E-Mag

1. It’s Electrifying! E-Mag

2. Electrostatics • Stored electricity, or electricity at rest • it involves the charges, forces between the charges and their behavior in various materials.

3. Electrical Forces • Comes from particles within the atom • unlike charges cause attraction, like charges cause repulsion

4. Properties of Charge • The metric standard unit of charge is the coulomb (C). • It is equal to the charge of 6.24 x 1018 electrons • or the charge that passes through a 100 watt bulb in one sec. • Charge is always conserved!

5. Rub a balloon on your hair, the balloon attracts you hair. Is the amount of charge in the balloon and your hair • A: more than before rubbing? • B: the same as before? • C: less than before?

6. The role that charge plays in electricity is similar to the role of mass when dealing with gravity. • As charge increases or distance between objects decreases, the force of attraction increases.

7. Electrostatic Force • Coulomb’s law -- the force (either attraction or repulsion) between two charged particles is a function of the amount of charge on each particle and the distance between them.

8. Coulomb’s Law • Calculated very similarly to Force, using Newton’s gravitational constant. • K = 8 987 551 788 Nm2/C2 or 8.99 x 109 Nm2/C2

9. Like gravity, electrostatic force is an inverse square function. • What has more effect on the force, amount of charge or distance? How do you know?

10. Think about it • Recall the value of G from Newton’s law of universal gravitation and compare it to the value of k (Coulomb’s constant). • Which is stronger, gravity or electrical force? • Explain!

11. Attraction/Repulsion • If an electron at distance X from a charged particle is attracted with force X, how big is the force at distance 2X? • Is the charged particle positive or negative?

12. Coulomb’s Law simplified

13. q is the symbol used to represent charge, while n is a positive or negative integer, and e is the electronic charge, 1.60 x 10-19 Coulombs.

14. “Watts” of Force!

15. Conductors • allow charges to flow through them easily • electrons are not tightly anchored to the nucleus

16. Electrical Conductor Not this kind of conductor!

17. Insulators • Electrons are tightly bound to the nucleus and cannot easily move. • Charges cannot easily flow through them. • Ex: plastic, styrofoam

18. Different Types of Material • Within a conductor charge can move freely • Charges in insulators do not move [much] • Conducting properties vary dramatically in semi-conductors depending on impurities and charge present

19. Static Electricity • A build up of electric charges in an object or in an area of an object. • Rubbing your feet on the carpet so that you can “shock” someone Phet simulation: http://phet.colorado.edu/simulations/sims.php?sim=Balloons_and_Static_Electricity 4.5 min, video: http://www.cbsnews.com/stories/2004/08/27/earlyshow/living/ConsumerWatch/main638947.shtml

20. How Do Refueling Fires Happen? • Static charge picked up when re-entering vehicle • Touching nozzle without discharging static electricity • A spark ignites fumes

21. How Often Do They Occur? • Fires are reported to the Petroleum Equipment Institute Web site • Most static fires have been reported since 1998 • No one knows how many – 175 have been reported since 1992

22. Three Causes of Static Fires • 50% happen when the refueler returns to the vehicle during refueling and doesn’t shut the car door • 29% happen when the refueler unscrews the gas cap • 21% happen for unknown reasons

23. More Safety Guidelines • DO NOT get back into your car when refueling • DO NOT top off your tank • If a flash fire occurs, back away, contact attendant, and most important: LEAVE NOZZLE IN VEHICLE!

24. Charging by Friction • Transfer of electrons by friction between two objects • rubbing a balloon so that it will “stick” to a wall Travolta simulation: http://phet.colorado.edu/simulations/sims.php?sim=John_Travoltage

25. Charging by Direct Contact • Conduction… transfer of electrons when one object touches another. • touching someone and “shocking” them

26. Charging by Induction • Transfer of electrons when objects are brought near each other, but do NOT touch.

27. How does Lightning start and end?

28. Balloon on wall Induction: Wall is neutral and an insulator + move slightly towards balloon - move slightly away from balloon Wall is neutral but has small surface charge, sufficient to hold balloon

29. Induction to Charge Object

30. Van De Graaff Generator • Robert Jemison Van de Graaff invented the Van de Graaff generator in 1931. • The device has the ability to produce extremely high voltages -- as high as 20 million volts.

31. 1) hollow metallic sphere (with positive charges)2) electrode connected to the sphere, a mesh in close proximity (but not contacting) the electrode and the belt3) upper roller 4) side of the belt with positive charges5) opposite side of the belt with negative charges6) lower roller (metal)7) lower electrode (ground)8) spherical device with negative charges, used to discharge the main sphere9) spark produced by the difference of potentials

32. Electroscope • When a negatively charged object is close to another object, electrons will be repelled from the first object. • Therefore, that end will have a negative charge. This process is called charging by induction.

33. When a negatively charged object touches a neutral body, electrons will spread on both objects and make both objects negatively charged. This process is called charging by conduction. • What happens if you touch the top of the electroscope?

34. Electric Fields • The space around every electric charge. This is similar to the gravitational field located around every mass. Vectors point away from a positive test charge and toward a negative test charge. Field line simulation: http://phet.colorado.edu/simulations/sims.php?sim=Charges_and_Fields

35. Field Lines