# Coulomb’s Law.pptx by hcj

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• pg 1
15.3
Coulomb’s Law
Remember:                                F           F
- Like charges repel each other.
- Unlike charges attract each other   F           F
- The repulsion and attraction are
F   F
due to electric force exerted by
charge on the other

Charles Coulomb Studied the
electric force between charges
and put forth Coulomb’s Law
Statement of Coulomb’s Law

The electrostatic force between two
point charges is directly proportional to
the product of charges and inversely
proportional to the square of distance
between them.
Electric force is directly proportional
to product of charges
F    q                                    q     F
r

2F           2q                                    q         2F
r
F     q                                   q         F
3F       r       3F
q                                   -3q
r
F                         F
-q                                   q
F       q                   r                q      F
r

6F                  2q                                   3q            6F
r
Electric force is inversely proportional
to the square of distance between
them
F   q                   q   F
r

F   q                                    q    F
2r

4F           q           q           4F
r
Mathematical formula of Coulomb’s
law

F   q1                  q2     F
r

F : Electric force each charge exerts on the other (N)
q1 : Magnitude of first charge (C)
q2 : Magnitude of second charge (C)
r : The distance between the two charges (m)
Ke : Coulomb’s constant (8.99 x 109 N.m2/C2
Remarks about electric force between
two charges
• It is directed along the line joining the two charges

• The force exerted by first charge on second charge (F12)
and the force exerted by second charge on first charge
(F21) are equal in magnitude and opposite in direction
(action and reaction, Newton’s third law), so

F12 = - F21
Electrostatic force Vs. gravitational force
Electrostatic force                   Gravitational force
Both of them are field forces (forces that are exerted by objects
without the need to a physical contact)
Both of them are mutual forces between two objects
Both of them are inverse square laws.
Their mathematical formulas are almost
the same

It is due to electric charges on     It is due to the masses of the two
the two objects                      objects
It could be attractive or repulsive It is only an attractive force
Very strong (K = 9 x 109 N.m2/C2) Very weak (G = 6.67x 0-11 N.m2/kg2)
Example on Coulomb’s law
• A 7.5 nC charge s located 1.80 m from a 4.20 nC charge.
a) Find the magnitude of the electrostatic force that
one charge exerts on the other.
b) Is the force attractive or repulsive?
Solution:
Given: q1 = 7.5 nC = 7.5x10-9 C
q2 = 4.2 nC = 4.2x10-9 C
r = 1.80 m

a)               = 8.99x109 x

= 8.74 x 10-8 N
b) The force is repulsive because the two charges has
like signs

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