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Electricity

Electricity. Have you ever stuck a balloon to the wall after rubbing it on your head? Have you ever gotten an electric shock off your door knob? Have you ever seen lightning?. All these things happen because of Static Electricity Chapter 20. Section 20.1 Objectives.

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Electricity

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  1. Electricity

  2. Have you ever stuck a balloon to the wall after rubbing it on your head? • Have you ever gotten an electric shock off your door knob? • Have you ever seen lightning?

  3. All these things happen because of Static Electricity Chapter 20

  4. Section 20.1 Objectives • Describe how and why an object can be charged • Define charging based on the separation of charges from atoms • Identify the differences between conductors and insulators

  5. What is Static Electricity? • Static electricity occurs when there is a build up of electric charge on the surface of a material. • It is called static electricity because the charges don’t move. • In contrast, the electricity we use everyday involves moving charges.

  6. Charge • Most things have the same number of electrons and protons in them. • They don’t have any overall charge. (neutral) • When do atoms have an overall charge? • If atoms/objects do have an overall charge interesting things can happen. • Video clip: http://www.youtube.com/watch?v=b89x8CAS6xU http://www.youtube.com/watch?v=tuZxFL9cGkI

  7. Static electricity • Static electricity is caused when certain materials are rubbed against each other. • Electrons can be rubbed off one material and on to another. • The material that has gotten extra electrons is now negatively charged • The material which has lost electrons is positively charged.

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  15. Static electricity • It is this imbalance of positive and negative charges that causes: • Balloons to stick to walls. • Your hair to stand on end when brush your hair on a dry day. • Static Cling • The electric shock you sometimes get from the door handle. • http://phet.colorado.edu/en/simulation/travoltage

  16. Static electricity Ben Franklin’s naming convention: Materials like vinyl, hard rubber, plastics become negatively charged They are actually gaining electrons Materials like fur, hair, glass and wool become positively charged Their electrons are getting stripped off leaving excess protons

  17. Conductors and Insulators Insulators: • A material through which a charge will not move easily is called an electric insulator. • Glass, dry wood, most plastics, cloth, and dry air are all good insulators. Conductors: • A material that allows charges to move about easily is called an electric conductor. • Metals are good conductors because at least one electron on each atom of the metal can be removed easily. • These electrons act as if they no longer belong to any one atom, but to the metal as a whole; consequently, they move freely throughout the piece of metal.

  18. Conductor vs. Insulator Conductor - allows charges to move freely and carries an electric current Insulator - does not transfer charge easily Conductor or Insulator depends on the inter-atomic bonding and on how tightly the atoms of a substance hold their VALENCE electrons.

  19. Electricity Conductors: Ex. Some Metals Each atom contributes one or more electrons to a general “sea” of electrons attached to no particular atoms Insulators: All electrons are attached to particular atoms.

  20. Electricity - electrons moving through a metal wire.

  21. Conductor vs. Insulator Conductor or Insulator?

  22. Conductors and Insulators • The figure belowcontrasts how charges behave when they are placed on a conductor with how they behave on an insulator.

  23. Concpet Check: Section 20.1 Question 1 What will happen if you rub two identical plastic rulers with a woolen cloth and bring them together? A. The rulers will attract each other. B. The rulers will repel each other. C. The rulers will neither attract nor repel. D. The rulers will partially attract and partially repel.

  24. Concept Check Section 20.1 Question 2 Which of the following statements about charges is true? A. Charges are created by gaining electrons. B. Charges are destroyed by losing electrons. C. Charges are separated through a transfer of electrons. D. Charges cannot be created, destroyed, or separated.

  25. Concept Check: Section 20.1 Question 3 A negative charge is generated in a rubber rod by rubbing it with wool. When two materials, A and B, are brought near the rubber rod, material A is attracted to the rod, whereas material B is repelled. What can you say about the charges on the two materials? A.A is positively charged, while B is negatively charged. B. Both A and B are negatively charged. C. Both A and B are positively charged. D. B is positively charged, while A is negatively charged.

  26. Electrical Charges • Homework: • Read pages 407 - 411 • Do Concept Review Problems: 1.1 – 1.3 • We will review tomorrow 5/9

  27. Electric Forces 20.2 Objectives: • Explain separation of charge and charging by induction • State Coulombs Law and how force depends on charges and distance • Understand the cause of attractive force on neutral objects

  28. Force Review • What is a Force? Answer: A push or a pull • Ex. Gravity • The Force of Gravity pulls (attracts) you to the Earth • We call it your Weight

  29. Forces • Four Fundamental Forces • Gravity • Electromagnetic • Which of the above is the strongest force? Nuclear Strong Force – binds protons & neutrons • 100 x stronger than Electromagnetic & many, many, many times stronger than the Weak Force Nuclear Weak Force – affects sub-sub atomic particles • Which is the weakest? • Strong Nuclear • Weak Nuclear Gravity!

  30. Electro-Static Force • Electrical Forces are billions upon billions of times stronger than Gravity… • Holds atoms together (protons & electrons) • It’s what attracts atoms to each other – creates a solid table that your hand can’t go through

  31. Static Electricity • Charge is… • the fundamental electric quantity • symbolized by q or Q • unit of measure is the Coulomb (C) • Electrons and Protons are referred to as elementary charges charge of an electron, qe = -1.62 x 10-19 C charge of a proton, qp = +1.62 x 10-19 C

  32. Electro-Static Force • An objects electric charge depends on the overall imbalance of its protons and electrons • The greater the imbalance the greater the overall charge • Most of the time Attractive & Repulsive Forces balance each other out, so… no noticeable effect… But if we can create an imbalance… • We get Electro-Static Force

  33. Electro-Static Force • Electric Force: • The force of attraction or repulsion between objects due to charge • like-charged things repel • oppositely-charged things attract.

  34. Electromagnetism • Strength of Electric Force Depends on • Charge • Distance • Gravity is a good analogy

  35. Recall… Gravity • Gravity = Force that pulls objects toward each other. • Universal Law of Gravitation: The force of gravity acts between all objects in the Universe

  36. Recall… Gravity Strength of the Gravity depends on… 1. The masses of the objects involved 2. The distance between the objects involved F = G where G = 6.67 x 10-11

  37. Coulombs Law Strength of the Electric Force depends on… 1. The CHARGES of the Particles involved 2. The distance between the Particles involved F = k where k = 9.0 x 109

  38. Coulombs Law • Strength of Electric Force Depends on • Charge Bigger Charge = Stronger Force • Distance Like Charges D , Frepulse Opposite Charges D , Fattract Just like Gravity This will play a big role in batteries

  39. Coulombs Law • Which subatomic particle is easiest to remove? • Why? According to Coulombs Law – the further apart the e- and p+, the less electromagnetic force between them. Valence Electron

  40. Recall... • 2 positive charges put together will repel each other. • A positive charge near a negative charge will attract each other.

  41. Concept Check Section 20.2 Coulomb’s Law If two positively charged objects, A and B, are brought near each other, the forces they exert on each other are repulsive. If, instead, B is negatively charged, the forces are attractive.

  42. Concept Check Section 20.2 Coulomb’s Law Problem: Sphere A, with a charge of +6.0 µC, is located near another charged sphere, B. Sphere B has a charge of −3.0 µC and is located 4.0 cm to the right of A. a. What is the force of sphere B on sphere A?

  43. Concept Check Section 20.2 Coulomb’s Law Problem: Identify the known and unknown variables. Known: qA = +6.0 µC qB = −3.0 µC rAB = 4.0 cm Unknown: FB on A = ?

  44. Concept Check Section 20.2 Coulomb’s Law Problem: Find the force of sphere B on sphere A.

  45. Concept Check Section 20.2 Coulomb’s Law Substitute qA = 6.0 μC, qB = -3.0 μC, rAB = 4.0 cm - - Because spheres A and B have unlike charges, they attract, the force of B on A is to the right and is negative (negative force sign = attraction, positive = repulsion)

  46. How do charges behave? • What is a water molecules overall charge? • Bending Water Demo – • If a charge does build up on an object, how does it affect other objects if most of them are neutral?

  47. Electric Fields • A charged particle produces an electric field around it • Electric Field: a region in space around a charged object that causes a stationary object to experience an electric force. • like a Gravitational Field only caused by charge, not mass

  48. Electric Fields • It is because of this electric field that …. • 2 positive charges put together will repel each other. • A positive charge near a negative charge will attract each other.

  49. Electric Fields • When neutral atoms encounter this electric field, the individual charges (p+, e-) within the atom are still affected by the electric field • When neutral molecules (like water) encounter this electric field, the individual charges within the molecule are still affected by the electric field

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