Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic...
-
Upload
sheila-madison-carr -
Category
Documents
-
view
219 -
download
0
Transcript of Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic...
![Page 1: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/1.jpg)
Current and ResistanceCurrent and Resistance
October 9, 2006October 9, 2006
![Page 2: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/2.jpg)
Notes Notes
New topic today – Current and New topic today – Current and ResistanceResistance
Quiz on FridayQuiz on FridayFriday 7:30 review sessiomn.Friday 7:30 review sessiomn.There is a new WebAssign.There is a new WebAssign.
Is it AM or PM … only the shadow knows Is it AM or PM … only the shadow knows for sure.for sure.
I know too.I know too.
![Page 3: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/3.jpg)
New Topic:Current and Resistance
![Page 4: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/4.jpg)
Physical Resistors
![Page 5: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/5.jpg)
More …
![Page 6: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/6.jpg)
What Happens?
“+”
“+”
“+”
“+”
REMEMBER, THE ELECTRONS
ARE ACTUALLY MOVING THE
OTHER WAY!-
-
![Page 7: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/7.jpg)
DEFINITION
Current is the motion of POSITIVE CHARGE through a circuit. Physically, it is electrons that move but …
Conducting material
Q,t
![Page 8: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/8.jpg)
Conducting material
Q,t
dt
dqi
ort
Qi
CURRENT
![Page 9: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/9.jpg)
UNITS
A current of one coulomb per second is defined as ONE AMPERE.
![Page 10: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/10.jpg)
A small sphere that carries a charge q is whirled in a circle at the end of an insulating string. The angular frequency of rotation is ω. What average current does this rotating charge represent?
Fraggen …..
![Page 11: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/11.jpg)
ANOTHER DEFINITION
A
I
area
currentJ
![Page 12: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/12.jpg)
Figure P27.8 represents a section of a circular conductor of non-uniform diameter carrying a current of 5.00 A. The radius of cross section A1 is 0.400 cm. (a) What is the magnitude of the current density across A1? (b) If the current density across A2 is one-fourth the value across A1, what is the radius of the conductor at A2?
![Page 13: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/13.jpg)
Ohm A particular object will resist the flow of current.
It is found that for any conducting object, the current is proportional to the applied voltage.
STATEMENT: V=IR R is called the resistance of
the object. An object that allows a
current flow of one ampere when one volt is applied to it has a resistance of one OHM.
![Page 14: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/14.jpg)
Graph
![Page 15: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/15.jpg)
A DIODE
Resistance Varies with Applied Voltage (actually with current)
![Page 16: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/16.jpg)
Let’s look at the atomic level ..
Conduction is via electrons. They are weak and small and don’t exercise
much. Positive charge is big and strong and doesn’t
intimidate easily. It’s an ugly situation … something like ……
![Page 17: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/17.jpg)
-
+
![Page 18: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/18.jpg)
Consider a metal conductor
So far, we have said that a metal is an equipotential because no charges were moving. Hence, no electric field in the metal You can move a charge freely in the metal BECAUSE there
is no electric field.
NOW we have a current. This can only happen if we allow an electric field to push
the charges. Thus, the metal is NO LONGER A TRUE
EQUIPOTENTIAL. But almost …. as we shall see in the next chapter.
![Page 19: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/19.jpg)
l
VV ab E
![Page 20: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/20.jpg)
The Current
Electrons are going the opposite way from the current. (WHY?)
They probably follow a path like …
Average “drift”speed - vd
OUTIN
![Page 21: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/21.jpg)
Notation
vd average drift velocity of the electron n number of electrons (mobile) per unit
volume. t interval of time x average distance the electron moves
in time t. Q total amount of CHARGE that goes
through a surface of the conductor in time t.
![Page 22: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/22.jpg)
dvJ ne
nevA
IJ
enAvt
QI
etnAvQ
davg
davg
d
)(
Often a Vector
The Diagram
![Page 23: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/23.jpg)
We return to the diagram ….. Consider an electron. Assume that whenever it
“bumps” into something it loses its momentum and comes to rest.
It’s velocity therefore starts at zero, the electric field accelerates it until it has another debilitating collision with something else.
During the time it accelerates, its velocity increases linearly .
The average distance that the electron travels between collisions is called the “mean free path”.
![Page 24: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/24.jpg)
Starting when the electron is at rest:
m
eEvv
m
eE
m
Fa
atatvv
d
0
Let n= number of charge carriersper unit volume (mobile electrons)
We showed two slides ago::
1
2
2
m
ne
so
Em
EneJ
orm
eEnenevnqvJ dd
![Page 25: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/25.jpg)
Finally
dv
![Page 26: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/26.jpg)
Reference
The average drift velocity of an electron is about 10-4 m/s
![Page 27: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/27.jpg)
Ponder
How can a current go through a resistor and generate heat
(Power) without decreasing the current itself?
![Page 28: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/28.jpg)
Loses Energy
Gets it back
Exit
![Page 29: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/29.jpg)
Conductivity
In metals, the bigger the electric field at a point, the bigger the current density.
EJ is the conductivity of the material.
=(1/) is the resistivity of the material
![Page 30: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/30.jpg)
Going to the usual limit …
JdAI
anddA
dIJ
![Page 31: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/31.jpg)
Example
A cylindrical conductor of radius R has a current density given by
(a) J0 (constant)
(b) r
Find the total current in each case.
![Page 32: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/32.jpg)
)(1 00 TT
![Page 33: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/33.jpg)
Range of and
![Page 34: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/34.jpg)
Ye old RESISTANCE
IRVVA
lR
IA
lV
A
I
l
VE
El
VEJ
ElV
11
![Page 35: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/35.jpg)
REMEMBER
IRVA
LR
![Page 36: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/36.jpg)
A closed circuit
![Page 37: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/37.jpg)
Power
R
EIVRIP
RIIRIIVPPower
VIt
QV
t
22
2
W
:Power
QVW
:isbattery by the done
workofamount The battery. by theresistor the
throughpushed is Q charge a t, In time
![Page 38: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/38.jpg)
Ponder
How can a current go through a resistor and generate heat
(Power) without decreasing the current itself?
![Page 39: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/39.jpg)
Loses Energy
Gets it back
Exit
![Page 40: Current and Resistance October 9, 2006 Notes New topic today – Current and Resistance New topic today – Current and Resistance Quiz on Friday Quiz on.](https://reader033.fdocuments.in/reader033/viewer/2022051622/56649cf95503460f949ca754/html5/thumbnails/40.jpg)
A Circuit
)( rR
EI
E=emf