Download Heat Transfer (Unit 6)

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Electrostatics (Chapter 10)
 Study of charges at rest
 Benjamin Franklin 1700's
Rubbed rods with silk or fur
 Two types of charge
POSITIVE and NEGATIVE
 Like Charges repel
 Opposite charges attract
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Example
Explain how positively charged rod can
be used to give metal sphere
a) negative charge?
b) positive charge?
c) How could positively charged rod
simultaneously give one metal sphere
positive charge and another metal
sphere negative charge?
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Coulomb's Law
 Charles Coulomb 1736 to 1806
 Charged spheres and measured
force between them
q1 q 2
F

4o r 2
 F is force in Newton
 q1 and q2 is charge of object one and
two measured in coulomb (C)
 r is distance between charges (m)
  o is permittivity of free space
= 8.85x10-12Fm-1 (F is Farad which
will be discussed later)
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Example
 In hydrogen atom electron separated
from proton by distance 5.3x10-11m.
Calculate magnitude of electrostatic
force on electron. Electron is in
circular orbit. How fast is it travelling?
Permittivity of free space is
8.85x10-12Fm-1. Electron charge is
-1.60x10-19C and mass 9.11x10-31kg.
Proton charge is +1.60x10-19C.
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Electric Field
 Electric field is region of space
where charge experiences
electrostatic force
 Electric field strength
E
defined
as force per charge
E
F
q
F
is Electrostatic force (N)
q
is charge (C)
E
NC-1
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Example
Calculate magnitude of electric field
strength 1cm from an electron.
Therefore what is force on a 5C charge
placed 1cm from the electron? Charge
of an electron is 1.60x10-19C.
Permittivity of free space is
8.85x10-12Fm-1.
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Electric Potential
 Electric potential V is defined to be
work done in bringing a unit positive
charge from infinity to that point

1
Q
V 
4 o r
Q is charge (C)
r is distance in (m)
 o is permittivity = 8.85x10-12Fm-1
V is electric potential (V)
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Example
What is the electric potential 1m from
an electron of charge -1.60x10-19C?
Permittivity is 8.85x10-12Fm-1.
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Potential Difference
 Difference in electric potential
between two points
 V  V2  V1
V1 electric potential at first point (V)
V2
"
" at second point (V)
V potential difference (V)
 Electric field lines point in direction of
decreasing potential (-ve potential
difference)
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Example
What is the potential difference due to
moving from a point that is 1mm from
an electron to a point that is 1cm from
the electron? Electron has charge
-1.60x10-19C. Permittivity is
8.85x10-12Fm-1.
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