Transcript Document
General Physics (PHY 2140) Introduction Syllabus and teaching strategy Electricity and Magnetism • Properties of electric charges • Insulators and conductors • Coulomb’s law Lecture 1. Chapter 15 7/18/2015 1 Syllabus and teaching strategy Lecturer: Prof. Alexey A. Petrov, Room 260 Physics Building, Phone: 313-577-2739, or 313-577-2720 (for messages) e-mail: [email protected], Web: http://www.physics.wayne.edu/~apetrov/ Office Hours: MWF 10:40 AM -11:35 AM, General Lectures, Room 150 Tuesday 1:00-2:00 PM, on main campus, Physics Building, Room 260, or by appointment. Grading: Reading Quizzes (bonus) Quiz section performance/Homework Best Hour Exam Second Best Hour Exam Final 5% 10% 25% 25% 40% Reading Quizzes: It is important for you to come to class prepared! BONUS POINTS: Reading Summaries Homework and QUIZ Sessions: The quiz sessions meet once a week; 5 quizzes will count towards your grade. BONUS POINTS: will be assigned by your quiz instructor. Hour Exams and Final Exam: There will be THREE (3) Hour Exams and one Final Exam. Additional BONUS POINTS will be given out for class activity. Online Content: Lecture Online will be made available to you as a supplemental reference. 7/18/2015 2 Introduction Knowledge of electricity dates back to Greek antiquity (700 BC). Began with the realization that amber (fossil) when rubbed with wool, attracts small objects. This phenomenon is not restricted to amber/wool but may occur whenever two non-conducting substances are rubbed together. 7/18/2015 3 15.1 Properties of Electric Charges Discovery Observation of “Static Electricity” A comb passed though hair attracts small pieces of paper. An inflated balloon rubbed with wool. “Electrically charged” Rub shoes against carpet/car seat to charge your body. Remove this charge by touching another person/a piece of metal. Two kinds of charges Named by Benjamin Franklin (1706-1790) as positive and negative. Like charges repel one another and unlike charges attract one another. 7/18/2015 4 15.1 Properties of Electric Charges Nature of Electrical Charge Origin of charge is at the atomic level. Nucleus : “robust”, positive. Electrons : mobile, negative. Usual state of the atom is neutral. Charge has natural tendency to be transferred between unlike materials. Electric charge is however always conserved in the process. 7/18/2015 Charge is not created. Usually, negative charge is transferred from one object to the other. 5 15.1 Properties of Electric Charges Quantization Robert Millikan found, in 1909, that charged objects may only have an integer multiple of a fundamental unit of charge. Charge is quantized. An object may have a charge e, or 2e, or 3e, etc but not say 1.5e. Proton has a charge +1e. Electron has a charge –1e. Some particles such a neutron have no (zero) charge. A neutral atom has as many positive and negative charges. Units 7/18/2015 In SI, electrical charge is measured in coulomb ( C). The value of |e| = 1.602 19 x 10-19 C. 6 15.2 Insulators and Conductors –Material classification Materials/substances may be classified according to their capacity to carry or conduct electric charge Conductors are material in which electric charges move freely. Insulator are materials in which electrical charge do not move freely. Glass, Rubber are good insulators. Copper, aluminum, and silver are good conductors. Semiconductors are a third class of materials with electrical properties somewhere between those of insulators and conductors. 7/18/2015 Silicon and germanium are semiconductors used widely in the fabrication of electronic devices. 7 Mini-quiz: Identify substances or materials that can be classified as 7/18/2015 Conductors ? Insulators? 8 15.2 Insulators and Conductors – Charging by Conduction. Consider negatively charge rubber rod brought into contact with a neutral conducting but insulated sphere. Some electrons located on the rubber move to the sphere. Remove the rubber rod. Excess electrons left on the sphere. It is negatively charged. This process is referred as charging by conduction. 7/18/2015 9 15.2 Insulators and Conductors – Earth/Ground. When a conductor is connected to Earth with a conducting wire or pipe, it is said to be grounded. Earth provides a quasi infinite reservoir of electrons: can accept or supply an unlimited number of electrons. 7/18/2015 10 15.2 Insulators and Conductors – Charging by Induction. Consider a negatively charged rubber rod brought near a neutral conducting sphere insulated from the ground. Repulsive force between electrons causes redistribution of charges on the sphere. Electrons move away from the rod leaving an excess of positive charges near the rod. Connect a wire between sphere and Earth on the far side of the sphere. Repulsion between electrons cause electrons to move from sphere to Earth. Disconnect the wire. The sphere now has a positive net charge. This process is referred as charging by induction. Charging by induction requires no contact with the object inducing the charge. 7/18/2015 11 15.2 Insulators and Conductors – Charging by Induction. Consider a negatively charged rubber rod brought near a neutral conducting sphere insulated from the ground. Repulsive force between electrons causes redistribution of charges on the sphere. Electrons move away from the rod leaving an excess of positive charges near the rod. Connect a wire between sphere and Earth on the far side of the sphere. Repulsion between electrons cause electrons to move from sphere to Earth. Disconnect the wire. The sphere now has a positive net charge. This process is referred as charging by induction. Charging by induction requires no contact with the object inducing the charge. 7/18/2015 Q: How does this mechanism work is we bring is a positively charged glass rod instead? 12 15.2 Insulators and Conductors – Polarization. Polarization is realignment of charge within individual molecules. Produces induced charge on the surface of insulators. how e.g. rubber or glass can be used to supply electrons. 7/18/2015 13 Mini-quiz A positively charged object hanging from a string is brought near a non conducting object (ball). The ball is seen to be attracted to the object. 1.Explain why it is not possible to determine whether the object is negatively charged or neutral. 2.What additional experiment is needed to reveal the electrical charge state of the object? ? 7/18/2015 + 14 Explain why it is not possible to determine whether the object is negatively charged or neutral. Two possibilities: Attraction between objects of unlike charges. + Attraction between a charged object and a neutral object subject to polarization. ++ ++ 7/18/2015 --- + 15 What additional experiment is needed to reveal the electrical charge state of the object? Two Experiments: Bring known neutral ball near the object and observe whether there is an attraction. ? Bring a known negatively charge object near the first one. If there is an attraction, the object is neutral, and the attraction is achieved by polarization. +- +++ - ++ -+- 7/18/2015 0 16 15.3 Coulomb’s Law - Observation Charles Coulomb discovered in 1785 the fundamental law of electrical force between two stationary charged particles. An electric force has the following properties: Inversely proportional to the square of the separation, r, between the particles, and is along a line joining them. Proportional to the product of the magnitudes of the charges |q1| and |q2| on the two particles. Attractive if the charges are of opposite sign and repulsive if the charges have the same sign. q1 q2 r 7/18/2015 17 15.3 Coulomb’s Law – Mathematical Formulation ke known as the Coulomb constant. Value of ke depends on the choice of units. SI units Force: the Newton (N) Charge: the coulomb ( C). Current: the ampere (A =1 C/s). Distance: the meter (m). Experimentally measurement: ke = 8.9875109 Nm2/C2. Reasonable approximate value: ke = 8.99109 Nm2/C2. 7/18/2015 18 Charge and Mass of the Electron, Proton and Neutron. 7/18/2015 Particle Charge ( C) Mass (kg) Electron -1.60 10+19 9.11 10+31 Proton +1.60 10+19 1.67 10+27 Neutron 0 1.67 10+27 19 Example 1e = -1.60 10-19 c Takes 1/e=6.6 1018 protons to create a total charge of 1C Number of free electrons in 1 cm3 copper ~ 1023 Charge obtained in typical electrostatic experiments with rubber or glass 10-6 C = 1 mc A very small fraction of the total available charge 7/18/2015 20 15.3 Coulomb’s Law – Remarks The electrostatic force is often called Coulomb force. It is a force: a magnitude a direction. Second example of action at a distance. 7/18/2015 21 Mini-Quiz Name the first action at a distance force you have encountered in physics so far. 7/18/2015 22 Example: Electrical Force Question: The electron and proton of a hydrogen atom are separated (on the average) by a distance of about 5.3x10-11 m. Find the magnitude of the electric force that each particle exerts on the other. 7/18/2015 23 Question: The electron and proton of a hydrogen atom are separated (on the average) by a distance of about 5.3x10-11 m. Find the magnitude of the electric force that each particle exerts on the other. Observations: We are interested in finding the magnitude of the force between two particles of known charge, and a given distance of each other. The magnitude is given by Coulomb’s law. q1 =-1.60x10+19 C q2 =1.60x10+19 C r = 5.3x10+11 m 7/18/2015 24 Question: The electron and proton of a hydrogen atom are separated (on the average) by a distance of about 5.3x10-11 m. Find the magnitude of the electric force that each particle exerts on the other. Observations: We are interested in finding the magnitude of the force between two particles of known charge, and a given distance of each other. The magnitude is given by Coulomb’s law. q1 =-1.60x10-19 C q2 =1.60x10-19 C r = 5.3x10-11 m Solution: Fe ke e r2 1.6 10 5.3 10 19 2 8.99 10 9 Nm 2 C2 11 m C 2 2 8.2 108 N Attractive force with a magnitude of 8.2x10-8 N. 7/18/2015 25 Superposition Principle From observations: one finds that whenever multiple charges are present, the net force on a given charge is the vector sum of all forces exerted by other charges. Electric force obeys a superposition principle. 7/18/2015 26 Example: Using the Superposition Principle Consider three point charges at the corners of a triangle, as shown below. Find the resultant force on q3 if q1 = 6.00 x 10-9 C q2 = -2.00 x 10-9 C q3 = 5.00 x 10-9 C y q2 F31 - 3.00 m q1 + 7/18/2015 4.00 m F32 + q3 37.0o x 27 Consider three point charges at the corners of a triangle, as shown below. Find the resultant force on q3. y q2 F31 - 3.00 m q1 + 4.00 m F32 + q3 37.0o x Observations: The superposition principle tells us that the net force on q3 is the vector sum of the forces F32 and F31. The magnitude of the forces F32 and F31 can calculated using Coulomb’s law. 7/18/2015 28 Consider three point charges at the corners of a triangle, as shown below. Find the resultant force on q3. y q2 3.00 m - F32 4.00 m F32 ke q3 q2 F31 ke q3 q1 r r 37.0o + q3 5.00 m q1 + Solution: F31 2 2 x 5.00 10 C 2.00 10 C 5.62 10 9 8.99 109 Nm C2 2 9 4.00m 5.00 10 C 6.00 10 C 1.08 10 9 8.99 109 Nm C2 2 5.00m 9 2 N 9 2 8 N Fx F32 F31 cos 37.0o 3.01109 N Fy F31 sin 37.0o 6.50 109 N F Fx2 Fy2 7.16 109 N 65.2o 7/18/2015 29