# The Electric Field, Electric Field Lines, and Electric Field Inside a Conductor - PowerPoint PPT Presentation

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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