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ELECTRIC CHARGE Chapter 16

ELECTRIC CHARGE Chapter 16. Just as the mass of the earth exerts an invisible force on you because of your mass. Any two objects which have an electric charge also exert a force on each other. The electron on the comb is not moving so it is called:. STATIC ELECTRICITY.

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ELECTRIC CHARGE Chapter 16

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  1. ELECTRIC CHARGE Chapter 16

  2. Just as the mass of the earth exerts an invisible force on you because of your mass. Any two objects which have an electric charge also exert a force on each other.

  3. The electron on the comb is not moving so it is called: STATIC ELECTRICITY the study of stationary charges is called: ELECTROSTATICS

  4. little comb BIG EARTH Gravity Which type of force is generally stronger? Electrical Force paper

  5. If I had two 1 kg piles of electrons separated by 1 m 1 m 1 kg e- 1 kg e- Their masses would attract each other due to gravity with a force of: .000000000067 N Their Charges would repel each other with an Electrical force of: 270,000,000,000,000,000,000,000,000,000,000 N

  6. In Mechanics, a fundamental concept was MASS. We never really said what it is, just how it behaves. Gravity Inertia Momentum Energy

  7. With Electrostatics, a fundamental concept is CHARGE We won’t define what it is or why it acts as it does, just how it behaves.

  8. There are ONLY 2 types of charges Positive (+) Negative (-)

  9. LIKE CHARGES REPEL + + - -

  10. OPPOSITE CHARGES ATTRACT - + Charge maze game applet

  11. - + + - All charge in the universe comes from two particles Electrons (-) Protons(+) How can an atom be neutral?

  12. positive 2 protons & 1 electrons - - - + + + + negative 2 protons & 3 electrons - + + - CATION ANION - Neutral 2 protons & 2 electrons How does an atom become a positive ion? Negative?

  13. The charge of an electron is equal & opposite the charge on a proton if an electron is -1 - a proton is exactly +1 +

  14. The smallest amount of charge possible is the charge on 1 electron or 1 proton the charge on any object is a multiple of this amount + - Our charge cannot not be split into a smaller piece of charge

  15. Conservation of Charge Charge is not created or destroyed, but it can move from 1 object to another

  16. Some materials are very good at holding onto their own electrons or stealing them from other objects. Like: Plastics and Rubber Others don’t hold on to their electrons well and tend to lose them. Like: hair or glass

  17. + - + - + - When two materials are rubbed, usually electrons get stolen by one the of the objects. They start out with NO NET CHARGE. Why are they neutral? + + cloth + - - - glass

  18. + + + - After the electrons move. What it the charge on each object? Negative - + + cloth + - - - - Positive glass

  19. + - + - + - If a piece of Plastic is rubbed on the same cloth (neutral again) + + cloth + - - - rubber

  20. + - + - + - If a piece of Plastic is rubbed on the same cloth (neutral again) + + cloth + - - - rubber

  21. - - + - + - + - They both end up charged + + Positive cloth + - Negative rubber

  22. CHARGE IS NOT CREATED It is only transferred between OBJECTS 2+ 0 + + + + - - + - + - 0 2- e-s + 2e-s - + - + - + - - + + - - - 2+ + 2- = 0 0 + 0 = 0

  23. thus CHARGE IS CONSERVED 0 + 0 = 0 (2+) + (2-) = 0 0 = 0 Initial = Final

  24. RECAP How does an object become negatively charged It GAINS Electrons positively charged It LOSES Electrons An object DOES NOT gain protons to become positively charged. This would be a NUCLEAR REACTION. John Travoltage applet

  25. BEN FRANKLIN FOUND THAT + + Repel + + + + + + - - - - Attract + + - + + - - - + + - - - - Repel + + - - + + - - So he knew that the charges were different - -

  26. I dub thou charge on the glass rod POSITIVE. and Rubber rod shall be NEGATIVE + + + - + - - This were arbitrary + - + - -

  27. - + 1- - The smallest charge an object can have is 1 electron + 1+ charge is QUANTIFIED (comes in chunks)

  28. The SI unit of CHARGE is a COULOMB (C) The smallest possible AMOUNT charge is (the charge on 1 electron) e = 1.60 x 10-19 C All Net charge is a multiple of this amount In other words charge is quantized, it only come in discrete packets or quantities.

  29. How many electrons do I need to have 1 full coulomb of charge? e = 1.60 x 10-19 C

  30. Millikan oil drop video clip ( in folder) Video via you tube

  31. The force exerted between two charged objects is Charge on object #1 (C) Charge on object #2 (C) F12 = F21 (N) k Q1 Q2 F = r2 distance between (m) r Q1 Q2

  32. Coulomb’s Constant= 8.988 109 N m2 C2 k Q1 Q2 F = r2 This equation is known as COULOMB’S LAW

  33. k Q1 Q2 F = r2 1 A fundamental constant known as THE PERMITTIVITY OF FREE SPACE = 8.85 10-12 C2/N m2 k = 4pe0

  34. So in some cases you will see Coulomb’s law written like this. If so I would think that you will be given eo Q1 Q2 1 F = 4pe0 r2

  35. You generally will need a direction for the force. I typically use a picture to determine direction and simply put in the charges without their signs. k Q1 Q2 F = r2

  36. Force of Gravity Coulombic Force Proportional to mass Proportional to charge 6.667 10-11 (little) 8.988 109(BIG) k Q1 Q2 G m1 m2 F = F = r2 r2 BOTH FOLLOW INVERSE SQUARE LAW

  37. Force of Gravity Coulombic Force Charge can be positive or negative Mass is always positive k Q1 Q2 G m1 m2 F = F = r2 r2 Always attractive Can be attractive or repulsive

  38. What is the magnitude of electrostatic force on a 1s electron in a helium atom due to the nucleus. The distance from electron to the nucleus is .53 x 10-10 m What is the direction of the force on the electron? What is the magnitude and direction of the force on the nucleus? How would the force change if the radius was doubled? How would the force change if the nucleus had two protons? What if the charge on both were doubled?

  39. A fixed proton and an electron are separated by some distance. When the electron is released released the ...... The force on the e- them will_______ increase increase The acceleration of the e- will_______ increase The velocity of the e- will_______

  40. A fixed electron and another electron are separated by some distance. When the electron is released released the ...... The force on the e- them will_______ decrease decrease The acceleration of the e- will_______ increase The velocity of the e- will_______

  41. Page 497: 1, 2, 5 2.7 N

  42. http://www.colorado.edu/physics/2000/waves_particles/wavpart2.htmlhttp://www.colorado.edu/physics/2000/waves_particles/wavpart2.html

  43. Coulomb’s Law only gives us the force between two particles. If more than 2 are present the forces just add or subtract + F2,3 = 36.9 N F2,1 = 44.6 N + FNET = 7.7 N Find the Resultant Force on Particle #2 Q3 = -21.5 mC Q1 = -15.4 mC .50 m .65 m - - + Q2 = +80.6 mC

  44. If the particles are not in a line, you can still add the forces together. Remember adding vectors? k Q1 Q2 F = r2 1 + 3 2 + -

  45. F2,1 2 + F2,3 Let’s look at the direction of the forces on PARTICLE #2 How would we find the Total Force on #2? 1 + 3 -

  46. F21 F23 How would we find the Total Force on #2? Resultant Force

  47. Find the Resultant Force on Particle #2 Q1 = -2.5 x 10-5 C - .65 m Q3 = +1.4 x 10-5 C + + Q2 = +8.6 x 10-5 C .50 m

  48. Problems: Honors Physics Page 497: 11, 14 2.96 x 105 N

  49. Electrons can travel easily through some materials but are STUCK in place in others

  50. Conductor- electrons travel easily Insulator- electrons are tightly bound

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