Chapter 31. Current and Resistance Chapter …physics.gsu.edu/dhamala/Phys2212/Slides/Chapter31.pdfTitle Microsoft PowerPoint - Chapter31 [Compatibility Mode] Author dhamala Created

Chapter 31. Current and Resistance

Lights, sound systems,

microwave ovens, and

computers are all connected

by wires to a battery or an

electrical outlet. How and

why does electric current

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.

why does electric current

flow through a wire?

Chapter Goal: To learn

how and why charge moves

through a conductor as what

we call a current.

1

Topics:

• The Electron Current

• Creating a Current

• Current and Current Density

Chapter 31.Chapter 31. Current and ResistanceCurrent and Resistance


• Conductivity and Resistivity

• Resistance and Ohm’s Law

2

Chapter 31. Reading QuizzesChapter 31. Reading Quizzes


Chapter 31. Reading QuizzesChapter 31. Reading Quizzes

3

What quantity is represented by the

symbol J ?

A. Resistivity


A. Resistivity

B. Conductivity

C. Current density

D. Complex impedance

E. Johnston’s constant

4

A. Resistivity

What quantity is represented by the

symbol J ?


A. Resistivity

B. Conductivity

C. Current density

D. Complex impedance

E. Johnston’s constant

5

The electron drift speed in a typical

current-carrying wire is

A. extremely slow (≈10–4 m/s).


A. extremely slow (≈10 m/s).

B. moderate (≈ 1 m/s).

C. very fast (≈104 m/s).

D. Could be any of A, B, or C.

E. No numerical values were provided.

6

A. extremely slow (≈10–4 m/s).

The electron drift speed in a typical

current-carrying wire is


A. extremely slow (≈10 m/s).

B. moderate (≈ 1 m/s).

C. very fast (≈104 m/s).

D. Could be any of A, B, or C.

E. No numerical values were provided.

7

All other things being equal, current will

be larger in a wire that has a larger value

of

A. conductivity.


A. conductivity.

B. resistivity.

C. the coefficient of current.

D. net charge.

E. potential.

8

All other things being equal, current will

be larger in a wire that has a larger value

of

A. conductivity.


A. conductivity.

B. resistivity.

C. the coefficient of current.

D. net charge.

E. potential.

9

The equation I = ∆V/R is called

A. Ampère’s law.

B.Faraday’s law.


B.Faraday’s law.

C. Ohm’s law.

D. Weber’s law.

10

The equation I = ∆V/R is called

A. Ampère’s law.

B. Farady’s law.


B. Farady’s law.

C. Ohm’s law.

D. Weber’s law.

11

The Electron Current

The electron current I is the number of electrons per second that pass through a cross section of a wire. The units of electron current are s-1.


12

electron current are s .

tiN e ∆=

The Electric Current

tnAvxnAnVN ∆=∆==

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.13

tnAvxnAnVN de ∆=∆==

dnAvi =The drift speed vd is the net speed with which the electrons move, not the speed at which any one electron is bouncing around.

tnAvtiN de ∆=∆=

The Electron Current

dnAvi =Typical V is about 10-4 m/s.


14

Typical Vd is about 10-4 m/s.

The Law of Conservation of Current: the electron

current is the same at all points in a current-

carrying wire.


15The electron current at A is exactly equal to the electron current at B.

How long does it take to discharge

a capacitor?

0.2 [m] /10-4 [m/s] = 2000 s?


16

How long does it take to discharge

a capacitor?

tnAvtiN de ∆=∆=

e

nAv

Nt =∆


ssmmm

t10

426328

11

109]/[10][10414.3][105.8

10 −

−−−⋅=

⋅⋅⋅⋅⋅=∆

dnAvt =∆

Establishing the Electric Field in a

Wire


18

A Model of Conduction

m

eE

m

Fax ==


tm

eEvtavv ixxixx ∆+=∆+=

The energy transfer is

the “friction” that raises

the temperature of the

wire.



tm

eEvtavv ixxixx ∆+=∆+=

τm

eEvvv ixd +==

τm

eEvd =


τm

eEvd =

dnAvi =


21

m

AEnei

τ=

The electron current is directly proportional to the electric field strength.

Current and Current Density

),( Eofdirectiontheindt

dQI

r≡

1 Ampere = 1 A = 1 coulomb per second = 1 C/s


22

eit

eN

t

QI e =

∆=

∆=

The direction of the current I in a metal is opposite the direction of motion of the electrons.

The current direction in a wire is from the positive terminal of a battery to the negative terminal.

Conservation of Current

∑∑ = outin II


The Current Density in a Wire

AneveiI d==

dnevA

IdensitycurrentJ ===


dnevA

densitycurrentJ ===

JAI =

Conductivity and Resistivity

neEe ττ 2

===

EJ σ=


Em

ne

m

EenenevJ d

ττ)( ===

m

netyconductivi

τσ

2

==

τσρ

2

1

ne

myresistivit ===

Conductivity and Resistivity


26

Resistance and Ohm’s Law

The resistance of a long, thin conductor of length L and

cross=sectional area A is


The SI unit of resistance is the ohm. 1 ohm = 1 Ω = 1 V/A.

The current through a conductor is determined by the

potential difference ∆V along its length:

27

Ohm’s Law

• Ohm’s law is limited to those materials whose

resistance R remains constant—or very nearly so—during

use.

• The materials to which Ohm’s law applies are

called ohmic.


called ohmic.

• The current through an ohmic material is

directly proportional to the potential difference. Doubling

the potential difference doubles the current.

• Metal and other conductors are ohmic devices.

28


29

Chapter 31. Summary Slides


Chapter 31. Summary Slides

30

General PrinciplesGeneral Principles


31

General Principles


32

General Principles


33

Important Concepts


34

Important Concepts


35

Important Concepts


36

Applications


37

Chapter 31. Questions


Chapter 31. Questions

38

These four wires are made of the same metal. Rank

in order, from largest to smallest, the electron

currents ia to id.


A. id > ia > ib > ic

B. ib = id > ia = ic

C. ic > ib > ia > id

D. ic > ia = ib > id

E. ib = ic > ia = id

39

These four wires are made of the same metal. Rank

in order, from largest to smallest, the electron

currents ia to id.


A. id > ia > ib > ic

B. ib = id > ia = ic

C. ic > ib > ia > id

D. ic > ia = ib > id

E. ib = ic > ia = id

40

Why does the light in a room come on

instantly when you flip a switch several meters

away?

A. Electrons travel at the speed of light through the

wire.

B. Because the wire between the switch and the bulb

is already full of electrons, a flow of electrons

from the switch into the wire immediately causes


from the switch into the wire immediately causes

electrons to flow from the other end of the wire

into the lightbulb.

C. The switch sends a radio signal which is received

by a receiver in the light which tells it to turn on.

D. Optical fibers connect the switch with the light, so

the signal travels from switch to the light at the

speed of light in an optical fiber.

41

Why does the light in a room come on

instantly when you flip a switch several meters

away?

A. Electrons travel at the speed of light through the

wire.

B. Because the wire between the switch and the

bulb is already full of electrons, a flow of

electrons from the switch into the wire


electrons from the switch into the wire

immediately causes electrons to flow from the

other end of the wire into the lightbulb.

C. The switch sends a radio signal which is received

by a receiver in the light which tells it to turn on.

D. Optical fibers connect the switch with the light, so

the signal travels from switch to the light at the

speed of light in an optical fiber.

42

What are the

magnitude and the

direction of the

current in the fifth

wire?


A. 15 A into the junction

B. 15 A out of the junction

C. 1 A into the junction

D. 1 A out of the junction

E. Not enough data to determine

43

What are the

magnitude and the

direction of the

current in the fifth

wire?


A. 15 A into the junction

B. 15 A out of the junction

C. 1 A into the junction

D. 1 A out of the junction

E. Not enough data to determine

44

Documents

Chapter 31. Current and Resistance Chapter …physics.gsu.edu/dhamala/Phys2212/Slides/Chapter31.pdfTitle Microsoft PowerPoint - Chapter31 [Compatibility Mode] Author dhamala Created