ElectrostaticsElectrostaticsthe study of electrical

charges at rest

ElectrodynamicsElectrodynamicsthe study of electrical

charges in motion

Two oppositeopposite types of charge exist, namedpositivepositive and negativenegative by Benjamin Benjamin

FranklinFranklin.

Charge is aproperty of

matter.

Charged particlesCharged particles exist in exist in atoms.atoms.

ElectronsElectrons are responsibleare responsiblefor for negativenegative charge; charge;

protonsprotons for for positivepositive charge; charge;neutronsneutrons have have nono charge. charge.

Small amountsSmall amounts of ordinary matter contain of ordinary matter containincredible amountsincredible amounts of subatomic particles! of subatomic particles!

ConductorConductormaterial that allows charges to move about easily

InsulatorInsulatormaterial through which charges will not easily move

Basic Law of ElectrostaticsBasic Law of ElectrostaticsLike charges repel; unlike charges attractLike charges repel; unlike charges attract

Link

link

link

Click here to view a simulation of thebehavior of pith ballsin the vicinity ofcharged rods. See a movie here.

Click Click here to read about charging to read about charging objects by friction.objects by friction.

View a simulationView a simulationof charging aof charging aballoon by rubbing balloon by rubbing it on your hair it on your hair and then stickingand then stickingit to a neutralit to a neutralwall wall here.

Learn all about Ben Franklin and his Learn all about Ben Franklin and his work with electricity here.work with electricity here.

charging a rodrod and electroscopeelectroscopepositivelypositively and negativelynegatively

by conductionconduction and inductioninduction

When charging by conductionconduction,the rod touchestouches the electroscope.

The electroscope gets the same chargesame charge as the rod.

When charging by inductioninduction, the rod does notdoes nottouchtouch the electroscope. The electroscope gets

the opposite chargeopposite charge of the rod.

Go Go here , here, here, and here to view simulations of charging to view simulations of charging an electroscope. Read more an electroscope. Read more here.

COULOMB’S LAWCOULOMB’S LAWThe force between two charged objects isThe force between two charged objects is

directly proportional to the product of theirdirectly proportional to the product of theircharges and inversely proportional to theircharges and inversely proportional to their

separation distance squared.separation distance squared.

link1, link2, link3, link4, link4

In equation form:In equation form:

FFF is the is the force of attractionforce of attraction, measured in , measured in NEWTONSNEWTONS,,between charges qbetween charges q11 and q and q22

k

kk is the is the Universal Electrostatic ConstantUniversal Electrostatic Constant, equal to , equal to

9.00 x 109.00 x 1099 N N mm22/coul/coul22

q1 q2

qq11 and and qq22 are the are the attracting chargesattracting charges, measured in, measured inCoulombsCoulombs

d2

dd is the is the distance between the chargesdistance between the charges, , and is measured in and is measured in METERSMETERS

The SI unit of chargeThe SI unit of chargeis the is the CoulombCoulomb,,named in honor ofnamed in honor of

Charles Augustin Coulomb..

1 C = charge on 1 C = charge on 6.25 x 106.25 x 101818 electrons (or protons) electrons (or protons)

1 e1 e-- = = 1.60 x 101.60 x 101919 CoulCoul = elementary charge = elementary charge

Electric force is a Electric force is a vectorvector and must be treated as such. and must be treated as such.

Electric FieldsElectric FieldsAn electric field exists in a regionAn electric field exists in a regionif space if a charge placed in thatif space if a charge placed in that

region experiences an electric force.region experiences an electric force.

The magnitude of an electric field at any The magnitude of an electric field at any given point is defined to be the ratio ofgiven point is defined to be the ratio ofthe force on athe force on acharge at thatcharge at thatpoint to the point to the amount of charge.amount of charge.

E = F/QElectric field strength has unitsElectric field strength has units

of Newtons/Coulomb (N/C).of Newtons/Coulomb (N/C).

The The directiondirection of the electric fieldof the electric field at any point is defined at any point is definedto be the same direction as the to be the same direction as the direction of force on a direction of force on a positive test chargepositive test charge placed in the region at that point. placed in the region at that point.

Field lines Field lines point away from positivepoint away from positive and and toward negative chargestoward negative charges..

Click Click here to view a simulationshowing the magnitude and direction of theshowing the magnitude and direction of the

electric force on a test charge whenelectric force on a test charge whenplaced near other charges.placed near other charges.

Click Click here to view a simulation of acharged particle moving through a regioncharged particle moving through a region

occupied by other charges.occupied by other charges.

Electric Potential DifferenceElectric Potential Difference

the change in the change in electric potential electric potential energyenergyper unit chargeper unit charge

V = W/QThe SI unit of electric potential

difference is the VOLTVOLT, named inhonor of Alessandro Volta.

One One VOLTVOLTis the is the electric potential differenceelectric potential difference

between two points when between two points when one Jouleone Joule of of workwork

is done in moving is done in moving one Coulombone Coulomb of charge of chargebetween the points.between the points.

electric cell - a device thatconverts one form of energy

to electrical energy

Chemical cells convert chemicalenergy into electrical energy.

Chemical cells can be“wet” or “dry”.

SolarSolar cellscells convert light energyinto electrical energy.

A generatorgenerator converts mechanicalenergy into electrical energy.

batterybattery - two or more cellsconnected in series or in parallel

the flow ofthe flow of charged particlescharged particles;

can be positive or negative,but usually negative (electrons)

through a conducting metal

Electric current is measuredin Amperes, in honor ofAndre Marie Ampere.

One Ampere is the flow ofOne Ampere is the flow ofone Coulomb of charge per second.one Coulomb of charge per second.

1 Amp = 1 Coulomb per second = 1 C/s1 Amp = 1 Coulomb per second = 1 C/s

II = QQ/tt

AmmeterAmmetera device that measures current

VoltmeterVoltmetera device that measures

electric potential difference

powerpower = = workwork//timetime

= (= (workwork//chargecharge))..((chargecharge//timetime))

== electric potential differenceelectric potential difference ..

currentcurrent

PP ( (WattsWatts) = ) = VV ( (VoltsVolts))..II ( (AmpsAmps))

Analogies of simple circuits are these links:Analogies of simple circuits are these links:

Water circuit analogy linkWater circuit analogy linkAir flow Air flow linkVarious Various link

Teaching with Analogies Teaching with Analogies link1, link2

ResistanceResistance

determines the amount of current flowdetermines the amount of current flow

= the ratio of potential difference to current= the ratio of potential difference to current

RR =VVII

The SI unit of resistance is the OhmOhm, , namedin honor of Georg Simon Ohm.

One OhmOne Ohm of resistance is the resistancesuch that one Voltone Volt of potential differenceis needed to obtain a current of one Ampone Amp.

The resistance of a circuit element depends on:

1. the length of the conductorthe length of the conductoras length increases, resistance increases proportionally

2. the cross-sectional area of the conductorthe cross-sectional area of the conductoras area increases, resistance decreases proportionally

3. the resistivity of the conductorthe resistivity of the conductoras resistivity increases, resistance increases proportionally

ResistivityResistivityThe resistivity, , of a conductor is equal to the

resistance of a wire 1 cm long and having a cross-sectional area of 1 cm2.

RR

RR = resistance, measured in Ohms = resistance, measured in Ohms

=

= resistivity, usually in units of = resistivity, usually in units of cmcm

ll

l = length, measured in cml = length, measured in cm

AA

AA = cross-sectional area, measured in cm = cross-sectional area, measured in cm22

Investigate resistivity hereInvestigate resistivity here

Ohm’s LawOhm’s LawThe ratio of potential difference to currentThe ratio of potential difference to current

is constant.is constant.

If R = V/I is a constant valueIf R = V/I is a constant valuefor a given resistor, then thatfor a given resistor, then that

resistor is said to obey Ohm’s Law.resistor is said to obey Ohm’s Law.

Click Click here and here to link to pages describing resistorcolor codes.color codes.

Click Click here and here to runsimulations of Ohm’s Law.simulations of Ohm’s Law.

Many circuit elements do notMany circuit elements do notobey Ohm’s Law. Resistorsobey Ohm’s Law. Resistorsthat get hot, like light bulbsthat get hot, like light bulbsand heating elements, do notand heating elements, do notkeep a constant resistance. keep a constant resistance. Resistance generally Resistance generally increases as increases as objects become hotter.objects become hotter.

Resistor CircuitsResistor CircuitsSeries

1. 1. total resistancetotal resistance is the sum of the is the sum of the separate resistorsseparate resistors

RRTT = R = R11 + R + R22 + R + R33 + ... + ...

2. 2. currentcurrent is the same through each resistor is the same through each resistor

IITT = I = I11 = I = I22 = I = I33 = ... = ...

3. 3. total potential differencetotal potential difference is the sum of each is the sum of each

VVTT = V = V11 + V + V22 + V + V33 + ... + ...

In other words, in a series circuit,In other words, in a series circuit,resistance and voltage add,resistance and voltage add,but current stays the same.but current stays the same.

RR11

RR22

RR33

EE = 12 V = 12 V

RRTT = =

VVTT = =

IITT = =

RR11

RR22

RR33

V,V,VV

PPTT = =

R,R,

I,I,AA

P,P,WW

8.0

5.0

2.0

RR11

RR22

RR33

EE = 12 V = 12 V

RRTT = 15 = 15 ΩΩ

VVTT = 12 V = 12 V

IITT = 0.80 A = 0.80 A

RR11

RR22

RR33

V,V,VV

PPTT = 9.6 W = 9.6 W

R,R,

I,I,AA

P,P,WW

8.0

5.0

2.0

0.80

0.80

0.80

6.4

1.6

4.0

5.1

1.3

3.2

Parallel 1.1. reciprocal of the total resistance reciprocal of the total resistance is is

the the sum of the reciprocals of the sum of the reciprocals of the

separate separate resistorsresistors

1/R1/RTT = 1/R = 1/R11 + 1/R + 1/R22 +1/R +1/R33 + ...+ ...2. 2. total currenttotal current is the sum of the current is the sum of the current

through each resistorthrough each resistorIITT = I = I11 + I + I22 + I + I33 + ... + ...

3. 3. potential differencepotential difference is the same is the same across across each resistoreach resistorVVTT = V = V11 = V = V22 = V = V33 = ... = ...

In other words, in a parallel circuit, resistance adds as reciprocals,

voltage stays the same, and current splits.

In other words, in a parallel circuit, resistance adds as reciprocals,

voltage stays the same, and current splits.

EE = 12 V = 12 V

RRTT = =

VVTT = =

IITT = =

RR11

RR22

RR33

R,R,

V,V,VV

I,I,

PPTT = =

RR33

RR11

RR22

P,P,WW

12

8.0

12

EE = 12 V = 12 V

RRTT = 3.42 = 3.42 ΩΩ

VVTT = 12 V = 12 V

IITT = 3.50 A = 3.50 A

RR11

RR22

RR33

R,R,

V,V,VV

I,I,

PPTT = 42 W = 42 W

RR33

RR11

RR22

P,P,WW

12

8.0

12

12

12

12

1.0

1.5

1.0

12

12

18

Go to Go to link1, link2, link3, , link2, link3, link4, , link5, and , and link6 to view to viewpages and simulations examining pages and simulations examining

Kirchhoff’s Loop and Junction Rules.Kirchhoff’s Loop and Junction Rules.

Kirchhoff’s RulesKirchhoff’s Rules

Loop Rule: Loop Rule: The sum of the potential differencesThe sum of the potential differencesaround any closed circuit loop is zero.around any closed circuit loop is zero.

Junction Rule: Junction Rule: The sum of the currentsThe sum of the currentsinto any circuit junction is zero.into any circuit junction is zero.

The sites linked The sites linked here andhere (click on “Circuit here (click on “Circuit Construction Kit”) allowConstruction Kit”) allowyou to build and test youryou to build and test yourown series, parallel, andown series, parallel, andcombination circuits.combination circuits.

For a complete For a complete interactive tutorial interactive tutorial

on electricity and on electricity and magnetism, go magnetism, go here.