1 ELECTRICITY I Statics 2 If you rub a piece of amber With the fur of a rabbit It will attract bits...

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ELECTRICITY IELECTRICITY I

StaticsStatics

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If you rub a piece of amberWith the fur of a rabbit

It will attract bits of stuff (paper, leaves, etc)

Electrostatics if the study of electrical charges that can be contained in one place

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Charges are measured in units Charges are measured in units

called Coulombscalled Coulombs..

• 1 C = 6.25 X 101 C = 6.25 X 101818 electrons electrons• The charge of one electron is 1.602 The charge of one electron is 1.602

x 10x 10-19-19

C. This magnitude is called C. This magnitude is called ee--..

• ee also can be used to represent a also can be used to represent a proton charge.proton charge.

• Charges are often expressed in Charges are often expressed in units of units of ee. (Ex: 2e, 3e. (Ex: 2e, 3e- - ))

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Charge and Mass of Atomic Charge and Mass of Atomic ParticlesParticles

electron -1.60x10-19

9.109 x 10-31

proton +1.60x10-19

1.673 x 10-27

neutron 0 1.675 x 10-27

Particle Charge (C) Mass (kg)

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The Law of Conservation of The Law of Conservation of Electric Charge…Electric Charge…

• Charge can neither be created Charge can neither be created nor destroyed, only transferred.nor destroyed, only transferred.

• Transferring charge creates Transferring charge creates ionsions…which are charged …which are charged particles.particles.

• Removing electrons creates a Removing electrons creates a positive charge.positive charge.

• Adding electrons creates a Adding electrons creates a negative charge.negative charge.

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Electric Charges

Like charges repel

Unlike charges attract

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PolarizationPolarization

• Bringing a charged object Bringing a charged object near (but not near (but not touching)touching) a neutral object a neutral object polarizespolarizes ((temporarily separatestemporarily separates)) the charge of the the charge of the neutral object.neutral object.Like charges in the neutral object are repelled Like charges in the neutral object are repelled by the charged object.by the charged object.Unlike charges in the neutral object are Unlike charges in the neutral object are attracted by the neutral object.attracted by the neutral object.The neutral object returns to normal when the The neutral object returns to normal when the charged object is removedcharged object is removed

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Electric DipolesElectric Dipoles

• An object that is An object that is electrically neutralelectrically neutral overall, but overall, but permanently polarizedpermanently polarized, is , is called an called an electric dipoleelectric dipole..Example: HExample: H220 molecule0 molecule

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Charging by ContactCharging by Contact

• If a charged object is brought in contact If a charged object is brought in contact with a neutral object, charges will be with a neutral object, charges will be repelled from (or attracted to) the repelled from (or attracted to) the charged object.charged object.The neutral object will gain a charge of The neutral object will gain a charge of the same sign as the charged object.the same sign as the charged object.

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Charging by InductionCharging by Induction

• Bring a charged object Bring a charged object near (but not near (but not touching)touching) a neutral object. a neutral object.Ground the neutral object.Ground the neutral object.Remove the ground.Remove the ground.Remove the charged objectRemove the charged objectThe neutral object now has a charge The neutral object now has a charge oppositeopposite to the charged object. to the charged object.

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GroundingGrounding

• Providing a path from a charged object Providing a path from a charged object to the Earth is called to the Earth is called groundinggrounding it. it.Charges will be attracted from (or Charges will be attracted from (or repelled to) the Earth by the charged repelled to) the Earth by the charged object.object.Since the Earth is so large, both the Since the Earth is so large, both the charged object and the Earth are charged object and the Earth are neutralized.neutralized.

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Electrons in SolidsElectrons in Solids

• When atoms are arranged in a solid they When atoms are arranged in a solid they “share” electrons“share” electrons– High electron mobility = conductorHigh electron mobility = conductor– Low electron mobility = insulatorLow electron mobility = insulator

• In order to produce a net flow of In order to produce a net flow of electrons, they must electrons, they must increaseincrease their their energyenergy

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SuperconductorsSuperconductors

• SuperconductorsSuperconductors are materials that are materials that lose all resistancelose all resistance to charge movement to charge movement at temperatures near absolute zero (0 K at temperatures near absolute zero (0 K or about -273or about -273ooC).C).Recently, “high temperature” (above 100 Recently, “high temperature” (above 100 K) superconductors have been K) superconductors have been discovered.discovered.

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The Periodic TableThe Periodic Table

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Static charges in NatureStatic charges in Nature

• A typical A typical thunder cloud thunder cloud has both + and – has both + and – chargescharges

• Lightning is Lightning is static electricitystatic electricity– Bolt travels Bolt travels

negative to negative to positivepositive

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LIGHTNINGLIGHTNING

Though air is not a conductor, the buildup of charges during a storm forces the electricity to flow through the air anyway, ripping apart gas molecules along the way. This “energized gas” state is called plasma - which is a good conductor.

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Question 1Question 1

• Explain from an atomic standpoint Explain from an atomic standpoint why charge is usually transferred why charge is usually transferred by electrons.by electrons.

• Protons are relatively fixed in the Protons are relatively fixed in the nucleus of an atom, while electrons nucleus of an atom, while electrons can be transferred from one atom can be transferred from one atom to another.to another.

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• Calculate the net charge on a Calculate the net charge on a substance consisting of a substance consisting of a combination of 7.0 x 10 combination of 7.0 x 10 1313 protons protons and 4.0 x 10 and 4.0 x 10 1313 electrons. electrons.

• 4.8 x 10 4.8 x 10 -6-6 C C

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• A negatively charged balloon has A negatively charged balloon has 3.5 3.5 C of charge. How many C of charge. How many excess electrons are on this excess electrons are on this balloon?balloon?

• 2.2 x 10 2.2 x 10 1313 electrons electrons

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Coulomb’s LawCoulomb’s Law

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Coulomb experimented with Coulomb experimented with charged spheres.charged spheres.

• He found that: He found that: – electrostatic force varied inversely electrostatic force varied inversely

with square of the distance with square of the distance between the spheres.between the spheres.

– electrostatic force varied directly electrostatic force varied directly with magnitude of the charge. with magnitude of the charge.

• He combined these discoveries He combined these discoveries to make his law.to make his law.

FFee = kqq’/d = kqq’/d22

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Electrical ForcesElectrical Forces

• The The electrical forceelectrical force between 2 charges between 2 charges depends ondepends on::The The sizesize of each charge of each chargeMore charge means more force.More charge means more force.The The distancedistance between the charges between the charges More distance means less force.More distance means less force.

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Comparing gravitational force to Comparing gravitational force to electric forceelectric force

• Gravity Gravity – Is a weak force Is a weak force – Acts over long distancesActs over long distances– Is attractive onlyIs attractive only

• ElectricityElectricity– Is a strong forceIs a strong force– Acts over short distancesActs over short distances– Is attractive and repulsiveIs attractive and repulsive

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What are What are q,dq,d and and kk??

• qq is the magnitude of the charge. is the magnitude of the charge.– q’ can indicate a test chargeq’ can indicate a test charge

• dd is the distance between the is the distance between the charged particles.charged particles.

• kk is Coulomb’s constant of is Coulomb’s constant of proportionality.proportionality.– kk = 8.99 x 10 = 8.99 x 1099 N N··mm22/C/C22

(Compare this to (Compare this to GG which is which is 6.67 x 106.67 x 10-11-11 N N··mm22 / kg / kg22 !) !)

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Sample Problem:Sample Problem:

• The electron and proton of a The electron and proton of a hydrogen atom are separated, hydrogen atom are separated, on average, by a distance of on average, by a distance of about 5.3x10about 5.3x10-11-11 m. m.

• Find the magnitudes of the Find the magnitudes of the electric forceelectric force and the and the gravitational forcegravitational force that each that each particle exerts on the other.particle exerts on the other.

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For the electric force, we use For the electric force, we use Coulomb’s Law:Coulomb’s Law:

FFee = kqq’/d = kqq’/d22

FFee = =

(8.99 x 10(8.99 x 1099 N N··mm22/C/C22) * (1.6 x 10) * (1.6 x 10-19-19 C) * (-1.6 x 10 C) * (-1.6 x 10-19-19 C) C)

(5.3 x 10(5.3 x 10-11-11))22

== -8.2 x 10 -8.2 x 10-8-8 N N

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For the gravitational force, we use For the gravitational force, we use Newton’s Law of Gravitation:Newton’s Law of Gravitation:

F = GmF = Gm11mm22/d/d22

FFgg = =

(6.67 x 10(6.67 x 10-11-11 Nm Nm22/kg2) * (9.11 x 10/kg2) * (9.11 x 10-31-31 kg) * (1.67 x 10 kg) * (1.67 x 10-27-27 kg) kg)

(5.3 x 10(5.3 x 10-11-11 m) m)22

== 3.6 x 10 3.6 x 10-47-47 N N

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• How does the electric force How does the electric force between two charges change when between two charges change when the distance between them is the distance between them is doubled?doubled?

• The force is quarteredThe force is quartered

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• What is the mathematical What is the mathematical representation of Coulomb’s Law?representation of Coulomb’s Law?

• F = kqq’/dF = kqq’/d22

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• What is the numerical value and What is the numerical value and unit of Coulomb’s constant?unit of Coulomb’s constant?

• 8.99 x 108.99 x 1099 N m N m22 / C / C22

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Electric FieldsElectric Fields

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The Electric FieldThe Electric Field• An electric field caused by a point An electric field caused by a point

charge exerts a force on any other charge exerts a force on any other charge in its environment.charge in its environment.

• A collection of all the forces makes A collection of all the forces makes up an electric field.up an electric field.

• Faraday (1791-1867) defined the Faraday (1791-1867) defined the electric field as the region of space electric field as the region of space around a charged object.around a charged object.

• When another charged object enters When another charged object enters the field, electrical forces arise.the field, electrical forces arise.

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Electric Field lines for a positive point

charge

Electric Field lines for a negative point

charge

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Electric DipolesElectric Dipoles

Field lines for two point charges of Field lines for two point charges of equal magnitudes, but opposite equal magnitudes, but opposite signs. This charge configuration is signs. This charge configuration is called an called an electric dipoleelectric dipole..

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Electric FieldElectric Field

A positive test charge A positive test charge experiences a force experiences a force directed away from directed away from the central chargethe central charge

+

+

As the charge is moved closer to the central charge the force increases in magnitude

++

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Effects of FieldsEffects of Fields

• NoNo effect on effect on neutralsneutrals..

• Electric fieldsElectric fields can can accelerateaccelerate charged charged particles or particles or repel repel them.them.

• Magnetic fieldsMagnetic fields can turn a charged can turn a charged particle in a particle in a circlecircle

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Electric Field IntensityElectric Field Intensity

is the ratio of the force on a test charge to its charge. is the ratio of the force on a test charge to its charge.

The units are Newtons per Coulomb (N/C)The units are Newtons per Coulomb (N/C)• E is the electric field intensityE is the electric field intensity• F is the forceF is the force• q’ is the test chargeq’ is the test charge

'q

FE

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Electric PotentialElectric Potential

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• It takes a force to raise an object in It takes a force to raise an object in the Earth’s gravitational field. When the Earth’s gravitational field. When we do raise an object in the field we we do raise an object in the field we change its gravitational potential change its gravitational potential energy. (mgh)energy. (mgh)

• When a charge is moved against an When a charge is moved against an electric field, its potential energy electric field, its potential energy also changesalso changes

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• A positive charge experiences A positive charge experiences a downward force in the a downward force in the direction of the E-fielddirection of the E-field

• It takes work to move the charge It takes work to move the charge against the electric field against the electric field increasing its PEincreasing its PE

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Electrical Potential (V)Electrical Potential (V)

• The electrical potential (V) is The electrical potential (V) is defined as the electrical potential defined as the electrical potential energy per unit charge.energy per unit charge.

• Potential (V)=Potential (V)= electrical potential energy electrical potential energy

charge charge

• Electric potential is a scalarElectric potential is a scalar

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Electrical Potential and Electrical Potential and CurrentCurrent

• A common analogy is to compare A common analogy is to compare potential to water pressure.potential to water pressure.

• The higher the potential, the higher The higher the potential, the higher the pressure to transfer charge.the pressure to transfer charge.

• A battery maintains a continuous A battery maintains a continuous potential difference.potential difference.

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Electrical Potential (Voltage)Electrical Potential (Voltage)

• The unit for electrical potential The unit for electrical potential is the Volt, named after is the Volt, named after Alessandro Volta, an Italian Alessandro Volta, an Italian scientist who developed the scientist who developed the batterybattery

• 1 Volt = 1 Joule / 1 Coulomb1 Volt = 1 Joule / 1 Coulomb

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Potential Difference (Voltage)Potential Difference (Voltage)

The voltage difference between 2 The voltage difference between 2 points is equal to the work done points is equal to the work done against the field to move a against the field to move a positive charge from A to B with positive charge from A to B with no accelerationno acceleration

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Uniform Electric FieldUniform Electric Field

• Two parallel conducting plates a Two parallel conducting plates a distance, d, apart constitute a uniform distance, d, apart constitute a uniform electric field.electric field.

• The uniform field emanates from the The uniform field emanates from the positive plate to the negative plate.positive plate to the negative plate.

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Voltage in a Uniform E-fieldVoltage in a Uniform E-field

• Voltage equals work per unit charge.Voltage equals work per unit charge.

• That is V = Nm/CThat is V = Nm/C

• N/C is E (electric field intensity).N/C is E (electric field intensity).

• m is distance between the plates.m is distance between the plates.

• Therefore…Therefore…

EdV

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Electron Volt (eV)Electron Volt (eV)

• When a particle with a When a particle with a charge equal to that of an electron charge equal to that of an electron moves through 1 volt in an E-field, it moves through 1 volt in an E-field, it changes energy by 1 eV.changes energy by 1 eV.

PE = qPE = qVV

• 1 eV = 1.6 x 101 eV = 1.6 x 10-19-19 J J

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CapacitanceCapacitance

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• Capacitance is the ability to store Capacitance is the ability to store charge.charge.

• A charge storing device is called a A charge storing device is called a capacitor. capacitor.

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• At a given potential (V), the At a given potential (V), the amount of charge (Q) that can be amount of charge (Q) that can be stored by a body depends on its stored by a body depends on its physical characteristics.physical characteristics.

• These physical characteristics are These physical characteristics are lumped together under the term lumped together under the term capacitance (C).capacitance (C).

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Capacitance (C)Capacitance (C)

• C = Q/VC = Q/V

• The unit of capacitance is the Farad The unit of capacitance is the Farad (F), named to honor Michael (F), named to honor Michael FaradayFaraday

• 1 Farad = 1 Coulomb / 1 Volt1 Farad = 1 Coulomb / 1 Volt

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• 1 Farad is a large amount of 1 Farad is a large amount of capacitance.capacitance.

• Usually a capacitor found in a piece Usually a capacitor found in a piece of electronic equipment will be rate of electronic equipment will be rate in microfarads (in microfarads (F) or picofarads F) or picofarads (pF)(pF)

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