Electrostatics – Coulomb’s Law
Tues / Wed Read Chapter 20 pp. 461-470
Homework: p. 477 1-3, 9-12
Wed/Thurs Do problems p. 478 20, 21, 22, 24, 25, 27
Thurs/Fri p. 478 28, 29, 30, 31, 32
Thurs / Fri Lab
Fri / Mon Homework: Read ch 21 pages 481-487
Do p. 502 23, 24, 26, 27, 28, 30, 31
Mon / Tues Homework: Read 488-501
Do 32, 33, 34, 42, 43, 47
Coulomb’s Law
• Charles Coulomb discovered that the
repulsive force due to similar charges or the
attractive force of dissimilar charges was
inversely proportional to the square of the
distance between the two charged bodies.
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F
1/r2
r
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Coulomb’s Law
• Coulomb further discovered that the force
was proportional to the product of the charge
on each of the objects.
q1
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Sphere 1
F  q1q2
q2
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Sphere 2
Coulomb’s Law
F = kq1q2
r2
Where:
k = 8.99 x 109 N•m2/C2
q1, q2 = the charge of each sphere in Coulombs.
r = the distance between the two charged bodies.
Applet 1 - Coulomb's Law
Applet II - Coulomb's Law
Example 1:
Two point charges,
q1 = 1C
q2 = 2C
are separated by 1 m. What is the force between them?
F = kq1q2 / r2
= (8.99E9N*m2/C2)(1C)(2C)/(1m) 2
= 1.8E10N
Where:
k = 8.99 x 109 N•m2/C2
q1, q2 = the charge of each sphere in Coulombs.
r = the distance between the two charged bodies.
Example 2:
Two point charges,
q1 = 1C
q2 = -2C
are separated by 1 m. What is the force between them?
F = kq1q2 / r2
= (8.99E9N*m2/C2)(1C)(2C)/(1m) 2
= -1.8E10N
Where:
k = 8.99 x 109 N•m2/C2
q1, q2 = the charge of each sphere in Coulombs.
r = the distance between the two charged bodies.
Example 3:
Two point charges,
q1 = 1E-19C
q2 = -2E-19C
are separated by 1E-10 m. What is the force between them?
F = kq1q2 / r2
= (8.99E9N*m2/C2)(1E-19C)(-2E-19C)/(1E-10m) 2
= -1.798E-8N
Where:
k = 8.99 x 109 N•m2/C2
q1, q2 = the charge of each sphere in Coulombs.
r = the distance between the two charged bodies.
Example 4:
At what distance does the electrostatic force between
two protons equal 899N?
F = kq1q2 / r2  kq1q2 / F = r2
(8.99E9)(1.6E-19)2 / 899 = (2.56E-31)  r= 5.05E-16
Where:
k = 8.99 x 109 N•m2/C2
q1, q2 = the charge of each sphere in Coulombs.
r = the distance between the two charged bodies.
Graphs (oh, no, not again.)
• Graph of F vs. r?
• Graph of F vs. q1q2?
Key Ideas
• Like charges repel.
• The electrostatic force is proportional to
the charge on the two objects and
inversely proportional to the square of
the distance between them.