Electricity & Magnetism Static Electricity, Electric Fields, Currents, Circuits Magnetic Fields & Electro Magnets Motors & Generators.

Electricity & Electricity & MagnetismMagnetism

Static Electricity, Electric Fields, Static Electricity, Electric Fields, Currents, CircuitsCurrents, Circuits

Magnetic Fields & Electro Magnetic Fields & Electro MagnetsMagnets

Motors & GeneratorsMotors & Generators

Atoms…Atoms…

•Have Have neutronsneutrons, , protonsprotons, and , and electronselectrons..

•Protons are Protons are positivelypositively chargedcharged

•Electrons are Electrons are negativelynegatively chargedcharged

Electrons…Electrons…

•Are located on the Are located on the outerouter edges of edges of atoms…they can be atoms…they can be movedmoved..

•A A concentrationconcentration of electrons in of electrons in an atom creates a net an atom creates a net negativenegative charge.charge.

• If electrons are If electrons are strippedstripped away, away, the atom becomes the atom becomes positivelypositively charged.charged.

+-

++

++

The world is filled with The world is filled with electrical chargeselectrical charges::

+

+

+

+

+

-

-- -

--

- --

What is this electrical What is this electrical potentialpotential called? called?

•Static ElectricityStatic Electricity

- --

--

-

- +++++

Static ElectricityStatic Electricity•The The build upbuild up of an electric charge of an electric charge

on the on the surfacesurface of an object. of an object.•The charge builds up but The charge builds up but does not does not

flow.flow.•Static electricity is Static electricity is potential potential

energyenergy. It does . It does notnot move. It is move. It is storedstored..

•Charge is source of all electric Charge is source of all electric force – both attractive and force – both attractive and repulsive.repulsive.

Static Discharge…Static Discharge…

•Occurs when there is a Occurs when there is a lossloss of of static electricity due to three static electricity due to three possible things:possible things:

•FrictionFriction - rubbing - rubbing•ConductionConduction – direct contact – – direct contact – charging by conduction is charging by conduction is charging by contact.charging by contact.

•InductionInduction – through an electrical – through an electrical field (not direct contact) – field (not direct contact) – charging by proximity.charging by proximity.

Conductors vs. InsulatorsConductors vs. Insulators

•ConductorsConductors – material – material through which electric through which electric current flows current flows easilyeasily..

• InsulatorsInsulators – materials – materials through which electric through which electric current current cannot movecannot move..

ExamplesExamples

•ConductorsConductors::– MetalMetal– WaterWater

•InsulatorsInsulators::– StyrofoamStyrofoam– RubberRubber– PlasticPlastic– PaperPaper

Ben FranklinBen Franklin

•Noticed Two types of chargeNoticed Two types of charge•Mobile chargeMobile charge•Stationary chargeStationary charge•Positive electricity – when Positive electricity – when

he rubbed glasshe rubbed glass•Negative electricity – when Negative electricity – when

he rubbed amberhe rubbed amber

More BenMore Ben

•Electrons move and leave Electrons move and leave a positive chargea positive charge

•Charge is caused by the Charge is caused by the transfer of electronstransfer of electrons

•Ionization energyIonization energy

The ElectroscopeThe Electroscope

•A device used to detect the A device used to detect the presence of electrical presence of electrical chargecharge

•It charges by inductionIt charges by induction•Cannot tell the type of Cannot tell the type of

charge – negative or positivecharge – negative or positive

Law of Conservation of Law of Conservation of ChargeCharge

•The total amount of charge in The total amount of charge in the universe remains constantthe universe remains constant

•Single charges can be neither Single charges can be neither created or destroyedcreated or destroyed

•Charge is an intrinsic property Charge is an intrinsic property of protons and electronsof protons and electrons

Coulomb’s LawCoulomb’s Law

221

d

qKqFG

•Action – reaction pairsAction – reaction pairs•Unit of charge – Coulomb (C)Unit of charge – Coulomb (C)•Distance is measured in Distance is measured in metersmeters

•K = 8.988 X 10K = 8.988 X 1099 Nm Nm22/C/C22

•1 e1 e-- = 1.609 X 10 = 1.609 X 10-19-19 C C•1 C = 6.25 X 101 C = 6.25 X 101818 e e--

4

1K

oo is the permittivity of free space is the permittivity of free space

• 8.854 X 108.854 X 10-12-12 C C22/Nm/Nm22

• Relates to how well a substance Relates to how well a substance allows the presence of a chargeallows the presence of a charge

Vector Nature of Electrical Vector Nature of Electrical ForcesForces

•Electrostatic forces add Electrostatic forces add vectoriallyvectorially

•Pythagorean theorem Pythagorean theorem and inverse tan.and inverse tan.

The Electric FieldThe Electric Field• Introduced by Michael FaradayIntroduced by Michael Faraday•Similar to a gravitational fieldSimilar to a gravitational field•A point charge generates a field A point charge generates a field

around it that has an effect on around it that has an effect on its environment.its environment.

•You can determine the effect on You can determine the effect on a test charge at any point in the a test charge at any point in the field. field.

Electric Field from positivePoint Charge

Charge eminates from theField

Electric Field from a negativepoint charge

Points into the point charge

Uniform Electric Uniform Electric FieldField

•Between two parallel platesBetween two parallel plates•Connected to a source of Connected to a source of

current – a voltagecurrent – a voltage•Field is uniform throughout Field is uniform throughout

the area between the platesthe area between the plates•A way of controlling a field A way of controlling a field

to do work.to do work.

Electric Field Intensity Electric Field Intensity (E)(E)

•Electric force per unit Electric force per unit test chargetest charge

•Test charge itself is Test charge itself is considered small considered small enough to not affect the enough to not affect the fieldfield

The EquationThe Equation

q

FE

•Electric field is a vector Electric field is a vector quantity with units, N/C.quantity with units, N/C.

•You can also find the electric You can also find the electric field intensity from the field intensity from the magnitude of the point magnitude of the point chargecharge

•Use Coulomb’s lawUse Coulomb’s law

The EquationThe Equation

2d

kQE

•Q means “test” chargeQ means “test” charge• q means “point” chargeq means “point” charge

Electric PotentialElectric Potential

•What causes charges to What causes charges to move?move?

•What causes a ball to What causes a ball to roll down hill?roll down hill?

•Why does heat travel Why does heat travel from hot to cold?from hot to cold?

More Electric PotentialMore Electric Potential

•Electric potential energy is Electric potential energy is analogous to mechanical or analogous to mechanical or gravitational potential energy.gravitational potential energy.

•Electrical potential – Electrical potential – voltage voltage – – allows us to describe allows us to describe physical situations easier in physical situations easier in terms of field or charge. terms of field or charge.

What is Voltage?What is Voltage?

•The The measure of energymeasure of energy given to given to the charge flowing in a circuit.the charge flowing in a circuit.

•The The greatergreater the voltage, the the voltage, the greater the force or “pressure”greater the force or “pressure” that drives the charge through that drives the charge through the circuit.the circuit.

•The units are joules/coulomb.The units are joules/coulomb.

•Electric potential – voltage Electric potential – voltage – comes from analyzing – comes from analyzing the work and energy from the work and energy from the motion of a charge in the motion of a charge in an electric field.an electric field.

The EquationThe Equation

0q

PEV Elec

Coulombs

JoulesVolts

r

kQqW 0

r

kQV

Think About It…Think About It…

•Note the four expressions Note the four expressions developed for force, developed for force, electric field intensity, electric field intensity, work and voltage.work and voltage.

•They were all derived They were all derived from Coulomb’s law.from Coulomb’s law.

Potential DifferencePotential Difference

•Potential difference is define Potential difference is define and the work per unit chargeand the work per unit charge

q

WVVV ABABAB

qVW

A Test Charge in a UniformA Test Charge in a UniformElectric Field Electric Field

•F=qEF=qE•W=FdW=Fd•W=qEdW=qEd•PE=qEdPE=qEd•V=PE/qV=PE/q•V=EdV=Ed

Electricity that Electricity that movesmoves……

•CurrentCurrent: The flow of : The flow of electrons from one place to electrons from one place to another.another.

•Measured in Measured in amperesamperes (amps) (amps)•KineticKinetic energy energy

How can we How can we controlcontrol currents?currents?

•With With circuitscircuits..•CircuitCircuit: is a : is a pathpath for the flow for the flow

of electrons. We use of electrons. We use wireswires..

There are 2 types of There are 2 types of currents:currents:

•Direct Current (DC)Direct Current (DC) – Where – Where electrons flow in the electrons flow in the samesame direction in a wire. direction in a wire.

There are 2 types of There are 2 types of currents:currents:

•Alternating Current (AC)Alternating Current (AC) – – electrons flow in electrons flow in differentdifferent directions in a wiredirections in a wire

There are 2 types of There are 2 types of circuits:circuits:

•Series CircuitSeries Circuit: the : the components are lined up components are lined up along along oneone path. If the circuit path. If the circuit is broken, is broken, all all components components turn off.turn off.

Series CircuitSeries Circuit

There are 2 types of There are 2 types of circuits:circuits:

•Parallel CircuitParallel Circuit – there are – there are severalseveral branching paths to branching paths to the components. If the the components. If the circuit is broken at any one circuit is broken at any one branch, branch, only the components only the components on that branch will turn offon that branch will turn off..

Parallel CircuitParallel Circuit

What is Resistance?What is Resistance?

•The The oppositionopposition to the flow of an to the flow of an electric current, producing electric current, producing heatheat..

•The The greatergreater the resistance, the the resistance, the lessless current gets through. current gets through.

•GoodGood conductors have conductors have lowlow resistance.resistance.

•Measured in Measured in ohms.ohms.

What Influences What Influences Resistance?Resistance?

• Material of wireMaterial of wire – aluminum and – aluminum and copper have low resistancecopper have low resistance

• ThicknessThickness – the thicker the wire – the thicker the wire the lower the resistancethe lower the resistance

• LengthLength – shorter wire has lower – shorter wire has lower resistanceresistance

• TemperatureTemperature – lower temperature – lower temperature has lower resistancehas lower resistance

Difference b/t Volts and Difference b/t Volts and AmpsAmps

•Example – you could say Example – you could say that…that…– AmpsAmps measure measure how muchhow much

water comes out of a hose.water comes out of a hose.– VoltsVolts measure measure how hardhow hard the the

water comes out of a hose.water comes out of a hose.

Ohm’s LawOhm’s Law

•Resistance = Voltage / Resistance = Voltage / CurrentCurrent

•Ohms = Volts / AmpsOhms = Volts / Amps

Practice with Ohm’s LawPractice with Ohm’s Law

OhmsOhms VoltsVolts AmpsAmps44 100100 2525

1515 150150 101022 3030 151599 4545 5566 4848 88

What is an What is an electromagnet?electromagnet?

•ElectromagnetElectromagnet – a magnet – a magnet made from a made from a current bearing current bearing coilcoil of wire wrapped around of wire wrapped around an an iron or steel core.iron or steel core.

What is a generator?What is a generator?

•GeneratorGenerator – a machine that – a machine that changes changes mechanicalmechanical energy energy to to electricalelectrical energy energy

•Usually use Usually use movingmoving magnets magnets to create to create currentscurrents in coils of in coils of wire.wire.

What is a motor?What is a motor?

•MotorMotor – a device that – a device that changes changes electrical electrical energy to energy to mechanical mechanical energy that can energy that can do do workwork..

That’s It !!!!That’s It !!!!