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Electrostatics History Ancient Greeks observed electric phenomena as early as 700 B.C. Amber, when rubbed, attracted pieces of straw or feather William Gilbert (1600) discovered electrical charge was not limited to amber More History Charles Coulomb (1785) Confirmed the inverse square relationship of electrical force. Hans Oersted (1820) Found that a compass needle deflects when near an electrical current. Michael Faraday (1830s) When a wire is moved near a magnet, an electric current is observed in the wire. Properties of Electric Charges Two types of charges: Positive and negative Named by Benjamin Franklin Like charges repel; unlike charges attract. Question #1 The charge on sphere 2 is three times the charge of sphere 1. Which force diagram is correct? (e) none of the above More Properties of Charge The positive charge carrier is the proton. Protons do not move because they are firmly held in the nucleus. The negative charge carrier is the electron. Electrons are found in the electron cloud surrounding the nucleus. An object becomes charged by gaining or losing electrons. Neutrons are electrically neutral and are found in the nucleus More Properties of Charge Electric charge is always conserved. Charge is not created, can only be transferred. Objects become charged because a negative charge is transferred from one object to another. The object that loses electrons becomes positive. The object that gains electrons becomes negative. Properties of Charge, final Charge is quantized Represented by the symbol e Electrons have charge –e Protons have charge +e The SI unit of charge is the coulomb (C) 1e = 1.602 x 10-19 C Conductors Materials in which electric charges move easily Usually metal (copper, silver, aluminum are good electrical conductors. When a conductor is charged in a small region, the charge readily (quickly) distributes itself over the entire surface of the material. Can be charged by induction, conduction Insulators Materials in which electric charges do not move easily. Non-metals are usually better electrical insulators (glass, plastic, rubber). When insulators are charged by friction, only the area rubbed becomes charged There is no tendency of charges to move to other regions. Can be charged by friction or polarization. Semiconductors and Superconductors Share characteristics of both conductors and insulators Ex: silicon, germanium Below certain temperature, resistance is zero Charging By Contact friction conduction Without contact induction polarization Friction Electrons will move to rubber, latex, most plastics. Electrons will be removed from glass. Conduction A charged object (rod) physically touches the uncharged object (sphere). The same type of charge is CONDUCTED from the rod to the sphere. Induction Induction Induction No physical contact between charged and uncharged objects OPPOSITE charge is INDUCED Induction occurs in conductors only Polarization Temporary charging of an insulator by bringing a charged object close to the surface. Question #2 An alpha particle with two positive charges and a less massive electron with a single negative charge are attracted to each other. The force on the electron is: a)Greater than that on the alpha particle b)Less than that on the alpha particle c) Same as that on the alpha particle d)I haven’t a clue… Answer #2 (c) The force on the electron is the same as that on the alpha particle – Newton’s 3rd law. Question #3 An alpha particle with two positive charges and a less massive electron with a single negative charge are attracted to each other. The particle with the most acceleration is: a)Alpha particle b)Electron c) Neither…they have the same acceleration d)I haven’t a clue… Answer #3 (b) The less massive electron has more acceleration – Newton’s 2nd law (F=ma). Question #4 An alpha particle with two positive charges and a less massive electron with a single negative charge are attracted to each other. As the particles get closer to each other, each experiences an increase in: a)Force b)Speed c) Acceleration d)All of these e)None of these Answer #4 As the particles get closer the Force increases, therefore the Acceleration increases and the Speed increases. Coulomb’s Law Warmup: follow the QR codes on page 3. Play with the apps and answer the questions. Coulomb’s law Governs forces and charges, ke is called the Coulomb Constant ke = 8.99 x 109 N·m2/C2 Typical charges are in the μC range. Remember that force is a vector quantity Question #5 If q1 = +20 C and q2 = +10 C and the two charges are 3 meters apart, what is the magnitude of the force between them? a)22 N b)200 N c) 2 x 1011 N d)6 x 1011 N e)I don’t have a clue Answer #5 If q1 = +20 C and q2 = +10 C and the two charges are 3 meters apart, what is the magnitude of the force between them? Known Formula Sub Answer Vector Nature of Electric Charges Two point charges are separated by a distance r The like charges produce a repulsive force between them The force on q1 is equal in magnitude and opposite in direction to the force on q2 Vector Nature of Electric Charges, cont. Two point charges are separated by a distance r The unlike charges produce an attractive force between them The force on q1 is equal in magnitude and opposite in direction to the force on q2 Question #6 If q1 = +20 C and q2 = +10 C and the two charges are 3 meters apart, what is the direction of the force between them? a)Away from each other b)Towards each other c) One chases the other d)Nothing…they don’t move at all e)I don’t have a clue Answer #6 (a) If q1 = +20 C and q2 = +10 C and the two charges are 3 meters apart, what is the direction of the force between them? a)Like charges repel Electrical Field An electric field is said to exist in the region of space around a charged object. When another charged object enters this electric field, the field exerts a force on the second charged object. Electric Field A charged particle, with charge Q, produces an electric field in the region of space around it A small test charge, q0, placed in the field will experience a force. Direction of Electric Field The electric field produced by a negative charge is directed toward the charge A positive test charge would be attracted to the negative source charge Direction of Electric Field The electric field produced by a positive charge is directed away from the charge A positive test charge would be repelled from the positive source charge Electric Field Lines A convenient aid for visualizing electric field patterns is to draw lines pointing in the direction of the field vector at any point These are called electric field lines and were introduced by Michael Faraday The number of lines per unit area through a surface perpendicular to the lines is proportional to the strength of the electric field in a given region Electric Field Line Pattern Point charge The lines radiate equally in all directions For a positive source charge, the lines will radiate outward. For a negative source charge, the lines will point inward. Electric Field Line Patterns An electric dipole consists of two equal and opposite charges. The high density of lines between the charges indicates the strong electric field in this region Electric Field Line Patterns Two equal but like point charges The low field lines between the charges indicates a weak field in this region The bulging out of the field lines between the charges indicates the repulsion between the charges. Unequal charges