Electronic Air Cleaners (Static Electricity)

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

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 • 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. Questions about electronic air cleaners • What keeps dust in the air? Why doesn’t gravity remove it? • If gravity can’t clear the air, is there another force around that can succeed? • Is there an electrostatic potential energy associated with electronic air cleaners? • How can dust acquire charge? Why is charged dust attracted to walls?

5. Questions about electronic air cleaners • What keeps dust in the air? Why doesn’t gravity remove it?

6. 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

7. 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

8. Questions about electronic air cleaners • If gravity can’t clear the air, is there another force around that can succeed?

9. Electric Charge (Part 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

10. Electric Charge (Part 2) • Charge is conserved • Charge is quantized • One fundamental charge is 1.6 x 10-19 coulomb • Charge is an intrinsic property of matter • Electrons are negatively charged • Protons are positively charged • Each has one fundamental charge

11. Net Charge • Net charge is the sum of an object’s charges • Most objects have zero net charge (neutral) • Neutral objects contain equal + & – charges

12. 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 • larger. • smaller. • the same size.

13. Questions about electronic air cleaners • Is there an electrostatic potential energy associated with electronic air cleaners?

14. rf Voltage • Charge has electrostatic potential energy (EPS) • Voltage 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 • Voltage is measured in joules/coulomb or volts

15. Questions about electronic air cleaners • How can dust acquire charge? Why is charged dust attracted to walls?

16. 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

17. 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

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

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 • with a negative electric charge. • with no negative electrically charged particles in your body. • with a positive electric charge. • electrically neutral.