lecture 2 ohmslaw

7/25/2019 Lecture 2 Ohmslaw

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Ohm’s Law, Energy,Ohm’s Law, Energy,

Power, Series andPower, Series andParallel CircuitsParallel Circuits

Circuits 1Circuits 1Fall 2005Fall 2005

arding !ni"ersityarding !ni"ersity

#onathan $hite #onathan $hite



OutlineOutline

►Ohm’s LawOhm’s Law %eorg Ohm%eorg Ohm

&elationshi' (etween resistance, current,&elationshi' (etween resistance, current,

and "oltageand "oltage►EnergyEnergy

►PowerPower

)n general)n general )n electrical circuits)n electrical circuits

►Series and 'arallel circuitsSeries and 'arallel circuits



%eorg Simon Ohm%eorg Simon Ohm

*1++-15. /*1++-15. /► %erman 'hysicist who%erman 'hysicist who

e'erimentally determined thate'erimentally determined that

the i the "oltage across athe i the "oltage across a

resistor is increased, the currentresistor is increased, the current

through the resistor will increasethrough the resistor will increase

► )deas were 'u(lished in 12+,)deas were 'u(lished in 12+,

(ut they were re3ected (y his(ut they were re3ected (y his

'eers e li"ed in 'o"erty or'eers e li"ed in 'o"erty or

se"eral years (eore ta4ing ase"eral years (eore ta4ing ateaching 'ositionteaching 'osition

► Called the o6art o electricityCalled the o6art o electricity



EnergyEnergy

► Energy is the undamental ca'acity to do wor4Energy is the undamental ca'acity to do wor4► < charge can ha"e 'otential energy *"oltage/< charge can ha"e 'otential energy *"oltage/

(ecause o its 'lace in s'ace(ecause o its 'lace in s'ace► )n a 'ower station, gas, coal, or nuclear energy is)n a 'ower station, gas, coal, or nuclear energy is

transormed into electrical energy = much li4e atransormed into electrical energy = much li4e a(attery(attery

► >ote that energy can neither (e created or>ote that energy can neither (e created ordestroyed, only transormeddestroyed, only transormed

► Energy is measured in 3oules and sym(oli6ed (y theEnergy is measured in 3oules and sym(oli6ed (y theletter w *lowercase w/letter w *lowercase w/

► Energy is the amount o 'ower consumed o"erEnergy is the amount o 'ower consumed o"ertime, which can (e written as w8time, which can (e written as w8?'?' dt dt rom trom t00 to tto t



PowerPower

► Power is the rate at which energy is usedPower is the rate at which energy is used► ' 8' 8 dW dW :dt, where:dt, where W W is energy and t is timeis energy and t is time

Since i8Since i8dq/dtdq/dt and " 8and " 8 dw/dqdw/dq, ' can (e rewritten, ' can (e rewritten

as *as *dw/dq) * (dq/dt)dw/dq) * (dq/dt) '8"9)'8"9)

► Power in electrical circuits is measured inPower in electrical circuits is measured inwatts and sym(oli6ed (y the letter $ >ote@watts and sym(oli6ed (y the letter $ >ote@

energy uses a lowercase letter and 'owerenergy uses a lowercase letter and 'oweruses an u''ercaseuses an u''ercase For eam'le, a A0 watt light (ul( uses A0 3oulesFor eam'le, a A0 watt light (ul( uses A0 3oules

o energy in 1 secondo energy in 1 second



Power FormulasPower Formulas

► P 8 7)P 8 7) >ote@ Power can (e negati"e ) the de"ice is a>ote@ Power can (e negati"e ) the de"ice is a

(attery, it is losing 'ower so that the loads can do(attery, it is losing 'ower so that the loads can dowor4 Can the 'ower e"er (e negati"e or awor4 Can the 'ower e"er (e negati"e or a

resistor Dy con"ention, i the current entersresistor Dy con"ention, i the current entersthrough the 'ositi"e terminal o an element, thethrough the 'ositi"e terminal o an element, the'ower is 'ositi"e ) the current lea"es an element'ower is 'ositi"e ) the current lea"es an element*enters through the negati"e terminal/, it is*enters through the negati"e terminal/, it is'roducing 'ower, and the 'ower will (e negati"e'roducing 'ower, and the 'ower will (e negati"e

►

!sing Ohm’s Law, you can state the a(o"e!sing Ohm’s Law, you can state the a(o"eormula in a num(er o ways@ormula in a num(er o ways@ P 8 )P 8 )22 9 &9 & P 8 7P 8 722 : &: &



Energy Con"ersion in aEnergy Con"ersion in a

resistorresistor► Bhe 'ower consumed (y a resistor can (e Bhe 'ower consumed (y a resistor can (e

written as w8written as w8?"9i?"9i dt dt rom trom t00 to t, since '8"9ito t, since '8"9i► <s you’"e seen, when there is a current<s you’"e seen, when there is a current

through a resistance, electrical energy isthrough a resistance, electrical energy is

con"erted to heat energycon"erted to heat energy► Bhis heat is caused (y the collisions o the ree Bhis heat is caused (y the collisions o the ree

electrons within the atomic structure o theelectrons within the atomic structure o theresisti"e materialresisti"e material

►

$hen a collision occurs, heat is gi"en o and$hen a collision occurs, heat is gi"en o andthe electron gi"es u' some o its acuiredthe electron gi"es u' some o its acuiredenergy as it mo"es through the materialenergy as it mo"es through the material



&e"iew&e"iew

►Ohm’s Law@Ohm’s Law@ 78 ) 9 &G ) 8 7:&G & 8 79)78 ) 9 &G ) 8 7:&G & 8 79)

►EnergyEnergy HeInition@ Ca'acity to do wor4HeInition@ Ca'acity to do wor4

easured in 3ouleseasured in 3oules

Sym(ol is wSym(ol is w

Energy can neither (e created orEnergy can neither (e created ordestroyed, only transormeddestroyed, only transormed

< (attery stores energy< (attery stores energy



&e"iew&e"iew

► Power@Power@ HeInition@ Bhe rate at which energy is usedHeInition@ Bhe rate at which energy is used

easured in wattseasured in watts

Sym(ol is $Sym(ol is $ P 8P 8 W W : t: t

)n electrical circuits,)n electrical circuits,►P 8 7 9 ), P 8 )P 8 7 9 ), P 8 )22 9 &, P 8 79 &, P 8 722 : &: &

Power can (e negati"e, i something is su''lyingPower can (e negati"e, i something is su''lying

'ower'ower

► 7oltage dro's@ Charge loses energy as it tra"els7oltage dro's@ Charge loses energy as it tra"els

through circuitsthrough circuits

lecture 2 ohmslaw

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