Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Coulomb Force The field model and the electric field Chapter 20 Electric Forces and Fields Topics: Sample question: In electrophoresis, what force causes DNA fragments to migrate through the gel? How can an investigator adjust the migration rate? Slide 20-1

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Nature of Electric Field Test charge is a small positive charge to sample the E-Field Charge of test charge is small compared to source charges (source charges are the charges that generate the field) E-field vectors E-field is the force per charge E-field vectors points away from + charges E-field vectors point towards - charges E-field for point charges gets weaker as distance from source point charges increases For a point charge E = F e / q = [k Q q / r 2 ] / q = k Q / r 2

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 1.Determine the magnitude and the direction of the electric field at point A. Slide E-field Superposition Example

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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)Sphere charge (nC) A B C D E Additional Clicker Questions Slide 20-61

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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)Sphere charge (nC) C Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Electric Field Vectors and Electric Field Lines E-field Applet 2 What observations can we make about E-field lines? What symmetries can you see?

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Electric Field Vectors and Electric Field Lines E-field Applet 3 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 Slide 20-45

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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 B.decrease the magnitude of the charge on both plates C.increase the distance between the plates D.decrease the distance between the plates E.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) Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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 Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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. Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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. Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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. 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. Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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. Slide Answer