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Chapter 2Chapter 2

© Copyright 2007 Prentice-HallElectric Circuits Fundamentals - Floyd

Chapter 2

Chapter 2Chapter 2


SummaryThe Bohr atom

•The nucleus is positively charged and has the protons and neutrons.

Electron Proton Neutron

•The atomic number is the number of protons and determines the particular element.•In the neutral atom, the number of electrons is equal to the number of protons.

•Electrons are negatively charged and in discrete shells.

The Bohr atom is useful for visualizing atomic structure.

Chapter 2Chapter 2


SummaryThe valence shell

Shell 3+ Shell 1 Shell 2

A neutral Si atom is shown. There are 4 electrons in the valence shell.

Is Si a conductor, insulator, or semiconductor? Semiconductor

The outer shell is called the valence shell. Electrons in this shell are involved in chemical reactions and they account for electrical and thermal conductivity in metals.

Chapter 2Chapter 2


SummaryElectrical charge

There is a force (F) between electrical charges. Like charges repel; unlike charges attract.• The force is directly proportional to charge.• The force is inversely proportional to square of distance.

+ + +_

Chapter 2Chapter 2


Summary

e-

Force is required to move a charge against the electric field.

Voltage is the work per charge done against the electric field.

Voltage

+++++++++

−−−−−−−−−

When force is applied over a distance, work is done. Work done in moving a charge against the electric field leads to the definition of voltage:

Chapter 2Chapter 2


SummaryVoltage

The defining equation for voltage is

WVQ

=

One volt is the potential difference (voltage) between two points when one joule of energy is used to move one coulomb of charge from one point to the other.

Chapter 2Chapter 2


SummaryVoltage

Voltage is responsible for establishing current.

Zinc(anode)

Zn + 2eZn2+

ZnSO4solution

Porousbarrier

Copper(cathode)Cu2+ + 2e

CuSO4solution

Cu

+e

e

-

- -

-Sources of voltage include batteries, solar cells, and generators. A Cu-Zn battery, such as you might construct in a chemistry class, is shown.

Chapter 2Chapter 2


SummaryVoltage

Ideally, a voltage source can provide a constant voltage for any current required by a circuit.

I

V

The IV curve for an ideal voltage source has a constant voltage for all current.In practice, ideal sources do not exist, but they can be closely approximated by actual sources.

Chapter 2Chapter 2


SummaryCurrent

Current (I) is the amount of charge (Q) that flows past a point in a unit of time (t). The defining equation is:

QIt

=

One ampere is a number of electrons having a total charge of 1 Cmoving through a given cross section in 1 s.

0.4 AWhat is the current if 2 C passes a point in 5 s?

Chapter 2Chapter 2


SummaryCurrent

Ideally, a current source can provide a constant current for any load.

I

V

The IV curve for an ideal current source has a constant current as indicated by the straight line.

Chapter 2Chapter 2


SummaryResistance

Resistance is the opposition to current. One ohm (1 Ω) is the resistance if one ampere (1 A) is in a material when one volt (1 V) is applied. Conductance is the reciprocal of resistance.

1GR

=

Components designed to have a specific amount of resistance are called resistors. Color bands

Resistance material(carbon composition)

Insulation coating

Leads

Chapter 2Chapter 2


SummaryColor

Black

Brown

Red

Orange

Yellow

Green

Blue

Violet

Gray

White

Gold

Silver

No band

0

1

2

3

4

5

6

7

8

9

±5%

± 10%

Digit

± 20%

100

101

102

103

104

105

106

107

108

109

10-1

10-2

Multiplier

1% (five band)

5% (four band)

Tolerance

2% (five band)

10% (four band)

Resistance value, first three bands:

First band – 1st digit

Second band – 2nd digit

*Third band – Multiplier (number of zeros following second digit)

Fourth band - tolerance

Resistance color-code

* For resistance values less than 10 Ω, the third band is either gold or silver. Gold is for a multiplier of 0.1 and silver is for a multiplier of 0.01.

Chapter 2Chapter 2


Summary

What is the resistance and tolerance of each of the four-band resistors?

5.1 kΩ ± 5%

820 kΩ ± 5%

47 Ω ± 10%

1.0 Ω ± 5%

Chapter 2Chapter 2


Summary

Alphanumeric Labeling

• Two or three digits, and one of the letters R, K, or M are used to identify a resistance value.

• The letter is used to indicate the multiplier, and its position is used to indicate decimal point position.

Chapter 2Chapter 2


SummaryVariable resistors

Variable resistors include the potentiometer and rheostat. The center terminal of a variable resistor is connected to the wiper.

13

2

Resistiveelement

Wiper

Shaft

R

Variable resistor (potentiometer)

R

Variable resistor (rheostat)

To connect a potentiometer as a rheostat, one of the outside terminals is connected to the wiper.

Chapter 2Chapter 2


SummaryWire resistance

Sometimes, the resistance of wires must be accounted for. The equation for wire resistance is: lR

Aρ

=

where ρ = resistivity in CM-Ω/ftl = length in feetA = cross sectional area in circular mils (CM)

What is the resistance of 400 feet of 22 gage copper wire? The area is 642 CM and the resistivity of copper is 10.37 CM-Ω/ft. The table value for resistance/1000 feet of 22 gage wire is 16.14 Ω/1000 feet.

By proportion, the resistance of 400 feet is 0.4 x 16.14 Ω = 6.46 ΩBy the equation, ( )( )10.37 -CM/ft 400 ft

642 CMlR

Aρ Ω

= = = 6.46 Ω

Chapter 2Chapter 2


SummaryThe electric circuit

A basic electric circuit consists of 1) a voltage source2) a path 3) a load.

Switch Metal strip

Metal reflector Spring

An example of a basic circuit is a flashlight, which has each of these plus a control element – the switch.

Chapter 2Chapter 2


SummaryThe electric circuit

Circuits are described pictorially with schematics. For example, the flashlight can be represented by

Switch

Battery (2 cells)

Lamp

Chapter 2Chapter 2


SummarySwitches

Switches are commonly used to control circuits by either mechanical or electronic means.

The pole refers to the movable arm of a switch.The throw refers to the number of contacts that are affected by a single switch action.

SPST SPDT DPST DPDT

Chapter 2Chapter 2


SummaryThe DMM

VHz

10 A

40 mA

OFF

mV

A

V

H

H

V H

COM

VΩ

The DMM (Digital Multimeter) is an important multipurpose instrument which can measure voltage, current, and resistance. Many include other measurement options.

Chapter 2Chapter 2


SummaryAnalog meters

Photo courtesy of Triplett Corporation

An analog multimeter is also called a VOM (volt-ohm-milliammeter). Analog meters measure voltage, current, and resistance. The user must choose the range and read the proper scale.

Chapter 2Chapter 2


Ampere

AWG

Charge

Circuit

The unit of electrical current.

(American Wire Gauge) A standardization based on wire diameter.An electrical property of matter that exists because of an excess or a deficiency of electrons. Charge can be either + or −.

An interconnection of electronic components designed to produce a desired result. A basic circuit consists of a source, a load, and an interconnecting path.

Selected Key Terms

Chapter 2Chapter 2


Conductance

Coulomb

Current

Electron

Ground

Ohm (Ω)

The ability of a circuit to allow current. The unit is the siemans (S).

The unit of electrical charge.

The rate of flow of electrical charge.

A basic particle of electrical charge in matter. The electron possesses a negative charge.

Selected Key Terms

The common or reference point in a circuit.

The unit of resistance.

Chapter 2Chapter 2


Potentiometer

Resistance

Rheostat

Siemens

Volt

Voltage

A three-terminal variable resistor.

The opposition to current. The unit is the ohm (Ω).

A two-terminal variable resistor.

Selected Key Terms

The unit of conductance.

The unit of voltage or electromotive force.

The amount of energy per charge available to move electrons from one point to another in an electric circuit.

Chapter 2Chapter 2


Quiz

1. The atomic number is the number of

a. protons in the nucleus

b. neutrons in the nucleus

c. protons plus neutrons in the nucleus

d. electrons in the outer shell

Chapter 2Chapter 2


Quiz

2. Valence electrons are

a. in the outer shell

b. involved in chemical reactions

c. relatively loosely bound

d. all of the above

Chapter 2Chapter 2


Quiz

3. The atomic particle responsible for electrical current in solid metallic conductors is the

a. proton

b. electron

c. neutron

d. all of the above

Chapter 2Chapter 2


Quiz

4. The symbol for charge is

a. C

b. Ω

c. Q

d. W

Chapter 2Chapter 2


Quiz

5. The definition for voltage is a.

b.

c.

d.

QVt

=

WVt

=

WVQ

=

V It=

Chapter 2Chapter 2


Quiz

6. A battery stores

a. electrons

b. protons

c. ions

d. chemical energy

Chapter 2Chapter 2


Quiz

7. The unit of conductance is the

a. ohm

b. coulomb

c. siemen

d. ampere

Chapter 2Chapter 2


Quiz

8. A four-color resistor with the color bands gray-red-black-gold is

a. 73 Ω

b. 82 Ω

c. 680 Ω

d. 820 Ω

Chapter 2Chapter 2


Quiz

9. A 330 kΩ ± 5% resistor has the color bands

a. red-red-brown-gold

b. orange-orange-yellow-gold

c. yellow-yellow-red-gold

d. yellow-yellow-green-gold

Chapter 2Chapter 2


Quiz

10. The circular mil is a unit of

a. length

b. area

c. volume

d. resistance

Chapter 2Chapter 2


Quiz

Answers:

1. a

2. d

3. b

4. c

5. c

6. d

7. c

8. b

9. b

10. b

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