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Electricity and Magnetism

Electricity and Magnetism. Review from Chem. Matter is made up of atoms, what are the parts of the atom? Which parts are relevant to Electricity?. Protons and Electrons carry charges, Neutrons do not. When I change the charge of an atom or molecule, what am I doing?.

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Electricity and Magnetism

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  1. Electricity and Magnetism

  2. Review from Chem. • Matter is made up of atoms, what are the parts of the atom? • Which parts are relevant to Electricity?

  3. Protons and Electrons carry charges, Neutrons do not. • When I change the charge of an atom or molecule, what am I doing?

  4. I am adding (or removing) electrons to the outermost shell of the atom. • Inner shells can not escape easily. • The nucleus is even harder to get at.

  5. For the majority of applications in this course we will talk about charge as an excess or dearth of electrons An object that has more electrons than protons will be negatively charged. An object that has fewer electrons than protons will be positively charged.

  6. Charge(q) is measured in Coulombs(C) after the French physicist Charles-Augustin de Coulomb (1736 –1806) for his work on the properties of charges. Electrons carry a charge of 1.602 x 10^-19 C How many electrons do I have if I have a Coulomb of charge?

  7. Electrons carry a charge of 1.602 x 10^-19 C How many electrons do I have if I have a Coulomb of charge? e- = 1.602x10^-19 C Charge (q) = Number of electrons * charge of each electron q = N*e- 1C = N x 1.602x10^-19 C N = 6.25*10^18 electrons

  8. We mentioned earlier that: An object that has more electrons than protons will be negatively charged. An object that has fewer electrons than protons will be positively charged. But why do we care whether something is charged? Charged objects exert forces on each other.

  9. Charged objects exert forces on each other. Opposites attract Like charges repel

  10. Yesterday we covered: • Charged particles of the atom • What it means for an object to be charged • How charges interact

  11. I have a sphere that’s positively charged. What can we say about it? A It has excess protons B It’s missing protons C It has excess electrons D It’s missing electrons E It’s made of protons

  12. I have a sphere that’s positively charged. What can we say about it? A It has excess protons B It’s missing protons C It has excess electrons D It’s missing electrons E It’s made of protons

  13. But how do we make an object charged? From chemistry you will remember that charge can not be created or destroyed. This implies that to make a charged object we must take or remove charge from somewhere else.

  14. To make a charged object we must take or remove charge from somewhere else. Once I have my rod charged up I can transfer the charge to another object.

  15. Depending on what materials are rubbed together, the rod will have a different charge

  16. If I charge 2 balls by touching each of them with a rubber rod, what will be their charge? Will they attract, repel, or do nothing?

  17. If I charge 2 balls by touching each of them with a rubber rod, what will be their charge? Will they attract, repel, or do nothing?

  18. Electroscope This is an electroscope It is a conductor with two leaves. If I charge the electroscope, what will happen to the leaves?

  19. Negative charge flows from the rod into the electroscope and onto the leaves. The leaves then repel each other. This works on people too!

  20. Using a Van Der Graaf Generator to separate charge, we can charge people up. This makes their hairs repel from each other.

  21. Van Der Graaf Generators

  22. Conductors and Insulators

  23. In sphere A the charges can not spread out, though they repel, they can not move. Sphere A is an insulator. In sphere B the charges are able to flow and repel until they are as far away from each other as they can get. Sphere B is a conductor.

  24. Electrostatic Induction Bending water demo When I bring a negatively charged rod near a stream of water, the stream bends. What’s going on here?

  25. Electrostatic Induction Bending water demo Is the water positively charged? In that case I should try a positively charged rod. Any guesses?

  26. Electrostatic Induction Bending water demo The water is attracted to both positive and negatively charged rods. This is because of something called induction.

  27. Neutral objects are generally made from. These charges will be pulled toward opposite charges and repelled by the same charge. When I bring a positive rod near a neutral object all the negative charges will be pulled toward the rod. All the positive charges in the neutral object will be pushed toward the opposite side. This causes objects to become “polarized”.

  28. If the object in the above picture is neutral, then the object has the same amount of positive charge as negative charge. Will the neutral object be attracted, or repelled by the rod when it becomes polarized?

  29. Will the neutral object be attracted, or repelled by the rod when it becomes polarized? Closer charges pull more strongly. The attraction between the positive rod and the negative charges will be stronger than the repulsion between the rod and the positive charges.

  30. Coulomb’s Law What will affect the force felt by a charge? Mass? Distance? Size? Charge? Conductivity?

  31. Coulomb’s Law

  32. Two point charges, each with a charge of +1.00 Coulomb are separated by a distance of 1.00 meter. What is the magnitude of the electrical force of repulsion between them.

  33. Two point charges, each with a charge of +1.00 Coulomb are separated by a distance of 1.00 meter. What is the magnitude of the electrical force of repulsion between them. Felect = (9.0 x 109 N•m2/C2) • (1.00 C) • (1.00 C) / (1.00 m)2 = 9.0 x 109 N

  34. Two point charges, each with a charge of +1.00 Coulomb are separated by a distance of 1.00 meter. 9.0 x 109 N This is an enormous force, and not a situation we are likely to ever see. It is very hard (next to impossible) to isolate that much charge and then get it so close together.

  35. Units for Charge SI (Système international) units have standard prefixes to denote order of magnitude. The ones we’ve seen so far are the more common ones: Kilo (k,10^3), Centi (c,10^-2) and Milli (m, 10^-3) In this unit we will be dealing with smaller units: Micro(μ, 10^-6), and nano (n, 10^-9)

  36. Micro(μ, 10^-6), and nano (n, 10^-9) Two balloons with charges of +3.37 µC and -8.21 µC attract each other with a force of 0.0626 Newton. Determine the separation distance between the two balloons.

  37. Micro(μ, 10^-6), and nano (n, 10^-9) Two balloons with charges of +3.37 µC and -8.21 µC attract each other with a force of 0.0626 Newton. Determine the separation distance between the two balloons. Felect = k • Q1 • Q2 / d2 d = SQRT(k • Q1 • Q2) / Felect d = SQRT [(9.0 x 109 N•m2/C2) • (-8.21 x 10-6 C) • (+3.37 x 10-6 C) / (-0.0626 N)] d = +1.99 m

  38. Electro statics in 2D Three balloons are arranged in an equilateral triangle. The top charge is 10 μC. The bottom left charge is 5μC and the bottom right charge is -5μC. Each is 2m apart. What is the force on the top balloon?

  39. I have two Styrofoam balls that are negatively charged, what is the charge on them?

  40. First we want to relate all the forces

  41. The force of gravity is equal to the vertical component of the tension force (Fy = 0.0108 N ). The Fy component is related to the Fx component and the angle theta by the tangent function.

  42. Tangent(theta) = Fx / Fy Tangent(15 degrees) = Fx / (0.01078 N) Fx = (0.01078 N) • Tangent(15 degrees) Fx = 0.00289 N

  43. Fx = 0.00289 N We are missing the distance: Sine(15) = (d/2)/(2.0 m) d/2 = 0.518 m d= 1.035 m

  44. Fx = 0.00289 N , d= 1.035 m Q = 5.87 x 10-7 C (negative)

  45. Q = 5.87 x 10-7 C (negative) How many extra electrons are on each ball? (charge on an electron is 1.6 x 10-19 C)

  46. Q = 5.87 x 10-7 C (negative) How many extra electrons are on each ball? (charge on an electron is 1.6 x 10-19 C) # excess electrons = (-5.87 x 10-7 C) / (-1.6 x 10-19 C/electron) # excess electrons = 3.67 x 1012 electrons

  47. Electric Fields Charges affect the area around them. At point A positive charges will be pushed right and negative charges will be pulled left. A

  48. Electric Fields When we draw Electric Vector Fields we draw the force that a positive charge would feel if it were placed at that location.

  49. Electric Fields When we draw Electric Vector Fields we draw the force that a positive charge would feel if it were placed at that location.

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