreases, and the suspended ball swings back a bit. Sharing a charge over two conductors decreases electric forces. It can be shown that the electrical forces are proportional to the quantity of charge on the charged objects. (2.4.1) If the metal rod in the above figure is now touched by hand, the small metal ball comes back and touches the rod. Almost all the charge goes from the rod to the hand and body, and to the ground. The earth is such a large conducting sphere that any charged object that touches the earth will lose almost all its charge to it. The sharing of charge between a small conductor and earth leaves no detectable charge on the conductor. This process of discharging is called grounding, or Earthing. Definition: to earth a charged object means to bring it in contact with a much large conductor, which may be or may not be the planet Earth. (2.4.2) Uncharged objects can be charged by rubbing them against other objects or by touching other charged bodies. Charging can be done without touching: a process called Electrostatic Induction or charging by influence. Experiment 1 ( induction and Separation) 1. Two uncharged metal spheres are put on insulating stands such that they touch each other. 2. A positively charged glass rod is brought near on of the metal spheres without actually touching it. 3. Without removing the glass rod, the two spheres are separated by pulling the stands apart. 4. The glass rod is moved far away. 5. The metal sphere that was nearer to the charged glass rod is found to have a negative charge, while the metal sphere that was farther away from the glass rod is found to have a positive charge. Interpretation Because charges can move in a conductor, the presence of the positively charged glass rod near the first metal sphere repels positive charges from that sphere away to the farther sphere, and attracts negative charges from the further sphere onto the nearer one. The charges stay separated while the inducing charge is nearby. Once the spheres are physically separated, the charges cannot return to their original locations because the air between the spheres is an insulator. The separation of positive and negative charges in a body induced (caused) by the presence of a charged object nearby is called electrostatic induction. The charge which appears on each conductor is called induced charge. The previous method is often called charging by induction and separation, referring to the mechanical separation of the conductor into two parts in step 3. There is another technique, called charging by induction and Earthing, which makes use of the same principle but works in a slightly different way. Experiment 2 (Induction and Earthing) 1. An uncharged metal sphere is put on an insulating stand. 2. A positively charged glass rod is brought near the sphere without actually touching it. 3. Without removing the glass rod, the sphere is momentarily earthed (by touching it with a finger, for example). Note the symbol = used to denote an earth connection. 4. The glass rod is moved far away. 5. The metal sphere is left with a negative charge. Interpretation Because charges can move in a conductor, the presence of the positively charged glass rod near the metal sphere repels positive charges from the nearer part to the further part of the sphere, and attracts negative charges from the further part into the nearer one. The charges stay separated while the glass rod is nearby. When the sphere is earthed, the positive charges attract more electrons from the conductor (your body), adding to the existing negative charge in the sphere. When the inducing charge is removed, the metal sphere is left with a negative charge that spreads over its surface. Chapter Opposite Charges When oppositely charged spheres produced by induction and separation are brought in contact together, they lose their charge, hence becoming neutral. Charges that cancel each other, resulting in neutral objects, are said to be equal in magnitude but opposite in sign or simply opposite (2.5) The fact that unlike charges attract each other and like charges repel each other, is used to interpret the attraction of neutral bodies by charged objects. This interpretation is also based on the fact that the electrostatic force between charged bodies decreases with an increase in the distance between the charges. Electrostatic force between two charges is 1) proportional to the charges, and 2) inversely proportional to the square of the distance separating the charges. This force is known as Coulomb’s Law , K is the electrostatic constant, or Coulomb’s constant  A) Attraction of a neutral conductor by a charged object It is observed that when a neutral conducting sphere is brought near a charged rod, the sphere is attracted to the rod. For instance consider a plastic rod and an uncharged conducting sphere. Positive charges within the conducting sphere are attracted towards the negatively charged plastic rod, while the negative charges are repelled away from the rod. This results in a positive charge nearer to the rod, and an equal negative charge farther away from the rod. Since the positive charge is nearer, the attractive force is greater than the repulsive force exerted on the equal negative charge, hence resulting in a net attractive force as shown in the diagram. B) Attraction of a neutral insulator by a charged object If an insulator, like a small piece of paper, is brought near a charged object, it is attracted to it. Although charge cannot move through the body of the insulator, charge can be slightly displaced within each molecule, resulting in a net induced opposite charge slightly nearer to the inducing charge. This is enough to cause a net attraction. Polarization The diagram below shows an uncharged metallic sphere and a charged pith ball. When the charged rod is brought close to the metallic sphere, the charged pith ball pendulum deviates, thus indicating the presence of a net repulsive electric force. Interpretation The large sphere did not lose or gain any charge (it is insulated) but it did repel the ball, hence it is electrified without being really charged. This kind of polarization is due to the redistribution of the electrons inside the body itself. A body is electrified when the natural distribution of its electrons is modified by: (A) Giving some electrons to this body. (B) Taking some electrons from this body,or (C)changing the distribution of its own electrons. This can be achieved by friction, contact or induction (influence) (2.6) Model is an explanation not a fact derived from direct observation. Atoms contain two types of charged particles: Electrons and protons Electrons carry a negative charge and move around a fixed nucleus that contains positive charges called protons in addition to neutrons having no charge. Protons and electrons have opposite charges that is (Charge of an electron)= -(charge of a proton)  Normally, atoms are neutral that is they have equal numbers of protons and electrons. An object which is neutral does not exert electrical forces on other neutral objects. An object that loses electrons becomes positively charged. It will exert a net repulsive force on another positively charged object and a net attractive force on a negatively charged object. On the other hand, a negatively charged object is one which has gained electrons. An atom which has gained an electron is called a negative ion. When an atom gains one electron or more, it becomes a negative ion. When an atom loses one electron or more, it becomes a positive ion. Note that an excess or a deficit in the number of electrons in a body turns it into a negatively charged body or positively charged body. Protons are out of the game! (2.6.1) Electric charge is a fundamental property of nature. Of the three building blocks of ordinary matter (the electron, the proton, and the neutron) two carry electric charge. All electrons carry the same charge and all protons carry the same charge. The protons charge has exactly the same magnitude as the electrons, but with opposite sign. Generalization: electric charge is quantized. It comes only in discrete amounts, any charge a must be an integer multiple of the basic unit e. 1 C is 1 coulomb Unit Coulomb is named after the French physicist, Charles Augustine de Coulomb. “.coulomb’ (small letter) is the name of the unit: C (CAPITAL LETTER) is the symbol. This principle is followed throughout the S.I. One coulomb is 6.25 x 10^18 elementary charges, making the elementary charge approx. 1.6 x 10^-19 C Charge of an electron is q = -e =-1.6 x 10^-19 C The quantity of electric charge of an object can be determined by knowing the excess or deficit of electrons in that object. The charge of an object having excess of N electrons can be expressed as q=-N.e The charge of an object having a deficit of N electrons can be expressed as q=N.e When an object gains a positive charge, an equal amount of negative charge will be in neighboring areas. This is the well-known Law of Conservation of electric charge, which states that the net amount of electric charge produced in any process is zero, or in other words, no electric charge can be created or destroyed. (2.6.2) The coulomb is a very large amount of electrostatic charge. A typical charge acquired by a rubbed body is 10^-8 C whereas lightning bolt may transfer as much as 20 C between the Earth and a cloud.  Role of charged particles in electric conduction 1. A conductor allows charge to spread through due to the ability of some of its electrons to move anywhere within it. 2. In metals only electrons can move. 3. In an insulator, all electrons are fixed to specific locations. 4. In conducting liquids, like molten salts and aqueous solutions containing ions, both positively and negatively charged particles can move. 5. In some semiconductors, gaps left by electrons (called holes) can move and caused conduction. (2.7) To detect a charge, tools such as the gold-leaf electroscope are needed. A gold-leaf electroscope is a device used to determine the electric state of an object utilizing the principle of repulsion of like charges. The most important part of the gold-leaf electroscope is a very thin and flexible strip or leaf of gold foil that is attached to a metal plate. Gold is used because it is a very good ductile conductor, it can be beaten into very thin sheets. These days aluminized plastic is used because it is cheaper and stronger than gold. Both the metal plate and the gold leaf are connected by means of a metal rod, which goes through the center of the Perspex insulating plug. When the metal cap receives some kind of charge, the latter spreads down to both the metal plate and the gold leaf. The plate and the leaf thus carrying like charges, repel each other causing the gold leaf to diverge. The function of the Perspex insulator is to prevent the charge passed to the metal cap from leaking away. The diagram below shows a simple cross section of an electroscope. Students should be able to draw a simplified diagram of an electroscope and to describe how to use it to detect (i) the presence, (ii) the sign, and (iii) the relative magnitude of charges. Experiment: Testing the nature (sign) of the charge on an object using an electroscope Get a gold-leaf electroscope and give it a negative charge by bringing it in contact with a charged polythene strip. The electroscope acquires a negative charge and the gold leaf diverges as shown in the figure. Now proceed as follows: Case (a) Bring a negatively charged polythene strip to the negatively charged electroscope and observe what happens.. Outcome of Experiment Since like charges repel each other, the negative charges (electrons) on the metal cap of the electroscope will be repelled down to the gold leaf. Hence, the metal plate and the gold leaf will acquire the same type of charge; repulsion will take place and the leaf will diverge more. Case (b) Now bring a positively charged cellulose acetate strip near the metal cap of the gold-leaf electroscope and observe what happens.  Out come of experiment A Outcome of Experiment Since unlike charges attract each other, the electrons on the metal plate and on the gold-leaf will be attracted to the metal cap towards the positively charged cellulose acetate strip. The divergence of the gold leaf will decrease due to a smaller repulsive force between the leaf and the metal rod. Outcome of B Case (c) Now bring your hand near the metal cap of the gold leaf electroscope that is negatively charged and observe carefully what happens to the gold leaf. Outcome of Experiment Your hand will acquire an induced positive charge because all the electrons will be repelled away to the Earth and the divergence of the leaf will decrease. The same results mentioned above will be obtained using positively charged bodies and electroscope. In this case (+) and (-) swap and electrons will flow in the opposite direction of the ones shown in the above diagrams. An increase in the divergence of the gold leaf shows that an object has the same charge as the electroscope. Extra note: An uncharged conductor behaves as if it has the opposite charge to the cap of the electroscope. In humid conditions, electrostatics experiments may give unexpected results due to changes being earthed by damp air, which is a conductor. Van de Graaff Generator The Van de Graaff generator is an electrostatic generator capable of producing enormously large static electric potentials. A simple Van de Graaff generator consists of a rubber belt, two rollers, two brushes, a motor and a dome (metallic hollow sphere made of aluminium or steel). The lower roller covered with silicon tape acts as a charger and the dome as a collector. When the motor is turned on, the lower roller acquires a negative charge from the rubber belt which builds a positive charge. By means of two brush addemblies, the negative charge is drained to earth leaving positive charges to accumulate an the dome.  (2.8) 1. The electrostatic paint spray gun To spread paint evenly on a surface, a highly efficient technology using electrostatics has been invented: the electrostatic paint spray gun. A fine ionizing electrode located at the atomizer tip of the gun is used to charge paint particles negatively by picking up additional electrons. On the other side by grounding the surface to be painted, charged particles are attracted to the surface, hence depositing on it a thin coating. The negative charge then leaks through the ground and returns to the power supple feeding the guns electrode: a circuit is established. This type of painting produces a even distributions of paint as well as it saves paint by reducing over spraying and wasted paint. Although this method is very efficient it is not suitable for all kinds of paint 2. Revealing fingerprint son surfaces Concealed fingerprints can be revealed using charged powder. A fine powder, for example silicon carbide, is coated on a metal plate and it is usually given a highly positive charge from a power supply. The specimen to investigate is on one plate and it is connected to the negative terminal of the power supply. The metal plate with the powder is connected to the positive terminal of a high voltage charger, making it acquire a positive charge. The positively charged powder is repelled from the metal plate and hits the specimen, sticking only to the ridges of the fingerprint. The other powder particles lose their positive charge and acquire a negative charge, so they are repelled back to the bottom plate. The same idea described above is implemented and applied in most modern photocopy machines and printers by using dark powder called toner that is attracted to charged areas on a metal plate and then transferred onto paper. 3. The Electrostatic Precipitator The function of an electrostatic precipitator is to remove smoke and dust from the waste gases going up the chimneys of factories. A fine wire grid inside the precipitator is kept highly charged so that a discharge will always occur between the fine grid and the two concave metal plates, which are earthed. When the smoke and dust particles carrying waste gases rise through the region between the grid and the concave metal plates, the charged dust particles are repelled from the wire grid and attracted to the earthed plates where they are deposited. To clean the precipitator, the metal plates are occasionally tapped so that the dust and smoke particles fall down the chimney where they can be removed. 4. The lightning conductor (the dangerous face of electrostatics) Lightning conductors help protect tall buildings from being struck by lightning. A lightning conductor consists or a thick copper strip on the outside of a building connecting metal spikes at the top of a metal plate in the ground. The conductor provides a path for electrons to flow easily in huge numbers from the top of the building to the ground. Thunderclouds carry charges, and if a negatively charged one passes overhead it repels electrons from the spikes to the earth. The points on the spikes are left with a high positive charge density. The air between the cloud an the thick copper strip will be ionized and the positive ions will be attracted to the cloud, leaving the negatively charged electrons to pass through the thick copper strip towards the metal plate which is buried deep in the ground. When charge leaks from a lightning conductor or sharp points on a metal object, you can sometimes see a faint glow of light and hear a hissing sound, this glow gives the process the name corona discharge.