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Static Electricity “ Electrostatics ”

Static Electricity “ Electrostatics ”. “ Static ” - not moving. Electric charges that can be collected an held in one place Examples: sparks on carpet, balloon against hair, lightning, photocopier

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Static Electricity “ Electrostatics ”

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  1. Static Electricity“Electrostatics”

  2. “Static”- not moving. Electric charges that can be collected an held in one place • Examples: sparks on carpet, balloon against hair, lightning, photocopier • History: ancient Greeks made little sparks when rubbing amber with fur (Greek word for amber: “elektron”) • Electric charge, “q”, is measured in Coulombs, C. One Coulomb is charge is a dangerously high charge. An average lightning bolt has about 10 Coulombs of charge.

  3. Atomic View • Proton: in nucleus • Positive charge • q = + 1.6 x 10-19 C • Electron: outside nucleus • Negative charge • q = - 1.6 x 10-19 C • Protons and Electrons have the same amount of charge but a proton has much more mass! • Neutron: in nucleus, has no charge • Molecules • 2 or more atoms bonded together • usually atoms and molecules are neutral, but if they have a net charge, they are called IONS

  4. Behavior of charges • Unlike charges attract • Like charges repel • A neutral object will attract both positive and negative charges

  5. Charles Coulomb, mid 1700’s, studied and published papers about the electrostatic force between 2 charged objects.

  6. Hmmm.. + + + - - - Ben Franklin was the first to use the terms “positive” and “negative” to describe electrical charge. Mid 1700’s

  7. Robert Millikan First determined the “elementary charge”- the charge on an electron or proton. (early 1900’s)

  8. Materials Conductors • Substances that have easily moveable electric charges • Most familiar conductors are metals that have “free electrons” • Positive ions may also be mobile • Insulators • Charges cannot move easily • Examples: plastic, wood, glass

  9. Semiconductor: used in computers Conduction is an intermediate magnitude between a conductor and an insulator Superconductor: NO resistance to the flow of electrons. So far, no material is a superconductor except at extremely low temperatures.

  10. Water: insulator or conductor? • PURE water does NOT conduct electricity • Impurities or ions in water can allow conduction • The purer the water, the lower the conductivity • (the conduction of electricity is called ELECTROLYTIC behavior- ) • Air: insulator or conductor? • Usually an insulator, thankfully • When strong forces are present, electron’s can be stripped from air molecules, creating ions • example: lightning

  11. Lightning An electrical discharge between the clouds and the ground or between two clouds. As the electrons flow through the ionized air, they generate so much heat that a PLASMA is produced. We see that plasma and call it LIGHTNING! The air around the lightning expands so rapidly from the heat that it creates a strong pressure wave of air molecules (that’s sound!) We call that THUNDER!

  12. How much electrical charge is flowing through a lightning bolt? Typically around 10 Coulombs of charge. How many electrons, each with a negative charge of 1.6 x 10-19 C, does it take to have 10 C of charge? 10 C / 1.6 x 10-19 C = 6.25 x 1019 electrons ! How many electrons are flowing in a 12 C lightning bolt? 7.5 x 1019 electrons

  13. The Earth is able to absorb much electrical charge. Touching a charged object to the Earth in order to discharge it is called GROUNDING

  14. Methods to electrically charge an object • Conduction: • Direct contact: will transfer electrons, such as touching your car door in the winter • Friction: rubbing your feet against carpet, hair against a balloon

  15. Induction: no direct contact • Start with a neutral object. Then, bring an electrically charged object near, but not in contact with, a neutral object • The charges in the neutral object will be “induced” to separate to get closer or farther from the charged object. • If provided a pathway, the separated electrons will leave. • The object is now positively charged.

  16. Static devices • Electroscope: the separation of metal leaves indicates the presence of static charge • Van de Graaff generator: charge is delivered by a rubber belt to a metal dome • Electrophorus a device used to transfer electric charge

  17. Coulomb’s Law • Calculates the magnitude of the electric force between two charges • Each charge experiences equal but opposite forces where k is a constant, k = 9 x 109 N·m2/C2

  18. Coulomb’s Law looks VERY similar to Newton’s Universal Law of Gravitation • Similarities: • Both act in a vacuum • Both are conservative • Both are inverse square laws • Both propagate with a finite speed, c, the speed of light • Differences: • Electrostatic forces are stronger than gravitational • FE = FG • 1036 • = age of the universe… • … in seconds!!!! • Gravity attracts on like charges, Electrostatic forces repel like charges and attract opposite charges • There are NO negative gravitational charges

  19. Both laws are INVERSE SQUARE LAWS “The Force varies with the inverse of the distance squared.” At twice the distance,  22 in denominator = ¼ the Force, At three times the distance, 32 in denominator, = 1/9 the Force At half the distance,  (1/2)2 in denominator = 4 times the Force Now if one CHARGE doubles…. The Force doubles since they are directly related.

  20. Force is a VECTOR!

  21. Electric Field A gravitational field surrounds all masses. An electric field surrounds all charges. The stronger the electric charge, the stronger the electric field surrounding it.

  22. One way to measure the strength of a gravitational field is to release a mass in the field and measure how strength of the force exerted on it. One way to measure the strength of an electrical fieldis to release a charge in the field and measure the strength of the force exerted on it.

  23. So… the strength of the electric field, E, is given by Electric Field = Force ÷ charge E = F ÷ q

  24. For example: A 0.5 C charge experiences a force of 20 N when placed in an electric field. What is the strength of the electric field, E? E = F ÷ q = 20 N ÷ 0.5 C = 40 N/C

  25. The electric field near a charged piece of plastic or styrofoam is around 1000 N/C. The electric field in a television picture tube is around 10,000 N/C. The electric field at the location of the electron in a Hydrogen atom is 500,000,000,000 N/C! The further you go from an electric charge, the weaker the field becomes.

  26. The electric field around a charge can be represented by Electric field lines Electric fields exist, but electric field lines don’t really exist but provide a model of the electric field.

  27. Electric Field Lines

  28. Electric field lines always point OUT of a positive charge and INTO a negative charge

  29. - 4q +2q To indicate a stronger electric field, just draw MORE lines. The farther apart the lines, the weaker the field. Since the electric field, E, has both magnitude and direction, it is a vector.

  30. The electric field INSIDE a hollow conductor is ZERO even if there are charges on the OUTSIDE of the conductor!

  31. Electric Shielding There is no way to shield from gravity, but there is a way to shield from an electric field…. Surround yourself or whatever you wish to shield with a conductor (even if it is more like a cage that a solid surface) That’s why certain electric components are enclosed in metal boxes and even certain cables, like coaxial cables have a metal covering. The covering shields them from all outside electrical activity.

  32. Faraday Cage

  33. Are you safe from lightning inside your car?Why or why not?

  34. Accelerating Charges A charge placed in an electric field will experience an electric force, F = Eq This force will make the charge accelerate according to Newton’s Second Law F = ma

  35. What direction will a charge accelerate? ++++++++ Positive charges will accelerate in the same direction as the electric field. Negative charges will accelerate in the opposite direction of the electric field. + -

  36. The Electric Field can also be determined by using Coulomb’s Law:

  37. d Electric Potential Energy Energy stored up between 2 charges separated by a distance d: Unit: Joules

  38. Changing the Electric Potential Energy If you raise or lower a mass in a gravitational field, you change the gravitational potential energy, UG. If you move a charge in an electric field, you change the electric potential energy, UE.

  39. Move a mass, m Through a gravitational field, g A distance, h Gravitational Potential Energy, mgh Move a charge, q Through an electrical field, E A distance, d Electrical Potential Energy, qEd

  40. +++++++++ The work energy required to move a charge through an electric field is given by W = qEd

  41. Two Ways to Find Electric Potential Energy ? Are these the same thing??? Which equation should be used??

  42. Conversion of energy Moving a mass or moving a charge takes work energy that is converted to potential energy Work = mgh Or Work = qEd

  43. If you release an object in a gravitational field, its gravitational potential energy is converted to kinetic energy.

  44. - If you RELEASE a charge in an electrical field, its potential energy is converted to kinetic energy! UE = ½ mv2 E

  45. Examples What is the potential energy stored between 2 charges of 3 C and 4 C separated by 2 m? 5.4 x 1010 J

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