physics 106 lesson #8 static electricity dr. andrew tomasch 2405 randall lab [email protected]
TRANSCRIPT
• Pressure P is the force perpendicular to a surface divided by the area of the surface:
Review: Pressure
Units of pressure: N/m2 ≡ Pascals (Pa)(also mm or inches of mercury and lbs/in2)
forcearea
FAP F PA
A difference in pressure across a surface or object exerts a net force perpendicular to the surface.
Pressure is a scalar. Area is a vector. The direction of an element of area is perpendicular to the surface.
“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 Principle
Review: Pascal’s Law
• At a depth h below the surface of an incompressible fluid:
absolute atm fluidP P gh
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
Gauge Pressure ≡ Difference From Atmospheric
Pascal
Review: The Equation of Continuity
222111 vAvA 2211 vAvA
Incompressible Fluid 1 2 (mass in) / time
(mass out) / timeThe product of the cross-sectional area and flow speed is everywhere the same.
What flows in must flow out.
• Flow in a horizontal pipe:
Bernoulli’s Equation: Speed and Pressure
2211 vAvA 21 vv 21 PP
As the speed of a fluid increases over a surface, the pressure of the fluid against the surface decreases.
Continuity Bernoulli
Properties of Electric Charges
• 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)
Franklin
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/courses/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:
• Strength constant:
k=8.99×109 Nm2/C2
1
1
1
2
2
2
F21
F21
F12
F12
F12
F21
F12 = -F21 according to Newton’s 3rd Law.
r
221
r
qqkF
Comparing the Electrostatic Force to Gravity
• Gravitational Force (Newton):
• Coulomb Force:
However, the gravitational force can only be attractive!
2r
GMmFG
2
21
r
qqkFC