Download Static Electricity

Coulomb’s
Law
Coulomb’s Law…
= the relationship among electrical
forces, charges, and distance.
It is like Newton’s law of gravity.
But, unlike gravity, electric forces
can be attractive or repulsive.
Discovered by French Scientist,
Charles Coulomb in the 18th
Century.
Coulomb’s Law
• F = K qAqB
d2
F = Force (N)
Kc = (Coulomb Constant)
9.0 X 109 N·m2/C2
q = charge (C for Coulomb)
d = distance (m)
Important Information…
• The SI Unit of measure for electrical
charge is a Coulomb (C)
• Charge of 1 proton is +1.6 x 10-19 C
• Charge of 1 electron is -1.6x 10-19 C
Remember…
• The charge (q) of a particle is NOT
a vector quantity.
• Therefore, when described as
“POSITIVE” or “NEGATIVE” that
tells whether it is a proton or
electron, but is NOT part of the
magnitude of the charge.
• DO NOT PUT THE NEGATIVES
IN THE FORMULA.
Sample Problem #1
• Two charges are
separated by 0.030m.
Object A has a charge of
+6.0 C while object B has
a charge of +3.0 C. What
is the force on object A?
Sample Problem #1
ANSWER
F = 1.8 x 1014 N
Sample Problem #2
• A negative charge of
-6x10-6 C exerts an
attractive force of 65 N
on a second charge
0.050 m away. What is
nd
the magnitude of the 2
charge?
Sample Problem #2
ANSWER
qb = +3.009 x 10-6 C
Electric Fields
• Electric Field – a vector
quantity that relates the
force exerted on a charge to
the size of the charge
• E=F/q or E=kc(q/d2)
• The magnitude of the
electric field is measured in
N/C (Newtons/Coulomb)
Electric Field Lines
• Direction of arrows indicate the
charge of the field
• Away from a positively charged
object
• Toward a negatively charged
object
• Strength of the field is denoted by
the spacing between the lines
(close = strong)
Electric Field Lines
Example
• Your hair acquired an equal
amount of positive charge when
you rubbed the balloon on your
head. What is the strength of the
electric field created by your head
at the location of your feet, 1.5
meters below if the charge is 2e9C?