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Chapter 20 Static Electricity

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Title: Chapter 21 Electric Fields Author: Tony Reiter Last modified by: reiter Created Date: 5/15/2003 7:00:45 PM Document presentation format: On-screen Show – PowerPoint PPT presentation

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Title: Chapter 20 Static Electricity


1
Chapter 20 Static Electricity
Charge by Conduction
2
Chapter 20 Static Electricity
Charge by Induction
3
Chapter 21 Electric Fields
  • Electric Field A property of space around a
    charged object that causes forces on other
    charged objects.
  • Vector quantity It has both direction and
    magnitude
  • The direction of the force is away from the
    positive and towards the negative.
  • The electric field is the strongest when the
    lines are close together
  • Field lines do not exist-only a pictorial guide.

4
Chapter 21 Electric Fields
-

5
Chapter 21 Electric Fields
6
Chapter 21 Electric Fields
7
Chapter 21 Electric Fields
8
Chapter 21 Electric Fields
Van de Graaf Generator
In the Van de Graaf generator, charge is
transferred onto a moving belt A, and then onto
the metal dome B, An electric motor does the work
needed to increase the electric potential energy.
9
Chapter 21 Electric Fields
Van de Graaf Generator
10
Chapter 21 Electric Fields
11
Chapter 21 Electric Fields
Electric Field Intensity
E Electric field intensity (N/C) F Force
(Newtons) q Test Charge (Coulombs)
Similar to Gravitational Field Intensity
g Gravitational field intensity (N/kg) F Force
(Newtons) m mass (kg)
12
Chapter 21 Electric Fields

Increase in electric potential energy
E
g

-
13
Chapter 21 Electric Fields
The electric potential difference (?V) is the
work done in moving a test charge in an electric
field divided by the magnitude of the test charge.
Electric potential difference is measured in
joules per coulomb. One joule per coulomb is a
volt.
14
Chapter 21 Electric Fields
_
HIGH V
_

LOW V
15
Chapter 21 Electric Fields
HIGH V


LOW V
16
Chapter 21 Electric Fields
Electric potential difference in a uniform field
And E F/q so
17
Chapter 21 Electric Fields
A force of .032 N is required to move a charge of
4.2 x 10-5 C in an electric field between two
points that are 25 cm apart. What potential
difference exists between the points?
V 190 volts
18
Chapter 21 Electric Fields
An electron is accelerated by a machine that
subjects it to a potential difference of 50
Megavolts. What energy has the electron
acquired?
W (50 x 106V)(1.6 x 10-19C) 8 x 10-12J
19
Chapter 21 Electric Fields
  • Electric current The flow of electrons
  • Electric current can be maintained only if the
    electrons are
  • returned to areas of high electron concentration


_ _ _ _ _ _
20
Chapter 21 Electric Fields
  • Millikans oil drop experiment
  • Early 1900s Determined electric charge
  • When the forces are balanced, F1 F2
  • Eq mg so q mg/E
  • Found that the charge is quantified
  • Multiples of 1.6 x 10-19C

21
Chapter 21 Electric Fields
  • An oil drop has a mass of 1.9 x 10-16 kg and is
  • suspended in an electric field with
  • intensity of 6000 N/C. Find the charge on
  • the drop and the number of excess
  • electrons.

F1 F2 so Eq mg
3.1 x 10-19C
one extra electron
22
Chapter 21 Electric Fields
All systems are in equilibrium when the energy of
the system is a minimum.
The ball comes to rest when the potential energy
is the least. It is the greatest at A and the
least at B.
23
Chapter 21 Electric Fields
A charged sphere
B neutral sphere
A is the charged sphere with high potential
energy. B is neutral with zero potential energy
  • - -
  • - -

The potential of A decreases and the potential of
B increases and both are at the same potential
  • - -
  • - -

24
Chapter 21 Electric Fields
What happens with a large sphere and a small
sphere?
  • -
  • -
  • -
  • -
  • -
  • -
  • -
  • -
  • -

High q Low q same V
Low V High V same q
25
Chapter 21 Electric Fields
  • Capacitor A device that stores a charge
  • As charge is added, the potential of the body
    increases.
  • For a given charge, the ratio of the charge to
    the potential
  • q/V is a constant.
  • Capacitance is the ability to store a charge.

C Capacitance (farads) q Charge (Coulombs) V
Potential (Volts)
26
Chapter 21 Electric Fields
A 3 x 102 pF capacitor has a potential difference
of 30 volts across it. What is the charge on the
capacitor?
C 3 x 102 pF 3 x 10-10 F q V 30 volts
q CV (3 x 10-10F)(30V) 9 x 10-9C
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