Physics 106 Lesson #8
Static Electricity
Dr. Andrew Tomasch
2405 Randall Lab
atomasch@umich.edu
Review: Pressure
Pressure is a scalar. Area is a vector. The direction of
an element of area is perpendicular to the surface.
• Pressure P is the force perpendicular to a
surface divided by the area of the surface:
P
force
area
A difference in
pressure across a
surface or object
exerts a net force
perpendicular to
the surface.

F
A
 F  PA
Units of pressure: N/m2 ≡ Pascals (Pa)
(also mm or inches of mercury and lbs/in2)
Review:
Pascal’s Principle
“Any change in the
pressure applied to a
completely enclosed
fluid is transmitted
undiminished to all
parts of the fluid and
vessel walls.”
Pascal
Review: Pascal’s Law
• At a depth h below the surface
of an incompressible fluid:
Pabsolute  Patm  fluid gh
Pascal
Gauge Pressure
≡ Difference From
Atmospheric
The pressure in a static fluid
is the same at all points that
have the same depth
regardless of the container’s
shape:
PA= PB= PC= PD
Review: The Equation of Continuity
(mass in) / time
Incompressible Fluid
1 A1v1  2 A2v2
1  2
A1v1  A2v2
(mass out) / time
What flows in must flow out.
The product of the crosssectional area and flow speed
is everywhere the same.
Bernoulli’s Equation: Speed and Pressure
• Flow in a horizontal pipe:
A1v1  A2v2
Continuity
v1  v2
P1  P2
Bernoulli
As the speed of a fluid increases over
a surface, the pressure of the fluid
against the surface decreases.
Properties of Electric Charges
Franklin
• Two types of charge: positive and negative
(Ben Franklin in early 1700’s)
• Like charges repel; unlike charges attract
• Charge is conserved
• Charge is quantized (comes in discreet units)
• Objects usually have as much negative
charge in them as they do positive charge →
the total charge is zero (electrically neutral)
Insulators and Conductors
We can classify materials
according to their ability to conduct
electrical charge:
– Conductors: charges (free
electrons) move freely (metal)
– Insulators: charge is not readily
transported (glass)
– Semiconductors: electrical
properties in between
Gold is the best
conducting metal
http://www.physicspost.com
Charging a Conductor by Conduction
Conduction = charging by contact
Rub a Teflon rod
with fur to
separate charge
Lightning
• Turbulence in the cloud
causes the charges to
separate (- down, + up)
• Some of the negative
charges on the ground
are pushed down away
from the surface
• A streamer of negative
charges approaches the
ground  lightning
http://regentsprep.org/Regents
/physics/phys03/alightnin/
The Lightning Rod (Ben Franklin)
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/estatics/u8l4e.html
The Electroscope
http://www.engr.uky.edu/~gedney/c
ourses/ee468/expmnt/escope.html
http://www.physicsclassroom.com/
mmedia/estatics/gen.html
Electric Forces
In 1785 Charles Coulomb established the
fundamental law of electric force between
two stationary charged particles:
– Force directed along the line joining the
particles
– Force inversely proportional to the square of
separation distance between particles
– Force proportional to the product of the two
charges
– Force attractive if particles have charges of
opposite sign and repulsive if charges have
same sign
Coulomb’s Law
• Size of force
depends on charge
and distance:
F21
F21
F
k q1 q2
r
2
• Strength constant:
k=8.99×109 Nm2/C2
r


1
2
-
-
1
2

1
F21
F12
F12
F12
2
F12 = -F21 according to
Newton’s 3rd Law.
Comparing the Electrostatic
Force to Gravity
• Gravitational
• Coulomb Force:
Force (Newton):
GMm
FG  2
r
FC 
k q1 q2
r
However, the gravitational
force can only be attractive!
2