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Electrostatics

Electrostatics. By Scott Adams. Physics I. Video #1. Like charges attract each other True b. False What are/is a good example of conductors? Copper b. Wood c. Gold d. A & C How do you get a negatively charged object? Gain electrons c. Lose electrons Neither d. a & b

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Electrostatics

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  1. Electrostatics By Scott Adams Physics I

  2. Video #1 • Like charges attract each other • True b. False • What are/is a good example of conductors? • Copper b. Wood c. Gold d. A & C • How do you get a negatively charged object? • Gain electrons c. Lose electrons • Neither d. a & b • A negatively charged rod touches a neutral spherical conductor and then removed. The neutral object is now • Negative b. positive c. still neutral

  3. Charge Like Charges Repel − − Like Charges Repel + + Forces are equal and opposite!!! Unlike Charges Attract − +

  4. Conservation of Charge: During any process, the net electric charge of an isolated system remains constant Conductors – allow electrons to flow easily – ex. Copper, Aluminum, Silver, Gold Insulators – resist the flow of electrons – ex. Rubber, plastic, wood, air

  5. Charging an object When a rubber rod is rubbed with fur or wool, it becomes negatively charged. (Cutnell & Johnson, 2004) Recall from Chemistry: a)How does an object become negatively charged? b)Positively charged? Gains Electrons Loses Electrons

  6. Objects at the top of the list have a greater affinity for electrons than objects at the bottom of the list.

  7. (Phet, 2010)

  8. The Electroscope Knob Leaves (Stw, 2005)

  9. Determine the Charge on the rod based on the activity of the leaves and electrons. Pos. Neg. Leaves Spread more Leaves Collapse (Stw, 2005)

  10. Charging by Conduction (Cutnell & Johnson, 2004)

  11. The Quantity of Charge The quantity of charge (q) can be defined in terms of the number of electrons, but the Coulomb (C) is a better unit for later work. A temporary definition might be as given below: The Coulomb: 1 C = 6.25 x 1018 electrons Which means that the charge on a single electron is: 1 electron: e- = -1.6 x 10-19 C Slide Author: (Tippens, 200a7)

  12. Units of Charge The coulomb (selected for use with electric currents) is actually a very large unit for static electricity. Thus, we often encounter a need to use the metric prefixes. 1 mC = 1 x 10-6 C 1 nC = 1 x 10-9 C Slide Author: (Tippens, 2007a)

  13. Charge Sphere A carries a charge of +3 C and an identical sphere B is -1C. If the spheres touch one another and then are separated, the charge on sphere B would be -1C +1C +3C +1C +3+-1 = +2C +3C -1C

  14. Another Example of Charge Sphere A carries a charge of -6C and an identical sphere B is -10C. If the spheres touch one another and then are separated, the charge on spheres A and B would be -6C -10C -6C -10 C ? ?

  15. Video #2 • During induction, the neutral charge gains what type of charge a. same b. opposite 2. The electric fields get stronger as you travel outward a. True b. False 3. A positive test charge of 3x10-5C is placed in an electric field. The force on it is 0.50N. What is the magnitude of the Electric field? a. 1.5x105 b. 6 c. 6,000 d. 17,000 4. When you have two equal but opposite charges the electric field lines _______________? a. leave the – charge and enter the + charge b. leave the + charge and enter the – charge c. Travel away from each other d. nothing

  16. Pith Ball Demo (No Sound)

  17. Charging by Induction

  18. Charging by Induction

  19. The Electrophorus 1. 2. -------- -------- 3. 4. How does the pith ball behave after being touched with the plate?

  20. Electrophorus

  21. Magnitude of Electric Fields E = Electric Field Intensity (N/C) Electric Fields Sample Problem 1. A positive test charge of 4 x 10-5 C is placed in an electric field. The force on it is 0.60 N. What is the magnitude of the electric field at the location of the test charge?

  22. - + + - + - + - Q + - + - + - + - -Q Electric Field Lines Electric Field Lines are imaginary lines drawn in such a way that their direction at any point is the same as the direction of the field at that point. Field lines go away from positive charges and towardnegative charges. Slide Author: (Tippens, 2007b)

  23. Examples of E-Field Lines Two identicalcharges (both +). Two equal but opposite charges. Notice that lines leave + charges and enter - charges. Also, E is strongest where field lines are most dense. Slide Author: (Tippens, 2007b) Various Electric Field Configurations

  24. The E-Field at a distance d from a single charge Q Inverse –Square Relationship E = Electric Field Intensity (N/C) E Field Strength Decreases

  25. The E-Field at a distance d from a single charge Q F is the force on the test charge, qo. -qo d F +qo +Q

  26. Draw Electric Fields at C in the square below: -q -q In which region(s) could the electric field be zero? I II III C r +q -2q +q +q

  27. Electric Fields and Conductors Summary •     The electric field inside a conductor is zero when the charges are at rest. • Electric fields do not cross each other • Electric fields do not loop together •   Any net charge on a good conductor is distributed equally on the surface. (Cutnell & Johnson, 2004)

  28. Electric Fields and Conductors Summary •   The electric field is always perpendicular to the surface of a conductor. •   Inside a nonconductor, an electric field can exist. •    On an irregularly shaped conductor, charge tends to accumulate where the radius of curvature of the surface is smallest, that is at sharp points. + + - - + - - + + + + + + ++ ++

  29. Video #3 • What is the value for K? • 1.6x10-19 Nm2/C2 b. -1.6x10-19 Nm/C c. 9x109 Nm2/C2 d. -9x109 Nm/C • Two charges double the distance between them, the new force will be what __________ the original force? • 4 b. ½ c. 2 d. ¼ • Two charged spheres both with the charge of 4x10-5 C are held a distance of 2 meters apart. What is the magnitude of the force? a. 3.6 b. 1x10-5 c. 2x10-5 d. 14.4 • A positive charge moves to the right through a magnetic field pointing up. What direction is the force on the charge? a. Down b. out of page c. into page c. left

  30. F Like charges experience forces away from each other while unlike charges experience forces towards each other k = 9x109 Nm2/C2 q(charge) = Coulombs (C) Coulomb’s Law: Inverse square relationship r (Cutnell & Johnson, 2004) -e = -1.60 x 10-19 C , mass of electron = 9.11x10-31 kg

  31. Coulomb’s Law - Sample Problems 1. Find the magnitude of the force between the two charges. 2 m --------------------------- +5C -3C Solution: q1= +5C = 5x10-6 C q2 = -3C = -3x10-6 C k = 9x109 Nm2/C2 r = 2m F = ? Formula: Substitute:

  32. 2. Find the resultant force on each charged particle. 5 cm --------------------------- +6C -2C

  33. 3. A force of 1 N exists between the two charges below. How far apart are the charges? ? --------------------------- +5C -20C

  34. Faraday Cage Video – Teacher Tube University of Tehran High Voltage Lab.Person in cage: Prof. H.Mohseniwww.eng.ut.ac.ir/hvlab

  35. Fun with a Giant Tesla Coil - Video

  36. Faraday Cage & Cars – You Tube

  37. References: Adams. S. (1999). Dilbert Cartoon. Received from 2007 AP Conference complimentary resource CD. (http://dilbert.com/strips/comic/1999-07-02 ) Cutnell & Johnson Physics. (2004). [Text Art CD]. John Wiley & Sons. Nave, R. (2010). Hyperphysics.[Illustration]. Permission granted to use illustrations. Retrieved from http://hyperphysics.phyastr.gsu.edu/hbase/hframe.html Phet (2010). Balloons and static electricity. Interactive Simulations. University of Colorado. Retrieved from http://phet.colorado.edu/en/simulation/balloons Ross Shepard Physics (n.d.) Electroscope Applet [video of Animation]. Retrieved from http://www.shep.net/resources/curricular/physics/p30/unit2/electroscope.html Sal44sal44. (2006). Faraday Cage. [Video]. Retrieved from http://www.youtube.com/watch?v=bZwlD-Z0zmE&feature=related. Studyvilla.com (n.d.) Equipotential surfaces. Electrostatics II. [Illustration]. Retrieved on December 22, 2010 from http://www.studyvilla.com/electrostaticsII.aspx Tippens, P. (2007a). Chapter 23 Electric Force. [PowerPoint Slides]. Received from 2007 AP Conference Complimentary Resource CD Tippens, P. (2007b). Chapter 26 Electric Field. [PowerPoint Slides]. Received from 2007 AP Conference Complimentary Resource CD.

  38. Tippens, P. (2007d). Electric Potential. [PowerPoint Slides]. Received from 2007 AP Conference Complimentary Resource CD. Tyrtle, T. (2008). Van de Graaf Generator – Boston Museum of Science. Retrieved on December 22, 2010 from http://www.flickr.com/photos/ttyrtle/2734669311/.

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