Questions to think about
• Why clothes catch hair
• How does dust stick to the ceiling
• Where the lightings come from
electric charge
Each object has charge, which is a scalar attribute describing electric
interactions of the object with other objects.
The SI unit of charge is the coulomb: 1C = 1A ⋅ 1s
Fe
E
The electrostatic force exerted on a particle
depends only on the charge q of the particles
and the local electric field E, produced by
other objects
Fe = q ⋅ E
E
Fe electric field
(lines)
The SI unit of electric field is the volt per meter. 1V/m = 1N / 1C
elementary particles
electron
mass: 9.1 ⋅ 10-31 kg
charge: -1.6 ⋅ 10-19 C
proton
mass: 1.7 ⋅ 10-27 kg
charge: 1.6 ⋅ 10-19 C
neutron
mass: 1.7 ⋅ 10-27 kg
charge: 0 C
Coulomb’s law
q2
q1
F
F
r
The electrostatic forces between two
particles are proportional to charge
of each object and inversely
proportional to the square of the
distance between them
F= k⋅
q1 ⋅ q 2
r2
Charles Augustin de Coulomb
(1596–1650) - French physicist,
described electrostatic interaction
The proportionality constant k = 9⋅109 Nm2/C2 is called the Coulomb constant.
electronic air purifier
Gravity is to weak to remove dust
particle carried in air by the drag force.
A much stronger electrostatic interaction
is used to clean air from dust.
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Charged objects produce electric fields.
A power supply removes electrons from
the corona wires and places electrons
on the collecting plates maintaining a
constant electrostatic field.
Dust particles acquire charge during a
corona discharge and are attracted to
the collecting plates.
electric potential
Electrostatic interaction is conservative. In
an electrostatic field, charged particles have
electrostatic potential energy.
200 V
300 V
Fe
Electric potential at a given location is
number, such that the electrostatic
potential energy of a particle at this
location is equal to the product of its
charge and the potential.
Fe
equipotential surfaces
The SI unit of potential is the volt: 1V = 1J / 1C
Example: An anion with charge 1.6 ⋅ 10-19 C is moved from a location with electric
potential of 200 V to a location with electric potential of 100 V. The change in the
potential energy of the anion is
∆Ue = (1.6⋅10-19 C) ⋅ 300 V - (1.6⋅10-19 C) ⋅ 200 V = 1.6 ⋅10-17 J
Puzzle: What would be the change in potential energy of an electron?
electric polarization
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Van de Graaff generator
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In an electric field, charged
particles within an object may shift,
accumulating different type of
charge on opposite sides of the
object.
The
object
becomes
electrically polarized.
An overall neutral object can
interact electrically with the objects
causing the electrical polarization.
lightning storm
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4. When the electric field
exceeds certain value
(dielectric strength) a
violent discharge
takes places
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1. The air motion in the
clouds separates
charged particles.
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2. The ground becomes
charged by induction.
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3. Charge separation
results in an electric
field.