1. Physics 106 Lesson #8 Static Electricity
Dr. Andrew Tomasch
2405 Randall Lab

2. 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:
A difference in pressure across a surface or object exerts a net force perpendicular to the surface.
Units of pressure: N/m2 ≡ Pascals (Pa)
(also mm or inches of mercury and lbs/in2)

3. 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

4. Review: Pascal’s Law
At a depth h below the surface of an incompressible fluid:
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

5. Review: The Equation of Continuity
Incompressible Fluid
(mass in) / time
(mass out) / time
The product of the cross-sectional area and flow speed is everywhere the same.
What flows in must flow out.

6. Bernoulli’s Equation: Speed and Pressure
Flow in a horizontal pipe:
Continuity
Bernoulli
As the speed of a fluid increases over a surface, the pressure of the fluid against the surface decreases.

7. 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)

8. 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

9. Charging a Conductor by Conduction
Conduction = charging by contact
Rub a Teflon rod with fur to separate charge

10. 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 10

11. The Lightning Rod (Ben Franklin) 11

12. The Electroscope

13. 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

14. Coulomb’s Law r Size of force depends on charge and distance:
Strength constant:
k=8.99×109 Nm2/C2 + - 1 2
F21
F12
F12 = -F21 according to Newton’s 3rd Law.

15. Comparing the Electrostatic Force to Gravity
Gravitational Force (Newton):
Coulomb Force:
However, the gravitational force can only be attractive!