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The Electric Field, Electric Field Lines, and Electric Field Inside a Conductor

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The Electric Field, Electric Field Lines, and Electric Field Inside a Conductor 18.6 to 18.8 18.6 The Electric Field We can use a test charge to determine how the ... – PowerPoint PPT presentation

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Title: The Electric Field, Electric Field Lines, and Electric Field Inside a Conductor


1
The Electric Field, Electric Field Lines, and
Electric Field Inside a Conductor
  • 18.6 to 18.8

2
18.6 The Electric Field
  • We can use a test charge to determine how the
    surrounding charges generate a force
  • Pick a small test charge so it doesnt change the
    surrounding charge orientation

3
Example Test Charge
  • A test charge (q0 1.0 x 10-10 C) experiences a
    force of 2 x 10-9 N when placed near a charged
    sphere. Determine the Force per Coulomb that the
    charge experiences and predict the force on a 2 C
    charge.
  • F/q0 20 N/C
  • F 40 N

4
Electric Field Definition
  • Force per charge
  • Vector
  • Same direction as the force on a positive test
    charge
  • Remember to add them as vectors!!!!
  • Unit N/C

5
Point Charges
  • Notice the q0 does not affect the E-field

6
Example
  • There is a point charge of q 2x10-8 C.
    Determine the E-field at 0.50 m away using a test
    charge of 1x10-10 C.
  • E 719 N/C

7
Example
  • There are two point charges of q1 4 C and q2
    8 C and they are 10 m apart. Find point where E
    0 between them.
  • d 5.85 m from q1 towards q2

8
Parallel-Plate Capacitor
  • Two plates are charged one positive and one
    negative with a space between
  • The E-field points from the positive to negative
  • Except at edges

9
18.7 Electric Field Lines
  • It would be nice to have some kind of map to show
    the E-field in space
  • Rules
  • Lines begin at positive charges only
  • Lines end at negative charges only
  • The number of lines entering or leaving a charge
    is proportional to the size of charge
  • Lines dont cross each other
  • Lines leave surfaces at 90 degrees

10
E-field Lines points charges
11
Curved E-Field Lines - Dipoles
12
Example Whats Wrong Here
13
18.8 Electric Field Inside a Conductor
  • If part of a conductor has a charge, the
    electrons immediately try to get as far apart as
    possible, thus
  • At equilibrium, any excess charge resides on the
    surface of a conductor
  • Since the electrons dont move under equilibrium
  • The E-field inside a conductor in equilibrium is
    zero

14
Shielding
  • Say an uncharged conducting cylinder is in the
    middle of a parallel-plate capacitor
  • The induced electrical charge is on the outside
    of the cylinder
  • The E-field inside the cylinder is zero
  • If we cut out the middle of the cylinder, nothing
    changes since the excess charge is on the outer
    surface
  • A test charge in the cavity would feel no force
    since E 0

15
Shielding
  • A conductor shields any charge within it from
    external electrical fields
  • Sensitive electrical equipment is shielded by
    putting in a metal box
  • Coaxial cable is shielded by a metal cylinder
    around the central metal wire. This reduces
    interference and signal loss

16
Practice Problems
  • Electrify your brain and answer these problems
  • 542 CQ 9-11, 14, P 25-27, 29, 30, 32
  • Total of 10 Problems
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