Chapter 20 Static Electricity

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Chapter 20Static Electricity
Charge by Conduction
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Chapter 20Static Electricity
Charge by Induction
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Chapter 21Electric 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.

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Chapter 21Electric Fields
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Chapter 21Electric Fields
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Chapter 21Electric Fields
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Chapter 21Electric Fields
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Chapter 21Electric 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.
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Chapter 21Electric Fields
Van de Graaf Generator
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Chapter 21Electric Fields
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Chapter 21Electric 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)
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Chapter 21Electric Fields

Increase in electric potential energy
E
g

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Chapter 21Electric 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.
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Chapter 21Electric Fields
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HIGH V
_

LOW V
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Chapter 21Electric Fields
HIGH V


LOW V
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Chapter 21Electric Fields
Electric potential difference in a uniform field
And E F/q so
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Chapter 21Electric 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
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Chapter 21Electric 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
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Chapter 21Electric Fields

• Electric current The flow of electrons
• Electric current can be maintained only if the
  electrons are
• returned to areas of high electron concentration

_ _ _ _ _ _
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Chapter 21Electric 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

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Chapter 21Electric 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
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Chapter 21Electric 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.
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Chapter 21Electric 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

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• - -

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

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• -
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High q Low q same V
Low V High V same q
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Chapter 21Electric 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)
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Chapter 21Electric 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