Static Electricity “Electrostatics”

“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.

Atomic View q = + 1.6 x 10-19 C q = - 1.6 x 10-19 C 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

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

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

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

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

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

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.

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

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!

How many electrons are flowing in a 12 C lightning bolt? 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

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

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

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.

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

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)

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.

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.

Get a calculator and let’s practice one… What is the magnitude of the electrostatic force between two charges, q1 = 3.2 mC and q2 = -24.8 mC separated 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

Remember…. Force is a VECTOR- it always points in a specific direction 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. + -

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

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

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.

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!)

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