Physics Chapter 17: Electric Forces and Fields

 Electrical Charge  Positive Charge  Caused by the Removal of Electrons  Negative Charge  Caused by an Excess of Electrons  Electrically Neutral  Balance of Nucleus and Electron Cloud Charges

Electric Forces and Fields  Electrical Charge  Like Charges Repel Each Other  Opposite Charges Attract Each Other

Electric Forces and Fields  Electrical Charge  Charge is Conserved  Some Atoms May Gain Electrons  Anions  Some Atoms May Lose Electrons  Cations  Charge is Transferred

Electric Forces and Fields  Electrical Charge  Quantity of Charge  Charges are Dependant on Addition or Removal of Electrons  Charges can be +1, +2, +3 … or -1, -2, -3 …  One Unit of Charge is 1.6x C

Electric Forces and Fields  Electrical Charge  Quantity of Charge  Charge of an Electron  - 1.6x C  Charge of a Proton  + 1.6x C

Electric Forces and Fields  Electrical Charge  Quantity of Charge  Units  Coulomb (C) Charles Augustin de Coulomb

Electric Forces and Fields  Transfer of Electrical Charge  Conductors  Materials that Allow Charge (electrons) to Move  Most Metals  Most Salts  Water (with electrolytes)

Electric Forces and Fields  Transfer of Electrical Charge  Insulators  Materials that Inhibit the Flow of Charge  Glass  Wood  Plastic  Water (pure)

Electric Forces and Fields  Transfer of Electrical Charge  Semiconductors  Insulator, but Dynamically Conductive by Temperature and Addition of Impurities  Silicon

Electric Forces and Fields  Transfer of Electrical Charge  Charging by Contact  Static Charge  Insulators  Conductors  Ground  Reservoir of Electrons Rub-eeRub-erCharge Glass Rodsilk+ Acrylic tubefur−

Electric Forces and Fields  Transfer of Electrical Charge  Charging by Induction  Distributing Charge

Electric Forces and Fields  Transfer of Electrical Charge  Conduction  Charging a Neutral Object Through the Contact of a Charged Object  Induction  Charging a Neutral Object Through a Charged Object without Contact

Electric Forces and Fields  Electrical Force  Force can Act Over a Distance  Force is Directly Proportional to Distance

Electric Forces and Fields  Electrical Force  Coulomb’s Law  Forces from Charge  The Greater the Charge, the Greater the Force  The Closer the Distance, the Greater the Force

Electric Forces and Fields  Electrical Force  Coulomb’s Law  F = Electrostatic Force  q = Magnitude of Point Charge (Not -)  r = Distance Separating Point Charges  k = 8.99x10 9 Nm 2 /C 2

Electric Forces and Fields  Electrical Force  Coulomb’s Law  Units  Coulomb (C)  1 C = 6.25x10 18 electrons  The Charge of One Electron = -1.60x C

Electric Forces and Fields  Electrical Force  Field Force (No Contact)  Can be Attractive or Repulsive  Gravity Only Attractive  Strong Force  Gravitational Force is Relatively Much Weaker

Electric Forces and Fields  Electrical Force  Superposition  More than Two Charged Particles  Vector Sum of the Forces +q 1 -q 2 -q 3 F 1-2 F 1-3

Electric Forces and Fields  Problem  Two electrons in an atom are separated by 1.5x10 –10 m, the typical size of an atom. What is the electrical force between them?

Electric Forces and Fields  Solution  Two electrons in an atom are separated by 1.5x10 –10 m, the typical size of an atom. What is the electrical force between them?

Electric Forces and Fields  Problem  Two negatively charged bodies each with charge of -5.0x10 –5 C are 0.20 m from each other. What force acts on each particle?

Electric Forces and Fields  Solution  Two negatively charged bodies each with charge of -5.0x10 –5 C are 0.20 m from each other. What force acts on each particle?

Electric Forces and Fields  Problem  Two charges, q 1 and q 2, are separated by a distance, d, and exert a force, F, on each other. What new force will exist if q 1 is doubled?

Electric Forces and Fields  Solution If q 1 is doubled it becomes 2q 1 which doubles F

Electric Forces and Fields  Problem  Two charges, q 1 and q 2, are separated by a distance, d, and exert a force, F, on each other. What new force will exist if q 1 and q 2 are cut in half?

Electric Forces and Fields  Solution If q 1 and q 2 are cut in half they become ½ q 1 and ½ q 2 which reduce F, 4 fold

Electric Forces and Fields  Problem  Two charges, q 1 and q 2, are separated by a distance, d, and exert a force, F, on each other. What new force will exist if d is tripled?

Electric Forces and Fields  Solution If d is tripled, F is reduced by 3 2

Electric Forces and Fields  Problem  Two charges, q 1 and q 2, are separated by a distance, d, and exert a force, F, on each other. What new force will exist if d is cut in half?

Electric Forces and Fields  Solution If d (r) is halved, F is increased by 2 2

Electric Forces and Fields  Problem  Two charges, q 1 and q 2, are separated by a distance, d, and exert a force, F, on each other. What new force will exist if q 1 is tripled and d is doubled?

Electric Forces and Fields  Solution If q 1 is tripled it becomes 3q 1 and if d is doubled the denominator becomes 3r 2, so F is reduced to ¾

Electric Forces and Fields  Problem  A force of 4.4x10 3 N exists between a positive charge of 8.0x10 –4 C and a negative charge of –3.0x10 –4 C. What distance separates the charges?

Electric Forces and Fields  Solution  A force of 4.4x10 3 N exists between a positive charge of 8.0x10 –4 C and a negative charge of –3.0x10 –4 C. What distance separates the charges?

Electric Forces and Fields  Problem  A positive charge of 3.0 µC is pulled on by two negative charges. One, –2.0 µC, is m to the north and the other, –4.0 µC, is m to the south. What total force is exerted on the positive charge?

Electric Forces and Fields  Solution  A positive charge of 3.0 µC is pulled on by two negative charges. One, –2.0 µC, is m to the north and the other, –4.0 µC, is m to the south. What total force is exerted on the positive charge?

Electric Forces and Fields  Solution  A positive charge of 3.0 µC is pulled on by two negative charges. One, –2.0 µC, is m to the north and the other, –4.0 µC, is m to the south. What total force is exerted on the positive charge?

Electric Forces and Fields  Solution  A positive charge of 3.0 µC is pulled on by two negative charges. One, –2.0 µC, is m to the north and the other, –4.0 µC, is m to the south. What total force is exerted on the positive charge?

Electric Forces and Fields  Homework  Page 655  Problems  18 (3.5x10 3 N)  19 (45N attractive)  21 (1x10 -7 N, 12 o below +x axis)  23 ?

Electric Forces and Fields  Electric Fields  Electric Fields Exist Around Any Object that Carries a Charge  The Potential Difference Between Two Charged Objects Can Transfer Energy in the Form of Electricity

Electric Forces and Fields  Electric Fields  Electrical Force is Measured in C/m, Units are Newtons  Electric Field (E) is Measured in F/q 0, Units are N/C

Electric Forces and Fields  A Charge Creates an Electric Field in All Directions  A Second Charge (test charge), Within the Electric Field of the First Charge, Will be Affected by the Force of the Electric Field +q 1 -q 2 -q 3 q0q0

Electric Forces and Fields  Electric Fields (E)  F - Electric Force

Electric Forces and Fields  Units  q`= Coulomb  F = Newton  E = Newton/Coulomb (N/C)

Electric Forces and Fields  The Magnitude of an Electric Field is Proportional to the Distance From the Charge Creating the Field  The Closer to the Charge, the Greater the Field  The Further from the Charge, the Weaker the Field

Electric Forces and Fields  Electric Fields Have Direction  Fields Extend Away from Positive Charges  Fields Extend Toward Negative Charges

Electric Forces and Fields  Electric Field Lines

Electric Forces and Fields  Electric Field Lines

Electric Forces and Fields  Conductors in Electrostatic Equilibrium  St. Elmo’s Fire  Charge Accumulation at Points of Conductive Surfaces  Corona St. Elmo’s Fire Video

Electric Forces and Fields  Conductors in Electrostatic Equilibrium  Van de Graff Generator  Rubber Belt Pulls e - from Felt  Electrons Transfer to Screen  Excess e - at Sphere  Seeking Equilibrium  No Points = No Corona

Electric Forces and Fields  Homework  Page  Problems  38 (12x10 3 N/C toward the 30nC Charge)  43 (a, 2x10 7 N/C +x b, 40N)  56 ?