1. Introduction to Electricity for CST 162 LAB
Chapter 1
Introduction to Electricity
for CST 162 LAB
Slide content from “Circuit Analysis: Theory and Practice” 4th Ed., by Robbins and Miller, 2007

2. System International (SI) System of Units
Electric Current
Ampere (A)
Electric Voltage
Volt (V)
Electrical Resistance
Ohm (Ω)

3. Prefixes
Metric Prefixes are used for convenience

4. Significant Digits and Numerical Accuracy
Digits that carry information
It is a common error to show more digits of accuracy than are warranted.

5. Circuit Diagrams Electric circuits
Use batteries and resistors as components
Circuit diagrams are used on paper
Three types of circuit diagrams are used
Pictorial, block, and schematic

6. Pictorial Diagrams
Help visualize circuits by showing components as they actually appear

7. Block Diagrams
Blocks represent portions of a system

8. Schematic Diagrams

9. Chapter 2
Voltage and Current

10. Atomic Theory Atom Electrons are negative, protons are positive
Contains a nucleus of protons and neutrons
Nucleus is surrounded by a group of orbiting electrons
Electrons are negative, protons are positive

11. Atomic Theory Electrically neutral atom Ion
Equal number of electrons and protons
Ion
An atom with an excess or deficit of electrons

12. Conductors Materials with a large numbers of free electrons
Metals are good conductors because they have few loosely bound valence electrons

13. Conductors
Excellent conductors
Silver
Gold
Copper
Aluminum

14. Electrical Charge Unit of charge is the coulomb (C) One coulomb =
6.24 × 1018 electrons (or protons)
The charge on one electron (or proton) =
1/ 6.24 × 1018 or 1.6 × C

15. Voltage When two objects have a difference in charges
They have a “potential difference”
or “voltage” between them
Unit of voltage is the volt
Thunderclouds
Millions of volts between them

16. Voltage Difference in potential energy Voltage between two points =
One volt, “if it requires one joule of energy to move one coulomb of charge from one point to another”

17. Voltage
V = Work/Charge
Voltage is always measured between two points

18. Current Movement of charge is electric current
More electrons per second passing through a circuit, the greater the current
Current is rate of flow of charge

19. Current Unit of current is ampere (A) One ampere =
Current in a circuit when one coulomb of charge passes a given point in one second
Current = Charge/time
I = Q/t

20. Current Electron current flow Conventional current flow
Electrons flow from the negative terminal of a battery to the positive terminal
Conventional current flow
We may also assume currents flow from positive to negative

21. Current
Conventional current flow is used in this course, and in our field of study

22. How to Measure Voltage Place voltmeter leads across components
Red lead is positive
Black lead is negative
If leads are reversed, you will read the opposite polarity

23. How to Measure Current Measurable current must pass through meter
Open the circuit (i.e. disconnect wires) and insert the ammeter, so that the current now flows through the meter
Connect with correct polarity

24. Chapter 3
Resistance

25. Resistance of Conductors
Resistance of material is dependent on several factors:
Type of Material
Length of the Conductor
Cross-sectional area
Temperature

26. Type of Material
Atomic differences of materials cause variations in how electron collisions affect resistance
Differences produce resistivity

27. Length Resistance of a conductor  = length
Directly proportional to its length
If you double the length of the wire, the resistance will double
 = length
In meters or feet

28. Area Resistance of a conductor If cross-sectional area is doubled
Inversely proportional to cross-sectional area of the conductor
If cross-sectional area is doubled
Resistance will be one half as much

29. Fixed Resistors
Resistance of a fixed resistor is constant over a wide temperature range
Rated by amount of resistance
Measured in ohms (Ω)
Also rated by power
Measured in watts (W)

30. Fixed Resistors Different resistors for different applications
Molded carbon composition
Carbon film
Metal film
Metal Oxide
Wire-Wound
Integrated circuit packages

31. Variable Resistors Resistance may be changed (varied)
Adjust volume, set level of lighting, adjust temperature
Have three terminals
Center terminal connected to wiper arm
Potentiometers (normally abbreviated to just “Pot”)
Rheostats

32. Color Code Colored bands on a resistor provide a code for determining
Value
Tolerance
Reliability

33. Reading color codes

34. Measuring Resistance Use an Ohmmeter
Remove all power sources to circuit
Isolate component to be measured
Connect probes across component
No need to worry about polarity
Ohmmeter determines shorts and opens in individual components

35. Chapter 4
Ohm’s Law
and Energy

36. Ohm’s Law Current in a resistive circuit
Directly proportional to its applied voltage
Inversely proportional to its resistance

37. Ohm’s Law For a fixed resistance For a fixed voltage
Doubling voltage doubles the current
For a fixed voltage
Doubling resistance halves the current

38. Ohm’s Law Also expressed as E = IR and R = E/I
Express all quantities in base units of volts, ohms, and amps or utilize the relationship between prefixes

39. Ohm’s Law in Graphical Form
Linear relationship between current and voltage
y = mx
y is the current
x is the voltage
m is the slope

40. Ohm’s Law in Graphical Form
Slope (m) determined by resistor conductance

41. Ohm’s Law in Graphical Form

42. Open Circuits Current can only exist where there is a conductive path
An “Open circuit” is defined when there is no conductive path

43. Open Circuits If I = 0 An open circuit has infinite resistance
Ohm’s Law gives R = E/I = E/0  infinity
An open circuit has infinite resistance

44. Voltage Symbols Voltage sources Voltage drops V = I*R Uppercase E
Uppercase V
V = I*R
“IR” drops

45. Voltage Polarities
Polarity of voltage drops across resistors is important in circuit analysis
Drop is + to – in the direction of conventional current
To show this, place plus sign at the tail of current arrow

46. Voltage Polarities

47. Current Direction
Current usually proceeds out of the positive terminal of a voltage source
If the current is actually in this direction, it will be supplying power to the circuit

48. Current Direction
If the current is in the opposite direction (going into the positive terminal), it will be absorbing power (like a resistor)

49. Current Direction
See two representations of the same current on next slide
Notice that a negative current actually proceeds in a direction opposite to the current arrow

50. Current Direction

51. Power Rating of Resistors
Resistors must be able to safely dissipate their heat without damage
Common power ratings of resistors are 1/8, 1/4, 1/2, 1, or 2 watts

52. Law of Conservation of Energy
Energy can neither be created nor destroyed
Converted from one form to another
Examples:
Electric energy into heat
Mechanical energy into electric energy

53. Law of Conservation of Energy
Energy conversions
Some energy may be dissipated as heat, giving lower efficiency

54. Chapter 5
Series Circuits

55. Series Circuits Two elements in a series
Connected at a single point
No other current-carrying connections at this point
A series circuit is constructed by connecting various elements in series

56. Series Circuits Normally
Current will leave the positive terminal of a voltage source
Move through the resistor(s)
Return to negative terminal of the source

57. Series Circuits Current is similar to water flowing through a pipe
Current leaving the element must be the same as the current entering the element
Same current passes through every element of a series circuit

58. Series Circuits
The laws, theorems, and rules that you apply to DC circuits
Also apply to AC circuits

59. Kirchhoff’s Voltage Law (KVL)
The algebraic sum of the voltage that rises and drops around a closed loop is equal to zero
ET - V1 - V2 - V3 - ∙∙∙ - Vn = 0

60. Kirchhoff’s Voltage Law (KVL)
Another way of stating KVL is:
Summation of voltage rises is equal to the summation of voltage drops around a closed loop
V1 + V2 + V3 + ∙∙∙ + Vn = ET

61. Resistors in Series Most complicated circuits can be simplified
For a series circuit
V1 + V2 + V3 = E
IR1 + IR2 + IR3 = E
I(R1 + R2 + R3 )= E
I(R1 + R2 + R3 )= IRtotal (Note: I’s cancel)

62. Resistors in Series
Total resistance in a series circuit is the sum of all the resistor values

63. Interchanging Series Components
Order of series components
May be changed without affecting operation of circuit
Sources may be interchanged, but their polarities can not be reversed
After circuits have been redrawn, it may become easier to visualize circuit operation

64. Circuit Ground Ground One type of grounding is chassis ground
Point of reference or a common point in a circuit for making measurements
One type of grounding is chassis ground
In this type of grounding
Common point of circuit is often the metal chassis of the piece of equipment

65. Circuit Ground Chassis ground Earth ground
Often connected to Earth Ground
Earth ground
Physically connected to the earth by a metal pipe or rod

66. Circuit Ground
If a fault occurs within a circuit, the current is redirected to the earth
Voltages are often measured with respect to ground

67. Ammeter Loading Effects
An ammeter is placed in a circuit to make a current measurement
Resistance in the meter will affect the circuit
Amount of loading is dependent upon the instrument and the circuit

68. Ammeter Loading Effects
If resistance of the meter is small compared to the resistance of the circuit, the loading effect will be small

69. Chapter 6
Parallel Circuits

70. Parallel Circuits House circuits contain parallel circuits
The parallel circuit will continue to operate even though one component may be “open”
Only the “open” or “defective” component will no longer continue to operate
A light bulb with a broken filament = “open”

71. Parallel Circuits

72. Parallel Circuits Elements in parallel Elements between nodes
When they have exactly two nodes in common
Elements between nodes
Any device like resistors, light bulbs, etc.
Elements connected in parallel
Same voltage across them

73. Parallel Circuits

74. Series - Parallel Circuits
Circuits may contain a combination of series and parallel components
Being able to recognize the various connections in a network is an important step in analyzing these circuits

75. Series - Parallel Circuits

76. Parallel Circuits To analyze a particular circuit
First identify the node
Next, label the nodes with a letter or number
Then, identify types of connections

77. Parallel Circuits

78. Kirchhoff’s Current Law (KCL)
The algebraic sum of the currents entering and leaving a node is equal to zero

79. Kirchhoff’s Current Law (KCL)
Currents entering the node are taken to be positive, leaving are taken to be negative
Sum of currents entering a node is equal to the sum of currents leaving the node

80. Kirchhoff’s Current Law (KCL)
An analogy:
When water flows in a pipe, the amount of water entering a point is equal to the amount leaving that point

81. Resistors in Parallel
Voltage across all parallel elements in a circuit will be the same

82. Resistors in Parallel
For a circuit with 3 resistors: IT = I1 + I2 + I3

83. Resistors in Parallel
Total resistance of resistors in parallel will always be less than resistance of smallest resistor

84. Equal Resistors in Parallel
Total resistance of equal resistors in parallel is equal to the resistor value divided by the number of resistors

85. Two Resistors in Parallel
For only two resistors connected in parallel, the equivalent resistance may be found by the product of the two values divided by the sum
Often referred to as “product over the sum” formula

86. Three Resistors in Parallel
For three resistors in parallel:
Rather than memorize this long expression
Use basic equation for resistors in parallel

87. Voltmeter Loading Effects
A voltmeter
Meter movement in series with a current-limiting resistance
If resistance is large compared with the resistance across which the voltage is to be measured, the voltmeter will have a very small loading effect

88. Voltmeter Loading Effects
If this resistance is more than 10 times the resistance across which the voltage is being measured, the loading effect can generally be ignored.
However, it is usually much higher.