Chapter27 Student
-
Upload
ieyra-shahera -
Category
Documents
-
view
218 -
download
0
Transcript of Chapter27 Student
-
7/25/2019 Chapter27 Student
1/29
Chapter 27. Current andChapter 27. Current and
ResistanceResistanceA PowerPoint Presentation byA PowerPoint Presentation by
Paul E. Tippens, Professor ofPaul E. Tippens, Professor ofPhysicsPhysics
Southern Polytechnic StateSouthern Polytechnic State
UniersityUniersity
!2""7
-
7/25/2019 Chapter27 Student
2/29
#b$ecties%#b$ecties% After co&pletin'After co&pletin'this &odule, you should bethis &odule, you should be
able to%able to%( )e*ne)e*ne electric currentelectric currentandand
electro&otie forceelectro&otie force..
( +rite and apply+rite and apply #h&s law#h&s lawto circuitsto circuitscontainin' resistance and e&f.containin' resistance and e&f.
( )e*ne)e*ne resistiityresistiity of a &aterial andof a &aterial and
apply for&ulas for its calculation.apply for&ulas for its calculation.
( )e*ne and apply the concept of)e*ne and apply the concept of
te&perature coe-cient of resistancete&perature coe-cient of resistance..
-
7/25/2019 Chapter27 Student
3/29
Electric CurrentElectric Current
Electric currentElectric current is theis the
rate of the /ow of char'erate of the /ow of char'e
00throu'h a cross1sectionthrou'h a cross1section
AA in a unit of ti&ein a unit of ti&e tt..
QI
t=
QI
t=
1C1 A
1 s=
1C1 A
1 s=
#ne#ne a&perea&
pereAAis char'e /owin'is char'e /owin'
at the rate of oneat the rate of one coulo&b percoulo&b per
secondsecond..
AA
11+ire+ire
00
tt
-
7/25/2019 Chapter27 Student
4/29
E3a&ple 4.E3a
&ple 4.The electric current inThe electric current ina wire is 5 A. 6ow &any electronsa wire is 5 A. 6ow &any electrons/ow past a 'ien point in a ti&e of/ow past a 'ien point in a ti&e of s8 s8
9 5 A 9 5 A
;q
I q It
t
= =
;q
I q Itt
= =
-
7/25/2019 Chapter27 Student
5/29
Conentional CurrentConentional Current
&a'ine a char'ed capacitor with&a'ine a char'ed capacitor with Q = CVQ = CVthat is allowed tothat is allowed to
dischar'e.dischar'e.Electron /ow%Electron /ow%TheThe
direction of edirection of e11 /owin'/owin'
fro& : to .fro& : to .Conentional current%Conentional current%The &otion of ;The &otion of ;
fro& to : has sa&efro& to : has sa&e
e.
11
1
Electron/ow
1 1
e1
Conentional/ow
-
7/25/2019 Chapter27 Student
6/29
CurrentCurrent
The number of charges that pass through the area in timeThe number of charges that pass through the area in timettN = n(|N = n(|vv||t)(|A|)t)(|A|)
The total charge that passes through is q times this numberThe total charge that passes through is q times this number
q = qnq = qnvv AAtt
The current I =The current I =q/q/ tt
Current , I = qnCurrent , I = qn v Av A
Current Density , J = I /A = qnCurrent Density , J = I /A = qn vv
-
7/25/2019 Chapter27 Student
7/29
Current DensityCurrent Density
The current density (associated with one type of chargeThe current density (associated with one type of charge
carriers) is defined as a product of the drift velocity, thecarriers) is defined as a product of the drift velocity, the
concentration of charge carriers and the charge of the carriers:concentration of charge carriers and the charge of the carriers:
J
J vd
=nq
-
7/25/2019 Chapter27 Student
8/29
-
7/25/2019 Chapter27 Student
9/29
Electric Current in a ConductorElectric Current in a Conductor
In a conductor, current density is proportional to the electricIn a conductor, current density is proportional to the electricfield vectorfield vector
J E= (h!"s law)(h!"s law)
The constant of proportionalityThe constant of proportionality is called the conductivityis called the conductivityof the conductor.of the conductor.
#nder a steady flow of charged particles#nder a steady flow of charged particles
along a conductor, the current across anyalong a conductor, the current across any
cross section of the conductor has thecross section of the conductor has thesa!e value$sa!e value$
II
II
II
%e assign this value to the current in the%e assign this value to the current in the
conductor$conductor$
-
7/25/2019 Chapter27 Student
10/29
Electro&otie BorceElectro&otie Borce
AA source of electro&otie force =e&f>source of electro&otie force =e&f>is a deice that uses che&ical,is a deice that uses che&ical,
&echanical or other ener'y to proide&echanical or other ener'y to proide
the potential di
-
7/25/2019 Chapter27 Student
11/29
+ater Analo'y to EDB+ater Analo'y to EDB
&ow
pressure
Pump%ater
'igh
pressure
alve%ater
low
Constriction
Source ofEDB
Resistor
'igh
potential
&ow
potential
*witch
E
RI
1
TheThe source of e&fsource of e&f=pu&p> proides the=pu&p> proides the
olta'eolta'e=pressure> to force=pressure> to force electronselectrons=water> throu'h electric=water> throu'h electric resistanceresistance
=narrow constriction>.=narrow constriction>.
-
7/25/2019 Chapter27 Student
12/29
Electrical Circuit Sy&bolsElectrical Circuit Sy&bols
Electrical circuitsElectrical circuitsoften contain one oroften contain one or
&ore resistors 'rouped to'ether and&ore resistors 'rouped to'ether and
attached to an ener'y source, such asattached to an ener'y source, such as
a battery.a battery.The followin' sy&bols are oftenThe followin' sy&bols are often
used%used%
+ - + -- + - + -
+round attery-+
-esistor
-
7/25/2019 Chapter27 Student
13/29
Electric ResistanceElectric Resistance
Suppose we apply a constant potentialSuppose we apply a constant potentialdi
-
7/25/2019 Chapter27 Student
14/29
#h&s Gaw#h&s Gaw
#h&s law#h&s lawstates that the currentstates that the current throu'h a 'ien conductor is directlythrou'h a 'ien conductor is directly
proportional to the potential di
-
7/25/2019 Chapter27 Student
15/29
E3a&ple 2.E3a&ple 2. +hen a+hen a 1@1@battery isbattery is
connected to a li'ht, a current ofconnected to a li'ht, a current of 55&A&Ais obsered. +hat is theis obsered. +hat is theresistance of the li'ht *la&ent8resistance of the li'ht *la&ent8
Source ofEDB
R
I
1
V = 3 @
5 &A
-
7/25/2019 Chapter27 Student
16/29
A&&eterA&&eter@olt&eter@olt&eter RheostatRheostatSourceSource
of EDBof EDB
-heostat
A
Gaboratory Circuit Sy&bolsGaboratory Circuit Sy&bols
@ .!f
-
+
-
7/25/2019 Chapter27 Student
17/29
Bactors A
-
7/25/2019 Chapter27 Student
18/29
Bactors A
. The. The te&perature Tte&perature Tof the &aterial.of the &aterial.The hi'her te&peratures usuallyThe hi'her te&peratures usually
result inresult in hi'herhi'herresistances.resistances.
. The Hind of. The Hind of &aterial&aterial. ron has &ore. ron has &ore
electrical resistance than aelectrical resistance than a
'eo&etrically si&ilar copper'eo&etrically si&ilar copperconductor.conductor.
RRoo R I RR I R
oo
RRiiI RI RccCopperCopper ronron
-
7/25/2019 Chapter27 Student
19/29
Resistiity of a DaterialResistiity of a Daterial
TheThe resistivityresistivity is a property of ais a property of a&aterial that deter&ines its electrical&aterial that deter&ines its electrical
resistanceresistance RR..Recallin' thatRecallin' that RRis directlyis directly
proportional to len'thproportional to len'th LLandandinersely proportional to areainersely proportional to areaAA,,
we &ay write%we &ay write%
or
L RA
R A L = = or
L RA
R A L = =
The unit of resistiity is theThe unit of resistiity is the oh&1&eteroh&1&eter
==
&>
-
7/25/2019 Chapter27 Student
20/29
E3a&ple .E3a&ple .+hat+hat len'thlen'thLLof copper wireof copper wire
is re;uired to produce ais re;uired to produce a & & resistor8resistor8Assu&e the dia&eter of the wire isAssu&e the dia&eter of the wire is 44
&&&& and that the resistiityand that the resistiity of copper isof copper is4.72 3 4"4.72 3 4"1J1J ..&&..
LR
A=
LR
A=
Re;uired len'th is%Re;uired len'th is%
-
7/25/2019 Chapter27 Student
21/29
Te&perature Coe-cientTe&perature Coe-cient
Bor &ost &aterials, the resistanceBor &ost &aterials, the resistance RRchan'es in proportion to the initialchan'es in proportion to the initial
resistanceresistance Roand to the chan'e inand to the chan'e in
te&peraturete&perature tt..
/R R t = /R R t = Chan'e inChan'e in
resistance%resistance%
TheThe te&perature coe-cient ofte&perature coe-cient of
resistance,resistance, is the chan'e in resistanceis the chan'e in resistance
per unit resistance per unit de'reeper unit resistance per unit de'reechan'e of te&perature.chan'e of te&perature.
/
/
1; #nits:
C
R
R t
=
/
/
1; #nits:
C
R
R t
=
-
7/25/2019 Chapter27 Student
22/29
E3a&ple .E3a&ple . The resistance of a copperThe resistance of a copperwire iswire is ."" &."" &atat 2"2"""CC. +hat will be its. +hat will be its
resistance if heated toresistance if heated to J"J"""CC8 Assu&e8 Assuðatthat 9 "."" ?C9 "."" ?Coo..
-
7/25/2019 Chapter27 Student
23/29
ConductiityConductiity
AA ConductiityConductiity &easures of a&aterialKs ability to conduct anelectric current
1 1 or
= =
1 1 or
= =
-
7/25/2019 Chapter27 Student
24/29
Electric PowerElectric Power
Electric powerElectric powerPP is the rate at whichis the rate at whichelectric ener'y is e3pended, or worH perelectric ener'y is e3pended, or worH per
unit of ti&e.unit of ti&e.
V q
V
To char'e C% +orH 9To char'e C% +orH 9
;@;@ and
or! qV q" I
t t t= = =
SubstituteSubstitute q = It ,q = It ,
then%then%VIt"
t= P = VI
I
-
7/25/2019 Chapter27 Student
25/29
Calculatin' PowerCalculatin' Power
Usin' #h&s law, we can *nd electricUsin' #h&s law, we can *nd electric
powerpowerfro& any two of the followin'fro& any two of the followin'
para&eters%para&eters% currentcurrent II,, olta'eolta'eVV, and, andresistanceresistanceRR..#h&s law%#h&s law% V = IRV = IR
0
0; ; V" VI " I R "R
= = =
0
0; ;V
" VI " I R "R
= = =
-
7/25/2019 Chapter27 Student
26/29
E3a&ple M.E3a&ple M. A power tool is rated atA power tool is rated at N AN Awhen used with a circuit that proideswhen used with a circuit that proides
42"[email protected]"1@.+hat power is used in operatin'+hat power is used in operatin'this tool8this tool8
E3a&ple 5.E3a&ple 5. A M""1+ heater draws aA M""1+ heater draws acurrent of 4" A. +hat is the resistance8current of 4" A. +hat is the resistance8
-
7/25/2019 Chapter27 Student
27/29
Su&&ary of Bor&ulasSu&&ary of Bor&ulas
QIt
=QIt
= 1C1 A1 s
=1C1 A1 s
=Electriccurrent%Electriccurrent%
; ;V V
I V IR R
R I
= = =; ;V V
I V IR R
R I
= = =
#h&s Gaw#h&s Gaw
1 volt-esistance: 1 oh!
1 a!pere=
1 volt-esistance: 1 oh!
1 a!pere=
-
7/25/2019 Chapter27 Student
28/29
Te&perature coe-cient ofresistance%
Te&perature coe-cient ofresistance%
Su&&ary =Cont.>Su&&ary =Cont.>
orL RARA L
= = orL RARA L
= =
/
/
1; #nits:
C
R
R t
=
/
/
1; #nits:
C
R
R t
=
00; ;
V" VI " I R "
R= = =
00; ;
V" VI " I R "
R= = =
/R R t =
ResistiityResistiityofof
&aterials%&aterials%
ResistiityResistiityofof
&aterials%&aterials%
ElectricPower P:
ElectricPower P:
-
7/25/2019 Chapter27 Student
29/29
C#OCGUS#O% Chapter 27C#OCGUS#O% Chapter 27
Current and ResistanceCurrent and Resistance