Coulomb’s Law
•
Methods to electrically charge an
object
– Conduction:
• Direct contact: will transfer
electrons, such as touching your
car door in the winter
• Friction: rubbing your feet against
carpet, hair against a balloon
– Induction: no direct contact
• Start with a neutral object. Then, bring an electrically charged object
near, but not in contact with, a neutral object
• The charges in the neutral object will be “induced” to separate to get
closer or farther from the charged object.
• If provided a pathway, the separated electrons will leave.
• The object is now positively charged.
•
Electrostatics devices
– Electroscope: the
separation of metal
leaves indicates the
presence of static
charge
– Van de Graaff
generator: charge is
delivered by a rubber
belt to a metal dome
– Electrophorus a device
used to transfer electric
charge
Coulomb’s Law
–
–
Calculates the magnitude
of the electric force
between two charges
Each charge experiences
equal but opposite forces
q1q2
F k 2
d
k is a constant, k = 9 x 109 N·m2/C2
(Since we are interested in the MAGNITUDE of the force, do not include
the signs of negative charges)
Coulomb’s Law looks VERY similar to
Newton’s Universal Law of Gravitation
FG
m1m2
d
2
Fk
q1q 2
d
2
Differences:
1. Gravitational Force is based on MASS.
Coulomb’s law is based on CHARGE.
2. Gravity is ALWAYS an attractive force.
The Electric Force can attract and repel.
3. “G” is a tiny number, therefore gravity force is a relatively
small force.
“k” is a huge number, therefore electric force is a
relatively large force.
Fk
q1q 2
d
2
Both laws are INVERSE SQUARE LAWS
“The Force varies with the inverse of the
distance squared.”
At twice the distance,  d2 = 22 in denominator
= ¼ the Force,
At three times the distance, 32 in denominator,
= 1/9 the Force
At half the distance,  (1/2)2 in denominator
= 4 times the Force
Now if one CHARGE, q, doubles…. The Force
doubles since they are directly related.
Get a calculator and
q1q 2
Fk 2
let’s practice one…
d
What is the magnitude of the electrostatic
force between two charges, q1 = 3.2 mC
and q2 = -24.8 mC separated by a distance
of 2.4 mm? (milli = 10-3, micro = 10-6)
USE THE EXPONENT BUTTON!!!
F = 9E9 * 3.2E-3 * 24.8E-6 ÷ 2.4E-3 2
F = 124000000 N
or, for Quest: 1.24E8
or 1.24e8
Remember….Force is a VECTOR- it
always points in a specific direction! If
more than two charges are present, we
must find the VECTOR sum of the
forces acting on an individual charge.
+
-
+
Get a clicker and calculator. Log in and get
ready to answer some questions.
1. Electric Charge is measured in
A. Amps
B. Volts
C. Coulombs
D. Farads
2. Opposite charges
A. Attract
B. Repel
C. May attract or repel, depending on their
relative size.
D. Neither attract nor repel.
3. Objects with a neutral charge are
A. Attracted only by objects with a net
positive charge.
B. Attracted only by objects with a net
negative charge.
C. Not attracted by any kind of charge.
D. Attracted by objects with either positive or
negative charge.
The charge of one electron
-19
= - 1.6 x 10 C
4. How much excess charge is present in
an object that has 3 x 1018 more electrons
than protons? (Use correct sign!)
5. If the distance between two charges
tripled, the magnitude of the electrostatic
force between them would change by a
factor of
A. 3
B. 6
q1q2
C. 9
F k 2
D. 1/3
d
E. 1/6
F. 1/9
6. Robert Millikan measured the elementary charge of an
electron by charging and then suspending oil drops using a
charged plate as shown:
-----------At the most basic level, which law of physics did Millikan
apply to measure the charge of an electron?
A. Faraday’s Law of mutual inductance
B. Coulomb’s Law
C. The law of conservation of Energy
D. Newtons 1st Law
E. Zeroth Law
7. The development of _______ at room
temerature could revolutionize the electric
industry?
A. Conductors
B. Insulators
C. Semiconductors
D. Superconductors
8. The force between a 150 kg dog and the
Earth is 1470 N. How close must an electron
and a proton be to generate the same force?
A. 9.90 X 10-7 m
B. 9.80 X 10-13 m
C. 3.96 X 10-16 m
D. 1.57 X 10-31 m