Static Electricity “Electrostatics”

1. Static Electricity“Electrostatics”

2. “Static”- not moving. Electric charges that can be collected and 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

15. 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

16. 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.

17. Electrostatics 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

18. Coulomb’s Law • Calculates the magnitude of the electric force between two charges • Each charge experiences equal but opposite forces k is a constant, k = 9 x 109 N·m2/C2 (Since we are interested in the MAGNITUDE of the force, do not include the signs of negative charges)

19. Coulomb’s Law looks VERY similar to Newton’s Universal Law of Gravitation • Differences: • Gravitational Force is based on MASS. • Coulomb’s law is based on CHARGE. • Gravity is ALWAYS an attractive force. • The Electric Force can attract and repel. • “G” is a tiny number, therefore gravity force is a relatively small force. • “k” is a huge number, therefore electric force is a relatively large force.

20. Both laws are INVERSE SQUARE LAWS “The Force varies with the inverse of the distance squared.” At twice the distance,  d2 = 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, q, doubles…. The Force doubles since they are directly related.

21. Get a calculator and let’s practice one… What is the magnitude of the electrostatic force between two charges, q1 = 3.2 mCand q2 = -24.8 mCseparated by a distance of 2.4 mm? (milli = 10-3, micro = 10-6) USE THE EXPONENT BUTTON!!! F = 9E9 * 3.2E-3 * 24.8E-6 ÷ 2.4E-3 2 F = 124000000 N or, for Quest: 1.24E8 or 1.24e8

22. Remember….Force is a VECTOR- it always points in a specific direction! If more than two charges are present, we must find the VECTOR sum of the forces acting on an individual charge. + - +

24. Get a clicker and calculator. Log in and get ready to answer some questions. • Electric Charge is measured in • Amps • Volts • Coulombs • Farads

25. Opposite charges • Attract • Repel • May attract or repel, depending on their relative size. • Neither attract nor repel.

26. Objects with a neutral charge are • Attracted only by objects with a net positive charge. • Attracted only by objects with a net negative charge. • Not attracted by any kind of charge. • Attracted by objects with either positive or negative charge.

27. The charge of one electron = - 1.6 x 10-19 C 4. How much excess charge is present in an object that has 3 x 1018 more electrons than protons? (Use correct sign!)

28. If the distance between two charges tripled, the magnitude of the electrostatic force between them would change by a factor of • 3 • 6 • 9 • 1/3 • 1/6 • 1/9