1. Electronic Air Cleaners

2. Question:
A woman rubs her feet on the carpet and gives a shock to her identical twin. If the twin also rubs her feet on the carpet before being touched, the shock will be
larger.
smaller.
the same size.

3. Observations About Air Cleaners
Dust doesn’t settle quickly on its own
Mechanical filters gradually plug up
Dust clings to things with static electricity
Air cleaners could be based on static electricity

4. Air Resistance Different from buoyancy
Opposes the relative motion of air and object
Acts to bring both to one velocity
Consists of a matched pair of forces:
The air pushes on the object
The object pushes on the air
Forces have equal magnitudes, opposite directions
Increases with relative speed and cross section
Demonstration: Drop a Baseball
Demonstration: Drop an Air-Filled Balloon
Demonstration: Drop a Helium-Filled Balloon

5. Terminal Velocity A real falling object experiences two forces:
Weight downward
Air resistance upward (while dropping in still air)
Air resistance affects descent
Object accelerates downward initially
Upward air resistance gradually increases
Terminal velocity occurs when net force is zero
Small objects have small terminal velocities
Demonstration: Drop a Baseball (again)
Demonstration: Drop a Feather
Demonstration: Drop a Chalk
Demonstration: Release Chalk Dust

6. Electric Charge 1 Two types: positive & negative
Like charges repel, opposites attract
Forces consist of a matched pair:
Each charge pushes or pulls on the other
Forces have equal magnitudes, opposite directions
Forces increase with decreasing separation
Charge is measured in coulombs
Demonstration: Negatively Charged Teflon Rod and Pith Ball Repel
Demonstration: Positively Charged Acrylic Rod and Pith Ball Repel
Demonstration: Opposite Charges Attract--First Rods, Then Balls

7. Electric Charge 2 Charge is conserved Charge is quantized
One fundamental charge is 1.6 · coulomb
Charge is an intrinsic property of matter
Electrons are negatively charged
Protons are positively charged
Each has one fundamental charge

8. Net Charge Net charge is the sum of an object’s charges
Most objects have zero net charge (neutral)
Neutral objects contain equal + & – charges
Demonstration: Rub Teflon Rod and Silk to transfer charge and produce two objects with non-zero net charges

9. Question:
A woman rubs her feet on the carpet and gives a shock to her identical twin. If the twin also rubs her feet on the carpet before being touched, the shock will be
larger.
smaller.
the same size.

10. Voltage Charge has electrostatic potential energy (EPS)
Voltage measures the EPS per unit of charge
Raising the voltage of positive charge takes work
Lowering the voltage of negative charge takes work
Voltage is measured in joules/coulomb or volts

11. Charging Objects Like charge separate whenever possible
They disperse on the outside of a conductor
They accumulate on an outside point or thin wire
Charging is limited by charge escape
Severe repulsion leads to corona discharge
Charges leap onto air molecules and escape
Air molecules often glow during corona discharge
If ionization occurs, a spark or arc forms
Demonstration: Electroscope with Charged Teflon Rod
Demonstration: Conductivity of Plastic and Metal with Electroscope
Demonstration: Faraday's Ice Bucket and Electroscope--Where is the Charge?
Demonstration: Van der Graaf Generator
Demonstration: Van der Graaf with Styrofoam Cup
Demonstration: Van der Graaf with Pom-Pom
Demonstration: Van der Graaf with Hair
Demonstration: Van der Graaf and a Sharp Point in Hand.
Demonstration: Ball Bouncing Between Charged Plates
Demonstration: Candle Flame in Charged Plates
Demonstration: Electrostatic Precipitator

12. Polarizing Objects Nearby charges can shift an object’s charges
When negatively charged dust nears a wall,
the wall’s positive charges move toward dust
the wall’s negative charges move away from dust
the wall becomes electrically polarized
Charged dust clings to walls and surfaces
Demonstration: Stream of Water and a Charged Comb or Balloon
Demonstration: Charged Balloon and the Wall