Eleanor Roosevelt High School Chin-Sung Lin. Electrostatics Electrostatics is the electricity at rest It involves electric charges, the force between

Electrostatics

Lesson 16ElectrostaticsEleanor Roosevelt High SchoolChin-Sung Lin

ElectrostaticsElectrostatics is the electricity at rest It involves electric charges, the force between them, and their behavior in the materials

HistoryThe word electricity comes from the Greek elektron which means amber The amber effect is what we call static electricity

HistoryBased on the experiment situations, there must be two types of charges. Benjamin Franklin arbitrarily call one positive and another one negative He also argued that when a certain amount of charge is produced on one body, an equal amount of the opposite charge is produced on the other body

Electric CHARGE

Electric Charges

Electric Charges

Electric ChargesElectric ChargesThe fundamental rule of all electrical phenomena is

Like charges repelopposite charges attract Thomsons Cathode Ray ExperimentDiscover electrons in a gas discharge cathode-ray tube, and prove that the charge of electrons are negative

Millikans Oil Drop ExperimentThe charge of oil drops are always the multiples of -1.60 x 10-19 C

Millikans Oil Drop ExperimentCharge histogram in units of e of the 42.5 million drops measured during the 36 month experimental run

Millikans Oil Drop ExperimentSummary as of January 2007Total mass throughput for all experiments- 351.4 milligrams of fluidTotal drops measured all experiments - 105.6 millionMillikans Oil Drop ExperimentWhat conclusion can you draw from these data?

Millikans Oil Drop Experiment Electric charges are quantized (discrete)Charges are always multiple of fixed chargesNo fractional charge particles was foundProve the existence of unit chargeElectric ChargesThe SI unit of charge is the coulomb (C)The charge of an electron is -1.60 x 10-19 C1 C is the charge of 6.24 x 1018 electronsSymbol: q, Q

Electric ChargesBy convention, electrons are negatively charged protons are positively chargedneutrons have no chargeAll electrons are identicalAll protons are identicalAll neutrons are identicalA protons charge is equal in magnitude to the negative charge of the electron

Electric ChargesParticleCharge (C)Mass (kg)electron-1.6x10-199.109x10-31proton+1.6x10-191.673x10-27neutron01.675x10-27CHARGE BY FRICTIONCharge by FrictionWhy is the rod got charged simplily by rubbing it with fur?

Charge by FrictionWhat decides which will gain or loss electrons during friction?

Charge by FrictionWhat happens when electrons are excited to such an extent that they leave the atoms?

Charge by FrictionWhy wont protons move?

Charge by FrictionElectrons are being transferred by friction when one material rubs against anotherThe one that gains electrons becomes negative. The one that lose electrons becomes positiveDifferent materials have different tendency to gain or lose electronsTriboelectric SeriesThe triboelectric series is a list that ranks various materials according to their tendency to gain or lose electronsThe tendency of a material to become positive or negative after triboelectric charging has nothing to do with the level of conductivity of the materialThe greater the relative position, the larger the expected electric chargeTriboelectric Series

Triboelectric Series

Triboelectric Series

Charging by Friction

CHARGE BY CONTACTCharge by ContactElectrons can be transferred from one material to another by simply touching

Charge by ContactA positively charged metal ball with charge Q contacts with an identical electrically neutral metal ballThe charges equally redistribute to both ballsSeparate these two balls and each one has charge Q/2

Electric Charges DistributionIf the object is good conductor, the charges will spread to all part of its surface because the like charges repel each other. Net charge inside is zero

Electric Charges DistributionIf we dump a bunch of electrons to the statue of liberty, and the statue is insulated from the ground, where will those electrons go?

Electric Charges DistributionOn symmetric objects, charge distributes uniformly, on non-symmetric objects, charges stay at sharp corners

Electric Charges DistributionWhy do electrons stay at sharp corners?

Electric Charges DistributionIf the object is a poor conductor, charge cannot move, so it remains localized around the contact region

LAW OF CONSERVATION OF CHARGELaw of Conservation of ChargeThe net amount of electric charge produced in any process is zeroIn an isolated system the total charge is conservedElectrons are neither created nor destroyed but are simply transferred from one material to another Law of Conservation of ChargeAn object that has unequal numbers of electrons and protons is electrically charged In a neutral atom, there are as many electrons as protons, so there is no chargeA charged atom is called an ion. An imbalance comes about by adding or removing electronsCHARGE BY INDUCTIONCharge by Induction

Why does the balloon stick to the wall?

Charge by InductionIf we bring a charged object near a conducting surface, even without physical contact, electrons will move in the conducting surface

Charge by InductionThe introduction of a charge into another body without contact being madeTemporary charging of this type produces Temporary Polarity

Charge by InductionTwo neutral metal balls A & B are placed next to each other and contact each otherApproach a negatively charged rod to ball A and different charges will be induced on each ball

Charge by InductionSeparate the two metal balls while the rod is still thereRemove the rod and the charges will be redistributed evenly on both balls

AIM: Charge by InductionDoNow:Started with a neutral ball with metal paint and a negatively charged rod, how can you charge the ball with positive charges by induction?

Charge by InductionApproach a negatively charged rod to a neutral ball with metal paint and different charges will be induced on each side of the ball

Charge by InductionWhile the rod is still there, ground the opposite side of the ball and electrons will flow to the ground

Charge by InductionA charged object is needed to charge an object by induction (without any contact)The object being charged ends up with a charge which is the opposite of the object being used to charge itA ground must be used to charge on the object. The ground allows for electron movement into or out of the object being charged

Charge by InductionTouch the negatively charged rod with the positive charged ball, then the excess electrons of the rod will flow to the ballNow like charges repel

Charge by InductionCharging by induction is not limited to conductorsWhen a charged rod is brought near an insulator, 0n side of the atom or molecular is more positive (or negative) than the other side

Charge by InductionThis explains why neutral bits of paper are attracted to a charged object or the negatively charged balloon attached to the neutral wooden wall

Charge by InductionMany molecules (H2O for example) are electrically polarized in their normal states. The distribution of electric charge is not perfectly even. Such molecules are said to be electric dipoles

Charge by InductionMicrowave oven uses oscillating electric field to make the water molecules oscillating. The flip-flopping H2O molecules impart thermal motion to surrounding food molecules

LightningThe negatively charged bottoms of clouds induced a positive charge on the surface of Earth below

Becomes very negativeBecomes very positiveLightning RodA metal rod mounted on top of a building and electrically connected to the ground through a wire, to protect the building in the event of lightning

Types of MaterialsTypes of materialsConductor: a material that transfers charge easily (ex. Metals)

Types of materialsInsulator: a material that does not transfer charge easily (ex. glass, water, air, ceramic, nonmetals)

Types of materialsSemiconductors: somewhere between 1 & 2 (ex. Silicon, carbon, germanium)

Types of materialsSuperconductors: some metals become perfect conductors below certain temperatures

Electrostatics InstrumentsElectrophorus

An electrophorus is used to produce electrostatic charge via the process of electrostatic inductionIt consists of a plastic plate and a metal disc with an insulating handleElectrophorus

Charge by friction, induction, and then conductionElectroscope

Electroscope

used to detect the presence and magnitude of electric charge on a bodyIt consists of a vertical metal rod, from the end of which hang two strips of thin gold leafA disk or ball terminal is attached to the top of the rod, where the charge to be tested is appliedElectroscope

Electroscope

Electroscope

Electroscope

AB

Electric Force

Electric Force The Coulombs LawCoulombs LawDescribing the electrostatic force between electrically charged particlesPublished in 1785 by French physicist Charles Augustin de CoulombTorsion balance experimentElectric Force The Coulombs Law

Torsion BalanceCylindrical Glass case & tubeTorsion head & fiberNeedle & spheresRemovable spheresScale

Electric Force The Coulombs Law

Torsion Balance Experiment(Video 4 minutes) Write down:Whats the purpose?Three major stepsWhats the conclusion?Fe ~ q1 q2

Coulombs LawThe electric force between two charges is proportional to the product of the two charges and

Electric Force The Coulombs Law76Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Fe ~ 1/r2

Coulombs LawThe electric force between two charges is inversely proportional to the square of the distance between the chargesElectric Force The Coulombs Law77Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Coulombs LawElectric Force The Coulombs LawFe ~ q1 q2

78Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Fe ~ 1/r2



Fe ~ 1/r2


Fe ~ q1 q2r280Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Fe ~ 1/r2


Fe ~ q1 q2r2Fe = k q1 q2r281Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Fe: electric force (N)k:electrostatic constant (N m2/C2)q1, q2: charge (C)r: distance between charges (m)Fe = k q1 q2r2Coulombs LawElectric Force The Coulombs Law

k A very large constant 8.99x109Whats the unit of k? (1 minute)Fe = k q1 q2r2Electrostatic constant k Electric Force The Coulombs Lawk =Fe r2

q1 q2k A very large constant 8.99x109 N m2/C2Whats the unit of k?Fe = k q1 q2r2Electrostatic constant k Electric Force The Coulombs Lawk =Fe r2

q1 q2[ k ] = [ ]N m2

C2

Charge 1Charge 2DistanceElectric Forceq1q2dFe2q1q2d3q1q2dq14q2dq15q2d2q13q2d4q1 q2dElectric Force The Coulombs Law85Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFe2q1q2d2Fe3q1q2dq14q2dq15q2d2q13q2d4q1 q2dElectric Force The Coulombs Law86Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFe2q1q2d2Fe3q1q2d3Feq14q2dq15q2d2q13q2d4q1 q2dElectric Force The Coulombs Law87Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFe2q1q2d2Fe3q1q2d3Feq14q2d4Feq15q2d2q13q2d4q1 q2dElectric Force The Coulombs Law88Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFe2q1q2d2Fe3q1q2d3Feq14q2d4Feq15q2d5Fe2q13q2d4q1 q2dElectric Force The Coulombs Law89Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFe2q1q2d2Fe3q1q2d3Feq14q2d4Feq15q2d5Fe2q13q2d6Fe4q1 q2dElectric Force The Coulombs Law90Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFe2q1q2d2Fe3q1q2d3Feq14q2d4Feq15q2d5Fe2q13q2d6Fe4q1 q2d2FeElectric Force The Coulombs Law91Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFeq1q22dq1q23dq1q24dq1q2 dq1q21/3 dq1q21/4 dElectric Force The Coulombs Law92Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFeq1q22dFe /4q1q23dq1q24dq1q2 dq1q21/3 dq1q21/4 dElectric Force The Coulombs Law93Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFeq1q22dFe /4q1q23dFe /9q1q24dq1q2 dq1q21/3 dq1q21/4 dElectric Force The Coulombs Law94Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFeq1q22dFe /4q1q23dFe /9q1q24dFe /16q1q2 dq1q21/3 dq1q21/4 dElectric Force The Coulombs Law95Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFeq1q22dFe /4q1q23dFe /9q1q24dFe /16q1q2 d4Feq1q21/3 dq1q21/4 dElectric Force The Coulombs Law96Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFeq1q22dFe /4q1q23dFe /9q1q24dFe /16q1q2 d4Feq1q21/3 d9Feq1q21/4 dElectric Force The Coulombs Law97Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFeq1q22dFe /4q1q23dFe /9q1q24dFe /16q1q2 d4Feq1q21/3 d9Feq1q21/4 d16FeElectric Force The Coulombs Law98Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Aim: Electric Force The Coulombs LawDoNow: (3 minutes)Write down the Coulombs Law. The particle A carries 1.2 x 10-4 C, the particle B carries 2.4 x 10-4 C, and the particle C carries 9.7 x 10-8 C. The three particles form a equilateral triangle with the measure of the side 0.45 m. Whats the ratio of the repelling forces between A-C and B-C?

Charge 1Charge 2DistanceElectric Forceq1q2dFe2q1q22dq13q22d2q12q22d3q12q22d3q1 q2 d q1 q2 dElectric Force The Coulombs Law100Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFe2q1q22dFe /2q13q22d2q12q22d3q12q22d3q1 q2 d q1 q2 dElectric Force The Coulombs Law101Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFe2q1q22dFe /2q13q22d3Fe /42q12q22d3q12q22d3q1 q2 d q1 q2 dElectric Force The Coulombs Law102Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFe2q1q22dFe /2q13q22d3Fe /42q12q22dFe3q12q22d3q1 q2 d q1 q2 dElectric Force The Coulombs Law103Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFe2q1q22dFe /2q13q22d3Fe /42q12q22dFe3q12q22d3Fe /23q1 q2 d q1 q2 dElectric Force The Coulombs Law104Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFe2q1q22dFe /2q13q22d3Fe /42q12q22dFe3q12q22d3Fe /23q1 q2 d6Fe q1 q2 dElectric Force The Coulombs Law105Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Charge 1Charge 2DistanceElectric Forceq1q2dFe2q1q22dFe /2q13q22d3Fe /42q12q22dFe3q12q22d3Fe /23q1 q2 d6Fe q1 q2 dFeElectric Force The Coulombs Law106Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Determine the electrical force of attraction between two balloons with separate charges of 3.5 x 10-8 C and -2.9 x 10-8C when separated a distance of 0.65 m.Electric Force The Coulombs LawFe = k q1 x q2/r2Fe = 8.99 x 109 Nm2/C2 x (3.5 x 10-8 C)(-2.9 x 10-8 C) / (0.65 m)2 = -2.16 x 10-5 NDetermine the electrical force of attraction between two balloons with separate charges of 3.5 x 10-8 C and -2.9 x 10-8C when separated a distance of 0.65 m.Electric Force The Coulombs LawElectric Force ExerciseEach of the two identical hot-air balloons acquires a charge of 3.2 x 10-6 C on its surface as it travels through the air. How far apart are the balloons if the electrostatic force between them is 4.5 x 10-2 N?

Electric Force ExerciseFe = k q1 x q2/r24.5 x 10-2 N = 8.99 x 109 Nm2/C2 x (3.2 x 10-6 C)(3.2 x 10-6 C) / r2 r = 1.43 mEach of the two identical hot-air balloons acquires a charge of 3.2 x 10-6 C on its surface as it travels through the air. How far apart are the balloons if the electrostatic force between them is 4.5 x 10-2 N?Electrical & Gravitational ForcesFg: gravitational force (N)G:Universal gravitational constant, 6.77x10-11 N m2/kg2m1, m2:mass (kg)r: distance between masses (m)Fg = G m1 m2r2Law of Universal GravitationElectric Force vs. Gravitational ForcesElectrical & Gravitational ForcesForceElectric ForceFeGravitational ForceFg

Fg = G m1 m2r2Law of Universal GravitationCoulombs LawFe = k q1 q2r2Electrical & Gravitational ForcesElectric ForceElectrostatic constant (k) = 8.99 x 109 Nm2/C2k is a very, very large number! Compare to G = 6.67 x 10 -11 N m2/kg2Electrical force is much stronger than the gravityThe reason why we dont feel it most of the time is because that the attracting force and repelling force cancel each other114Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Electric ForceComparison of four basic forcesForceStrongElectromagneticWeakGravityStrength11/13710-66x10-39Range10-15 m10-18 m115Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Electric ForceModel of a helium atom: Based on the atomic model proposed by Rutherford and Bohr, a positively charged nucleus is surrounded by electrons Electrons are attracted to protons in the nucleus, but electrons repel other electrons--++Model of a Helium atom116Figure 5-1If the spheres radius is doubled, the spheres surface increases by a factor of 4.

Electric ForceWhy dont protons pull the oppositely charged electrons into the nucleus?--++Model of a Helium atom


Electric ForceThe reason why dont protons pull the oppositely charged electrons into the nucleus is in the domain of quantum physics. An electron behaves like a wave and has to occupy a certain amount of space related to its wavelength--++Model of a Helium atom


Electric ForceWhy the protons in the nucleus do not mutually repel and fly apart?--++Model of a Helium atom


Electric ForceThe reason why the protons in the nucleus do not mutually repel and fly apart is that in addition to electrical forces in the nucleus, there are even stronger forces (strong forces) that are non-electrical in nature--++Model of a Helium atom


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Eleanor Roosevelt High School Chin-Sung Lin. Electrostatics Electrostatics is the electricity at rest It involves electric charges, the force between