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: 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)

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: 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 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.

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.

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/ 10

The Lightning Rod (Ben Franklin) http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/estatics/u8l4e.html 11

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 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.

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