Electrostatics
Basic Structure of the Atom:
Negative Charge(-)
no mass
Positive Charge(+)
1 amu
Neutral Charge(0)
1 amu
Electrostatics: Electrical charges at rest
Law of Electrostatics: Like charges repel,
Unlike charges attract
Conductors: Free electrons allow
conduction of charge
emetal
+
+
+ +
+
+
+
+
+ +
+
+
Insulators (non-Conductors): Tightly
bound electrons do not conduct
charge readily
wood
+
+
+ +
+
+
No electron
transfer!
Semi-Conductors: Silicon and Carbon,
among others that fall in between good
conductor and good insulators. Often used
in computer chips and other electronics.
Electroscope: Device used for detecting
charges:
+
+
+
-
-
-
Coloumb’s Law: Electric Charge (Q) exerts a
force that depends directly on the charges
and inversely on the distance between the
charges!
Charge is determined by the surplus or
deficiency of electrons in relation to protons
and is measured in coloumbs (C)
1 C = 6.25 X 1018 electrons
This is a lot of charge-- most charges are
measured in microcoloumbs (µC)
1 µC = 10-6 C
Coloumb’s Law:
F=
kQ1Q2
r2
k = 9 X 109 N m 2/C2
this value is for charges
separated by air
Demo’s
An electrostatic charge of 60.0 µC exerts a
force of 175 N on a charge of 50.0 µC. How
far apart are the charges?
Q1 = 60.0 µC
Q2 = 50.0 µC
r=
kQ1Q2
F
F = 175 N
r=?
= (9 x 109 N
m 2/C2)(60.0 x 10-6 C)(50.0 x 10-6 C)
175 N
= .391 m
Electric Field and Electric Field Intensity
Lines of Force
q
m
q
q
q
Gravitational Field
Electric Field
Electric Field Lines of Force:
Electric Field Intensity (E): Force that acts on
a small test charge (+) in the field
F
E=
q
Units: N/C
1) Calculate the electrostatic force acting on a
proton and an electron that are 1.00 x 10-10 m
apart.
2) A pith ball with a charge of 6.0 µC is
placed 12 cm from a second ball that is
charged -4.3 µC. A) Find the magnitude of
the force between them. B) Explain if this is
a force of attraction or repulsion.
3) Two pith balls are charged and separate so
that the angle between their threads is 20.0˚.
If the mass of each ball is .15 g, what
electrostatic force is acting on each ball?
(FBD!)
4) If the threads in the previous problem are
15.0 cm long, a) how far apart are the pith
balls and b) what is the magnitude of the
charge acting on the pith balls?
5) An object with a charge of 12.3 µC is acted
upon by a force of 6.4 N when placed in an
electric field. What is the field’s intensity at this
point?
6) An electron moving through an electric field
experiences an acceleration of 6.3 x 103 m/s2.
A) How much electrostatic force is acting on it?
B) What is the electric field intensity?
Two pith balls are equally charged and
separate so that the strings they hang from
form an angle of 20.0˚. If the strings are 15.0
cm long, and the electrostatic charge on each
ball is +0.100 µC, what must be the
electrostatic force acting on each?
k = 9 X 109 Nm2/C2