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Physics Beyond 2000

Physics Beyond 2000. Chapter 13 Electrostatics. http://www.sciencejoywagon.com/physicszone/lesson/07elecst/default.htm. Electrostatics. The charges are at rest. There is not any electrical current. The charges have forces on each other. A neutral atom.

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Physics Beyond 2000

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  1. Physics Beyond 2000 Chapter 13 Electrostatics http://www.sciencejoywagon.com/physicszone/lesson/07elecst/default.htm

  2. Electrostatics • The charges are at rest. There is not any electrical current. • The charges have forces on each other.

  3. A neutral atom • Each electron carries one unit of negative charge. • One unit of negative charge = -1.6 × 10-19C

  4. A neutral atom • The nucleus contains protons. • Each proton carries one unit of positive charge. • One unit of positive charge = 1.6 × 10-19C

  5. A neutral atom • For a neutral atom, the number of electrons = the number of protons. • Total charge of a neutral atom = 0

  6. A charged atom • If a neutral atom gains electrons, it becomes negatively charged.

  7. A charged atom • If a neutral atom loses electrons, it becomes positively charged.

  8. A charged atom • If a neutral atom loses electrons, it becomes positively charged.

  9. + + + + + + + + Charges on insulators • Excessive charges on insulators will stay on the insulator. • Example: a plastic ruler with positive charges on it.

  10. + + + conductor + + + + + + insulated handle Charges on insulated conductor • The excess charges on the insulated conductor remains constant unless the conductor touches another conducting medium.

  11. Charges on insulated conductor • When a conducting medium touches the conductor, the excess charges flow away. + + + conductor + + + + conducting rod + + insulated handle

  12. Charges on insulated conductor • If all the charges flow away, the conductor becomes neutral. • This is called earthing. conductor conducting rod insulated handle

  13. + + + conductor + + + + + conducting rod + insulated handle Charges on insulated conductor • In some cases, only part of the charges flow away. Some charges still remain. • This is called sharing.

  14. + + + + + + + + + + + + Charges on isolated conductor • The conductor is completely isolated from any other object. Its excess charges remain constant.

  15. Two kinds of charges • Positive charges • Negative charges • Unit: coulomb ,C. • One proton contains 1.6 × 10-19 C. • One electron contains - 1.6 × 10-19 C.

  16. + + + + + + + + Two kinds of charges • Like charges repel.

  17. - - - - - - - - Two kinds of charges • Like charges repel.

  18. - - - - + + + + Two kinds of charges • Unlike charges attract. http://www.colorado.edu/physics/2000/waves_particles/wavpart2.html

  19. Charging objects • Use a power supply • By rubbing • By induction

  20. earthed Charging by power supply • Use a light conducting sphere and a EHT. • Ground the negative terminal of the EHT. • Let the sphere touch the positive terminal of the EHT. • What kind of charges is on the sphere? Positive charge

  21. earthed Charging by power supply • Use a light conducting sphere and a EHT. • Ground the positive terminal of the EHT. • Let the sphere touch the negative terminal of the EHT. • What kind of charges is on the sphere? Negative charges

  22. Charging by power supply • Explain why the sphere is charged. Hint: sharing the charges.

  23. Charging by rubbing • Use a piece of dry cloth to rub a polythene rod. • What kind of charges is on the polythene rod? Negative charges

  24. Charging by rubbing • Explain why the polythene rod is negatively charged

  25. Shuttling ball experiment positive negative

  26. Shuttling ball experiment 3. An isolated light conducting sphere. 2. Metal plate A connected to the negative terminal of the EHT. • Metal plate B connected • to the positive terminal • of the EHT.

  27. Shuttling ball experiment 3. An isolated light conducting sphere. • Connect to • the positive terminal • of the EHT. 2. Connect to the negative terminal of the EHT. 4. Allow the ball to touch one plate

  28. Shuttling ball experiment 3. An isolated light conducting sphere. • Connect to • the positive terminal • of the EHT. 2. Connect to the negative terminal of the EHT. 5. The ball is shuttling.

  29. Shuttling ball experiment Explain the experiment. 3. An isolated light conducting sphere. 2. Metal plate connected to the negative terminal of the EHT. • Metal plate connected • to the positive terminal • of the EHT.

  30. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Charge distribution in a conductor • Inside the conductor: the excess charges do not stay inside the conductor because of the repulsion. • On the surface of a conductor: the excess charges reside on the surface of the conductor.

  31. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Charge distribution on the surface of a conductor • If the surface of the conductor is flat, the charge distribution is uniform except at the edge.

  32. + + + + + + + + + + + + + + Charge distribution on the surface of a conductor • If the surface of the conductor is spherical, the charge distribution is uniform.

  33. + + + + + + + + + + + + + + Charge distribution on the surface of a conductor • If the surface of the conductor is of pear-shaped, the charge distribution is dense on curved surface than on flat surface.

  34. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Charge distribution on the surface of a conductor • On the surface of a conductor, the surface charge density describes the distribution of charges. • Surface charge density is the charge per unit surface. where Q is the amount of charge on an area A.

  35. Example 2 • The surface area of a sphere is 4r2. • The SI unit of charge Q is coulomb (C) • The SI unit of surface charge density  is coulomb per m2 (C m-2) .

  36. Electric fields • There are forces (attraction or repulsion) between charges. • How to describe these forces? (How large is the force and what is its direction if I put a charge, say 1 C, at a point?) • Electric field is the basic concept. http://www.colorado.edu/physics/2000/applets/nforcefield.html http://www.colorado.edu/physics/2000/waves_particles/wavpart3.html

  37. Electric fields • An electric field is a region in which an electric charge experiences a force. Place a charge at this point. There is not any force on it. So the electric field is zero at this point. +

  38. Electric fields http://www.sciencejoywagon.com/physicszone/lesson/07elecst/fieldint/efield.htm • An electric field is a region in which an electric charge experiences a force. Place a charge at this point. There is force on it. So there is an electric field here. + + + +

  39. Electric fields • There are two ways to describe the electric fields. (i) Lines of forces (graphical) ; (ii) Electric field strength (numerical).

  40. Lines of Force • Draw directed lines. • Some electric field patterns: http://www.sciencejoywagon.com/physicszone/lesson/07elecst/static/fieldmap.htm

  41. Lines of Force • A field line always directs away from a positive charge and ends at a negative charge. • The tangent to the line at any point gives the direction of force acting on any positive test charge. • The number of field lines drawn per unit cross-sectional area is proportional to the strength of the electric field.

  42. + - Lines of Force • A field line always directs away from a positive charge and ends at a negative charge.

  43. + - Lines of Force • The tangent to the line at any point gives the direction of force acting on any positive test charge. + + +

  44. + - Lines of Force • The force is in opposite direction if the test charge is negative. - - -

  45. + - Lines of Force • The tangent to the line at any point gives the direction of force acting on any positive test charge. + + +

  46. Lines of Force • The number of field lines drawn per unit cross-sectional area is proportional to the strength of the electric field. 2. Strong electric field 1. Weak electric field

  47. q = 3 C F = 18 N Electric field strength E • The electric field strength E at a point is the force per unit test charge placed at that point. where F is the force on test charge q. Unit of E: N C-1 or V m-1 • Example • Find the electric field strength • at the point of the charge.

  48. + + + Electric field strength E • Note that the test charge q must be small. • For a big test charge, the original electric field may be changed. 2. The electric field changes due to the presence of another big charge. • The electric field of a • single charge

  49. + - Ionization of air by strong electric field • An air molecule consist of equal number • of positive and negative charges.  forces 2. In a strong electric field, positive charges are pulled to one side and negative charges to the opposite side. E 3. Charges are completely separated. There forms an ion-pair. + - E

  50. + + + + + + + + + + + + + + + Ionization of air by strong electric field • Sharp point carrying charges may create a very strong electric field. • The electric field strength that causes ionization is called the breakdown field strength Eb. For air, Eb 106 NC-1.

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