1. Electronic Air Cleaners (Static Electricity) Physics 10 Kenneth N. Barish

2. K. Barish, Electronic Air Cleaners 2 Introductory 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 1.larger. 2.smaller. 3.the same size.

3. K. Barish, Electronic Air Cleaners 3 Observations About Air Cleaners oDust doesn’t settle quickly on its own oMechanical filters gradually plug up oDust clings to things with static electricity oAir cleaners could be based on static electricity

4. K. Barish, Electronic Air Cleaners 4 Questions about electronic air cleaners oWhat keeps dust in the air? Why doesn’t gravity remove it? oIf gravity can’t clear the air, is there another force around that can succeed? oIs there an electrostatic potential energy associated with electronic air cleaners? oHow can dust acquire charge? Why is charged dust attracted to walls?

5. K. Barish, Electronic Air Cleaners 5 Questions about electronic air cleaners oWhat keeps dust in the air? Why doesn’t gravity remove it?

6. K. Barish, Electronic Air Cleaners 6 Air Resistance oDifferent from buoyancy oOpposes the relative motion of air and object oActs to bring both to one velocity oConsists of a matched pair of forces: –The air pushes on the object –The object pushes on the air –Forces have equal magnitudes, opposite directions oIncreases with relative speed and cross section

7. K. Barish, Electronic Air Cleaners 7 Terminal Velocity oA real falling object experiences two forces: –Weight downward –Air resistance upward (while dropping in still air) oAir resistance affects descent –Object accelerates downward initially –Upward air resistance gradually increases –Terminal velocity occurs when net force is zero oSmall objects have small terminal velocities

8. K. Barish, Electronic Air Cleaners 8 Questions about electronic air cleaners oIf gravity can’t clear the air, is there another force around that can succeed?

9. K. Barish, Electronic Air Cleaners 9 Electric Charge (Part 1) oTwo types: positive & negative oLike 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 oCharge is measured in coulombs

10. K. Barish, Electronic Air Cleaners 10 Electric Charge (Part 2) oCharge is conserved oCharge is quantized –One fundamental charge is 1.6 x 10 -19 coulomb oCharge is an intrinsic property of matter –Electrons are negatively charged –Protons are positively charged –Each has one fundamental charge

11. K. Barish, Electronic Air Cleaners 11 Net Charge oNet charge is the sum of an object’s charges oMost objects have zero net charge (neutral) oNeutral objects contain equal + & – charges

12. K. Barish, Electronic Air Cleaners 12 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 1.larger. 2.smaller. 3.the same size. Introductory Question:

13. K. Barish, Electronic Air Cleaners 13 Questions about electronic air cleaners oIs there an electrostatic potential energy associated with electronic air cleaners?

14. K. Barish, Electronic Air Cleaners 14 Voltage oCharge has electrostatic potential energy (EPS) o oVoltage measures the electrostatic potential energy per unit of charge –Raising the voltage of positive charge takes work –Lowering the voltage of negative charge takes work o oVoltage is measured in joules/coulomb or volts rfrf

15. K. Barish, Electronic Air Cleaners 15 Questions about electronic air cleaners oHow can dust acquire charge? Why is charged dust attracted to walls?

16. K. Barish, Electronic Air Cleaners 16 Charging Objects oLike charge separate whenever possible –They disperse on the outside of a conductor –They accumulate on an outside point or thin wire oCharging 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 oIf ionization occurs, a spark or arc forms

17. K. Barish, Electronic Air Cleaners 17 Polarizing Objects oNearby charges can shift an object’s charges oWhen 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 oCharged dust clings to walls and surfaces

18. K. Barish, Electronic Air Cleaners 18 Questions about electronic air cleaners oWhat keeps dust in the air? Why doesn’t gravity remove it? –Air resistance & terminal velocity oIf gravity can’t clear the air, is there another force around that can succeed? –Yes, the electric force oIs there an electrostatic potential energy associated with electronic air cleaners? –Yes, Voltage is energy held by each unit of charge. oHow can dust acquire charge? Why is charged dust attracted to walls? –Corona discharge and polarization.

19. K. Barish, Electronic Air Cleaners 19 Quiz Question You are standing on a plastic bench that insulates you from your surroundings. Both you and the helium balloon you are holding are electrically neutral. You now rub the balloon against your sweater, so that the balloon becomes negatively charged, and then let the balloon float away. You are left a)with a negative electric charge. b)with no negative electrically charged particles in your body. c)with a positive electric charge. d)electrically neutral.

20. K. Barish, Electronic Air Cleaners 20