Shocking: Charges at Rest and Electric Fields PDF

SHOCKING Charges at Rest and Electric Fields  Electrostatics As early as 600 BC, the Greeks were already aware that amber a hardened translucent yellowish tree resin-when vigorously rubbed with a piece of cloth, could attract nearby objects. In 1600, William Gilbert,...

SHOCKING Charges at Rest and Electric Fields  Electrostatics As early as 600 BC, the Greeks were already aware that amber a hardened translucent yellowish tree resin-when vigorously rubbed with a piece of cloth, could attract nearby objects. In 1600, William Gilbert, who served as physician to Queen Elizabeth I of England, found out that many other substances possess the same ability as that of amber when rubbed against another substance.  Electrostatics He called these substances electrics and this ability of amber electricity, from the Greek word elektron, which means "amber." Subsequently, it was shown that every object acquires this ability to attract small pieces of matter after being rubbed against another object. The object attracted is said to have acquired charge or electrified. Because the charge is at rest, it is often referred to as static electricity. Electrostatics is the study of all phenomena associated with electric charges at rest.  Structure of the Atom To understand electrostatics, the structure of the atom must first be understood. An atom is made up of subatomic particles-protons, neutrons, and electrons. Protons and neutrons constitute the nucleus of an atom, while electrons orbit an atom's nucleus.  Structure of the Atom The mass of a proton is almost equal to the mass of a neutron. The mass of an electron is very small compared to that of a proton or a neutron. Thus, the mass of an atom is concentrated at the nucleus. In an electrically neutral atom, the number of protons is equal to the number of electrons.  Structure of the Atom In terms of charge, the proton is positively charged, the electron is negatively charged, and the neutron carries no charge. The charge of a proton is equal, but opposite to the charge of an electron.  The elementary charge (represented by e) is the electric charge carried by a single proton. It is a fundamental physical constant. Thus, a proton has a charge of +e, while an electron has a charge of -e.  Electrical charges Electrical charges are usually represented by q. The International System of Unit (SI) unit of charge is the coulomb (C), named after French physicist Charles-Augustin de Coulomb, who made important discoveries on electricity. A coulomb is approximately equal to 𝟔. 𝟐𝟒𝒙𝟏𝟎𝟏𝟖 e. Equivalently,1 e= 𝟏. 𝟔𝟎𝟐𝒙𝟏𝟎#𝟏𝟗 C. Summarizes the properties of proton, neutron, and electron Conductors and Insulators  Conductors Charges can exist in materials and move through them. However, the ease with which charges move through them differs. Conductivity is the measure of the ease at which an electric charge moves through a material. Materials that readily allow the flow of charges through them are called conductors. Metals are good conductors because they have plenty of free electrons that can easily move in the material.  Insulators Insulators are materials that resist the flow of charges. The conductivity of insulators is low. Some examples of insulators are rubber, plastic, mica, paper, glass, and air.  Semiconductors Semiconductors are materials which have a conductivity between conductors (generally metals) and nonconductors or insulators (such as most ceramics). Semiconductors can be pure elements, such as silicon or germanium, or compounds such as gallium arsenide or cadmium selenide. What are semiconductors used for? Semiconductors are an essential component of electronic devices, enabling advances in communications, computing, healthcare, military systems, transportation, clean energy, and countless other applications.  Superconductors A superconductor is any material that can conduct electricity with no resistance. In most cases, materials such as metallic elements or compounds offer some resistance at room temperature, but offer less resistance at a temperature known as its critical temperature.  Superconductors Heike Kamerlingh Onnes discovered superconductivity by cooling mercury to a temperature of about 4 K. Most superconductors only work at temperatures close to absolute zero. Process of Charging  Process of Charging The number of protons and electrons in an atom is equal; hence, an atom is neutral. However, an atom may gain or lose electrons. If the atom gains electron, it becomes negatively charged; if it loses electrons, it becomes positively charged. There are three processes by which a neutral body may be charged. These are charging by friction, conduction, and induction.  Changing by friction Charging by friction results when two different materials are rubbed together. The material that will either become positively charged or negatively charged depends on its electron affinity. Electron affinity is a measure of the attraction of an atom to an electron, or the tendency of an atom to become negatively charged. Materials with higher electron affinity are capable of gaining electrons from those of lower electron affinity. Triboelectric series Triboelectric series is arranged in the order of increasing electron affinity from top to bottom. In general, when two different materials are rubbed together, the one that is higher on the list will become positively charged.  Try me! Identify which of the two materials will become negatively charged if charged by friction. 1. Wood or PVC PVC 2. Paper of Plastic cover plastic cover 3. Cotton or glass Cotton  Sample Problem 1 The human hair is combed using a rubber comb. What charge is acquired by the hair and by the comb? In the triboelectric series, the electron affinity of human hair is lower than rubber. Therefore, the hair becomes positively charged and the comb becomes negatively charged.  Sample Problem 2 When a glass rod is rubbed with a silk What are nC and C in cloth, the rod acquires a charge of physics? Nanocoulombs (nC) and coulombs (C) are units to magnitude 3.45 nC. designate electric charge. The (a) Did the glass rod gain or lose electrons? standard unit used to indicate charge is the Coulomb. (b) How many electrons were transferred 1 nanocoulomb is equal to 1 * during the process? 10-9 coulombs. (c) What is the change in the mass of the glass rod? (d) What is the change in the mass of the silk cloth?  Sample Problem 2 (a) Did the glass rod gain or lose electrons? Since the rod acquires a positive charge (the problem states that the rod has a positive charge of 3.45 nC), this means the rod lost electrons. The silk cloth, on the orther hand, would gain those electrons and become negatively charged.  Sample Problem 2 (b) How many electrons were transferred during the process? -to determine the number of electrons transferred, we can use the formula: 𝒒 = 𝒏 % 𝒆 - q is the charge on the glass rod (3.45 nc = 3.45 𝑥 10!"𝐶 ) - n is the number of electrons transferred, - e is the elementary charge (the charge on a single electron, which is approximately 1.602 𝑥 10!#"𝐶) $ - Rearranging the formula to solve for n : 𝑛 = (substituting the % &.() + #,!" known values. 𝑛 = #.-,. + #,!#" ≈ 2.15 𝑥 10#,  Sample Problem 2 (c) What is the change in the mass of the glass rod? -when electrons are transferred, there is a change in the mass of the object because electrons have mass, even though its incredibly small. The mass of a single electron is approximately: 𝒎𝒆 = 𝟗. 𝟏𝟎𝟗 𝒙 𝟏𝟎!𝟑𝟏 𝒌𝒈 The change in the mass of the glass: ∆𝒎 = 𝒏 % 𝒎𝒆  Sample Problem 2 (d) What is the change in the mass of the silk cloth? The mass of the silk cloth is increased by = 𝟏. 𝟗𝟔𝟐 𝒙 𝟏𝟎!𝟐𝟎 𝒌𝒈  Charging by Conduction is a process of charging wherein electrons are transferred from one object to another by direct contact.  Process of Charging a neutral body may also be charged without physical contact with a charged body. This process called induction Coulomb’s Law Charles-Augustin de Coulomb Charles-Augustin de Coulomb invented a device, dubbed the torsion balance, that allowed him to measure very small charges and experimentally estimate the force of attraction or repulsion between two charged bodies.  Torsion balance The torsion balance consists of two identical metal spheres, which are connected by an insulating rod. This rod is suspended at the middle by a thin wire. All these are enclosed in a glass container to avoid the effects of wind. Another charged metal sphere is inserted through a hole in the glass case. The force between the spheres creates a torque, which in turn twists the wire. The force is then calculated from the amount of twist. COULOMB’S LAW  Coulomb’s Law Like charges repel and unlike charges attract, but how great is this force of attraction or repulsion between charges?  Coulomb’s Law Through experimentation, Coulomb discovered that the magnitude of the electrical force between two charged particles is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them. This statement is called Coulomb's law. Mathematically, this can be written as 𝒒𝟏 𝒒𝟐 𝑭𝑬 = 𝒌 𝒓𝟐  Coulomb’s Law 𝒒𝟏 𝒒𝟐 𝑭𝑬 = 𝒌 𝟐 Where 𝑭𝑬 𝒓 is the magnitude of the electric force in newtons, 𝒒𝟏 𝒂𝒏𝒅 𝒒𝟐 charges in coulombs and these are the magnitudes of the two charges r is the distance between the center of two charges, in meters 𝓀 is the proportionality constant called Coulomb’s constant with an approximate value of 𝟗𝒙𝟏𝟎𝟗 𝑵. 𝒎𝟐 /𝑪𝟐 Note that only the absolute value of the product of 𝒒𝟏 𝒂𝒏𝒅 𝒒𝟐 is determined because only the magnitude of the force is to be computed. The force magnitude F is always positive.  COULOMB’S LAW The direction of the force depends on the signs of the charges: If both charges are the same sign (both positive or both negative), the force is repulsive If the charges have opposite signs, the force is attractive.  Coulomb’s Law The electric force F is a vector quantity, which may be positive or negative depending on its direction. Coulomb’s law bears a strong resemblance to Newton’s law of universal gravitation. Both equations have the same form. Recall that the gravitational force 𝐹% between two masses 𝑚& 𝑚' is given by 𝒎𝟏 𝒎𝟐 𝑭𝑮 = 𝑮 𝟐 𝒓 Where G is the universal gravitational constant approximately equal to 6.674𝑥10#&& 𝑁. 𝑚' /𝑘𝑔'  Sample Problem 1 Two protons are separated by a distance of 3.8𝑥10#&( m in air. (a) Find the magnitude of the electric force one proton exerts on the other. Is this force attractive or repulsive? (b) Find the magnitude of the gravitational force one proton exerts on the other. Is this force attractive or repulsive?  Sample Problem 2 Two small conducting and identical spheres A and B have charges -25 nC and +35 nC, respectively. They are separated by a distance of 0.05m. (a) What is the magnitude of the electric force between the two spheres? Is this attractive or repulsive? (b) The spheres are then allowed to touch each other and then separated. What is the magnitude of the force between the two spheres? Is this repulsive force or attractive force?  Hence we can conclude that two bodies under the action of mutual force between the two, the attractive force brings the bodies towards each other while the repulsive force moves them away from each other. Note: It is to be noted that the gravitational force is always attractive in nature. ELECTRIC FIELD  Electric field Electric force is a noncontact force. An electric charge q can exert force on other charged objects even though they are at some distance away. The space surrounding a charged body is called an electric field. An electric field causes any charged particle placed in it to experience an electric force. Every charge has an electric field associated with it. Michael Faraday (22 September 1791 – 25 August 1867 Michael Faraday, an English scientist who made important discoveries on electricity and magnetism, introduced the use of electric lines of force to map out an electric field. Lines of force have the following properties: What did Faraday discover in 1821? Electromagnetic rotation experiment of Faraday, 1821, the first demonstration of the conversion of electrical energy into motion.  1. Lines of force start from positively charged particles and end on negatively charged particles or continue toward infinity. 2. Lines of force neither intersect nor break as they pass from one charge to another. 3. The greater the number of lines of force, the stronger the electric field. The neutral point is the point where no lines of force pass. The electric field is zero at the neutral point. Thus, neutral points are points where the resultant field is subtractive and the electric fields are equal bur oppositely directed. A neutral point between two like charges is a point between the two charges and nearer the smaller charge. For two unlike charges, lines of force can pass from positive to negative charge. The neutral point cannot be between them; it is an external point along the line joining them and nearer the small charge.  Electric field due to a point charge An electric field exists in the region of space around a charged object or a source charge. When another charged object enters this electric field, it will experience an electric force. The strength of the electric field at a point due to the source charge is called electric field intensity. (In this text, electric field will simply refer to electric field intensity.) Electric field is defined as the force that a test charge will experience when placed at that point. Physicists use a unit positive charge as the test charge in defining an electric field. This test charge and the electric field are usually represented by 𝒒𝟎 , and E, respectively. The magnitude of the electric field due to the point charge is !! " 𝐸 = = 𝓀 "" ## It follows that E has the unit of newton/coulomb (N/C). Like electric force, electric field is also a vector quantity and has the same direction as the electric force on a positive charge placed at a point. The electric field also follows the superposition principle.  Sample Problem Page 25 THANK YOU