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Electrostatics
Static Charges and Related
Electric Fields
Charged Particles
• Proton:
– Rest mass- 1.6726 x 10-27 kg
– Electric charge - 1.60 x 10-19 C
• Electron:
– Rest mass- 9.1094 x 10-31 kg
– Electric charge - 1.60 x 10-19 C
• Neutron:
– Rest mass- 1.6749 x 10-27 kg
– Electric charge - 0.00 C
Point Charges
• Any charge that is present on the
surface of any isolated sphere.
– Acts as if it were located in the center of
the sphere.
• The same way gravity acts from the center,
even though mass is all around us.
• The quantity of charge at that location
can be represented by “Q”, and is
measured in Coulombs.
• Named after Charles Augustin de Coulomb
(1736-1806)
Coulomb’s Law
• Established proportionality between
factors dealing with forces between
charged particles.
Q1Q2
Force  k 2
d
Where k = 9 x 109 Nm2/C2
Example:
• Two charged particles each charged to 10-2 C
are placed 10m apart. Calculate the force on
them:
Q1Q2
Fk 2
d
2
2
Nm (10 C)
F  9x10
x
2
2
C
(10m)
2
9
F  9x10 N
3
Repulsive Force
Electric Fields
• Exist in a region of space if an electric
charge placed inside it feels a force.
Field Lines
• The path taken by a
positive test charge
within a region of
space surrounding a
charged particle.
• The electric line of force is a line so
drawn that a tangent to it at any point
indicates the orientation of the electric
field at that point.
Electric Field Intensity
• The electric field intensity is
proportional to the number of lines
of force per unit area normal to the
field.
• High intensity = lots of lines.
• Low intensity = few lines.
• Also known as potential gradient.
Field Intensity Equation
• E = F/q
• F is the force in
Newtons acting
on the test
charge q in
coulombs.
Combine with Coulomb’s law,
E = kQ/r2
Example:
• An electrostatic
charge is placed in an
electric field that has
an intensity of 1.50 x
105 N/C. The charge
experiences a force of
2.10 N Calculate the
magnitude of the
charge.
F = 2.10 N
1.50 x 105 N/C
F
2.10N
q 
E 1.50x10 5 N
C
Electric Fields Between
Charges
• Find “E” for each
charge, add all
together.
ET=E1+E2+E3…
Classwork
Modern Physics (Holt, Rinehart, Winston)
• Questions Group B(for Proficiency)
– Questions #1-11
• Problems Group A
Understanding)
– Questions #1-11.
(to show Advanced