Transcript Document
Chapter 20 Electric Forces and Fields Topics: • • Coulomb Force The field model and the electric field Sample question: In electrophoresis, what force causes DNA fragments to migrate through the gel? How can an investigator adjust the migration rate? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-1 Why is Electric Field important? • Helps us understand action at a distance • Often we can measure electric field when we can’t measure or know source charges directly • Tells us how much force per charge we can expect when we place a charged object at a point in space • Lets us analyze what would happen for different charges • Later, E-field will help us better understand circuits Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Additional Clicker Questions A small sphere is suspended from a string in a uniform electric field. Several different cases of sphere mass and sphere charge are presented in the following table. In which case is the angle at which the sphere hangs the largest? Sphere mass (g) A. 2.0 B. 3.0 C. 2.0 D. 3.0 E. 4.0 Sphere charge (nC) 4.0 4.0 6.0 8.0 9.0 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-61 Answer A small sphere is suspended from a string in a uniform electric field. Several different cases of sphere mass and sphere charge are presented in the following table. In which case is the angle at which the sphere hangs the largest? Sphere mass (g) C. 2.0 Sphere charge (nC) 6.0 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-62 Electric Field Vectors and Electric Field Lines E-field Applet 2 http://qbx6.ltu.edu/s_schneider/physlets/main/efield.shtml What observations can we make about E-field lines? What symmetries can you see? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Consider an infinite sheet of charge • What kind of symmetry would we expect? • What will the field look like? • Is the field (A) converging, (B) diverging, or (C) neither -- (D) can’t tell • What can we say about E-field strength? • A charged sheet can be considered to be like an infinite sheet when we look at points a distance d away where d << L, where L is the length of a side of the sheet Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Consider an infinite sheet of charge Q E where 2 0 A • Epsilon nought, 0 8.85 10 12 C2 N m2 is electric permitivity of free space • Electric permitivity is a measure of how well electric field can pass through space or material Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Consider two infinite sheets of charge What is the E-field at points A, B, and C ? A B Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. C Electric Field Vectors and Electric Field Lines E-field Applet 3 http://www.falstad.com/vector2de/ What observations can we make about the E-field for parallel Plates? Define capacitor as any two conductors with equal and opposite charges Discuss electric permittivity, spacing and charge density Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Dipole and Uniform Electric Fields Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-45 Checking Understanding Two parallel plates have charges of equal magnitude but opposite sign. What change could be made to increase the field strength between the plates? A. B. C. D. E. increase the magnitude of the charge on both plates decrease the magnitude of the charge on both plates increase the distance between the plates decrease the distance between the plates increase the area of the plates (while keeping the magnitude of the charges the same) F. decrease the area of the plates (while keeping the magnitude of the charges the same) Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-50 Answer Two parallel plates have charges of equal magnitude but opposite sign. What change could be made to increase the field strength between the plates? A. increase the magnitude of the charge on both plates Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-51 Checking Understanding A dipole is held motionless in a uniform electric field. For the situation below, when the dipole is released, which of the following describes the subsequent motion? A. The dipole moves to the right. B. The dipole moves to the left. C. The dipole rotates clockwise. D. The dipole rotates counterclockwise. E. The dipole remains motionless. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-57 Answer A dipole is held motionless in a uniform electric field. For the situation below, when the dipole is released, which of the following describes the subsequent motion? D. The dipole rotates counterclockwise. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-58 E-fields and Conductors Inside => E=field is zero Outside => E-field is perpendicular to the surface Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Checking Understanding A dipole is held motionless in a uniform electric field. For the situation below, when the dipole is released, which of the following describes the subsequent motion? A. The dipole moves to the right. B. The dipole moves to the left. C. The dipole rotates clockwise. D. The dipole rotates counterclockwise. E. The dipole remains motionless. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-55 Answer A dipole is held motionless in a uniform electric field. For the situation below, when the dipole is released, which of the following describes the subsequent motion? A. The dipole moves to the right. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-56