Physics Chapter 2.docx

. 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 h...

. 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** 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** A. **Giving some electrons to this body.** B. **Taking some electrons from this body,or** 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! 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. 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. **[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. 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.** 1. **The electrostatic paint spray gun** 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** 4. **The lightning conductor (the dangerous face of electrostatics)**