Coulomb’s Law
p. 538 in your book
• Two electrically charged objects exert a
force on each other.
• Opposite charges ATTRACT
• Notice the forces are “equal but opposite”
Charged objects & electrical force
• Two electrically charged objects exert a
force on each other.
• Like charges REPEL
• Notice the forces are “equal but opposite”
Charged objects & electrical force
• Coulomb’s law quantifies the magnitude of the
force between two charged particles.
𝒒𝟏 ∙𝒒𝟐
• 𝑭=𝒌 𝟐
𝒓
• k is a proportionality constant
• 𝑘 = 8.99𝑥109 𝑁 ∙ 𝑚2 /𝐶 2
• q1 is the magnitude (no sign) of the charge of
point 1
• q2 is the magnitude (no sign) of the charge of
point 2
• r is the distance between the two charges
Coulomb’s Law
• Coulomb’s law quantifies the magnitude of the
force between two charged particles.
• 𝑭=
𝒒𝟏 ∙𝒒𝟐
𝒌 𝟐
𝒓
• F is a vector, directed along the line joining the
charges
• F is attractive if the charges are opposite sign
• F is repulsive if the charges are the same sign
• Assumption is that the charges are very small compared
to the distance separating them (i.e., point charges)
Coulomb’s Law
• Two objects, whose charges are +1.0C and
-1.0C are separated by 1.0km. Find the
magnitude of the attractive force that either
charge exerts on the other.
• F = 9.0x103 N
Example 1 p. 539
• Newton’s Law of
Universal
Gravitation:
•𝐹=
• Coulomb’s law:
•𝐹=
𝑞1 ∙𝑞2
𝑘 2
𝑟
𝑚1∙𝑚2
𝐺 2
𝑟
Similarities:
Both are inverse square laws
The force is directed along a line between the two objects
Difference:
Electrostatic force can be either attractive or repulsive
Gravitational force is always attractive
Notice anything familiar?
• Newton’s Law of
Universal Gravitation:
•𝐹=
𝑚1∙𝑚2
𝐺 2
𝑟
• Shows how the
gravitational attraction
between the earth and a
satellite provides the
centripetal force to
keep the satellite in
orbit
• Coulomb’s law:
•𝐹=
𝑞1 ∙𝑞2
𝑘 2
𝑟
• Used by Niels Bohr
to explain his
model of the atom
Notice anything familiar?
• We can use Coulomb’s law to determine the
net electrostatic force acting on a point
charge (q1) due to multiple other point
charges (q2 and q3).
More than 2 point charges
• First determine the force (magnitude and
direction!) that q3 exerts on q1 (ignore q2)
• Then determine the force (magnitude and
direction!) that q2 exerts on q1 (ignore q3)
More than 2 point charges
• The net force on q1 is the vector sum of
these two forces.
More than 2 point charges
• Determine the magnitude and direction of
the net electrostatic force on q1.
• +5.7N
Example 4 p. 541
• Coulomb’s law worksheet
Assignment