This document provides a lesson on electrical resistance. It defines resistance and discusses its characteristics such as how resistance is determined by material, length, cross-sectional area, and temperature. It also covers topics like resistor color codes, different types of resistors, and sample resistance calculation problems. The objectives are to define resistance, explain how wire sizes are identified, define resistors and their applications, learn to read resistor color codes, and differentiate between conductance and resistance.

1. LESSON 2:
RESISTANCE
Engr. Onofre E. Algara, Jr.
BS Electrical Engineering (DLSU-D)
MS Electrical Engineering (Mapua University)
Subject Instructor
COLEGIO DE MUNTINLUPA
ELECTRICAL ENGINEERING DEPARTMENT

2. Objectives:
1. Define resistance and discuss its characteristics
2. Explain the different ways Circular Wires are
identified
3. Define what resistors are and state some of its
application
4. Learn to read resistor color codes and other
types of resistance identification
5. Differentiate conductance with resistance

3. Resistance
• the property of a material to restrict or oppose the flow of electrons
• expressed in ohms, symbolized by the Greek letter omega (Ω)
• One ohm (1 Ω) of resistance exists if there is one ampere (1 A) of
current in a material when one volt (1 V) is applied across the material
Schematic Symbol
for Resistance

4. Resistance
• The resistance of any material with a uniform cross-sectional area is
determined by the following four factors:
• Material
• Length
• Cross Sectional Area
• Temperature
⚫ At a fixed temperature of 20°C (room temperature), the resistance
is related to the other three factors by
Where:
ρ - (Greek letter rho) resistivity of the material
l - length of the material
A - cross-sectional area of the material
EQN. 1

5. Resistance
• Resistivity - (specific resistance) resistance of a unit length and unit
cross section of a certain material
Resistivity () of various materials @ 20°C
Material -CM/ft Material -cm

6. Circular Wires
a)the higher the resistivity, the more the resistance
b)the longer the length of the conductor, the more the resistance
c) the smaller the area of the conductor, the more the resistance
d)the higher the temperature of the conductor, the more the resistance

7. d = 1mil
Circular Wires
• Cross-sectional Area of Conductors
• Circular Mil (CM) - the area of a circle
whose diameter is in mil
• Mil - is a unit of measurement for length
and is related to the inch by
• Square Mil (SM) - the area of a square
whose side is in mil
A = 1CM
A = 1 mil2 s = 1mil

8. Circular Wires
• Conversion of SM to CM and Vice Versa
• Note:
• to find the area in circular mils (CM), the diameter must be converted to mils
1 mil2
1mil
1CM
EQN. 2

9. Circular Wires
• Wire sizes are usually based on the wires cross-sectional area or its
diameter
• English System
• Wire sizes are expressed in terms of the AWG (American Wire Gage)
which uses circular mil (CM)
• Metric System
• Wire sizes are expressed in terms of millimeters (mm) diameter for
solid wires and square millimeters (mm2) area for stranded wires

10. Circular Wires
• American Wire Gage (AWG) Sizes

11. Circular Wires
• American Wire Gage (AWG) Sizes

12. Circular Wires
• From the PEC (Philippine Electrical Code)

13. Circular Wires
• From the PEC (Philippine Electrical Code)

14. Resistors
• component that is specifically designed to have a certain amount of
resistance
• principal applications of resistors are to limit current in a circuit, to divide
voltage, and, in certain cases, to generate heat
• Main categories:
• Fixed resistors are available with a large selection of
resistance values that are set during manufacturing and
cannot be changed easily
• Variable resistors are designed so that their resistance
values can be changed easily with a manual or an automatic
adjustment

15. Resistor Color Code: Four Band
• Four Band Resistor
Color Code
• fixed resistors with value
tolerances of 5% or 10%
are color coded with
four bands to indicate
the resistance value and
the tolerance
BAD
BOYS
RAGING
OUTRAGEOUS
YOUNG
GIRLS
BUT
VIOLETA
GOT
WISE
 20% No Band

16. Resistor Color Code: Four Band
• Start with the band closest to one end of the resistor, The first band is
the first digit of the resistance value, If it is not clear which is the
banded end, start from the end that does not begin with a gold or silver
band.
• The second band is the second digit of the resistance value.
• The third band is the number of zeros following the second digit. or the
multiplier.
• The fourth band indicates the percent tolerance and is usually gold or
silver. 1st band = black = 0
2nd band = brown = 1
3rd band = red = 2
4th band = gold = 10% tolerance
The resistance R = 100Ω +/- 10%
( the resistance is from 90Ω to 110Ω)

17. Resistor Color Code: Five Band

18. Resistor Color Code
1st band = gray = 8
2nd band = red = 2
3rd band = black = 0
4th band = yellow = 0000
5th band = brown = 1%
the first two bands gives 82
the third band makes 820
the fourth band gives a result of 8200000
The fifth bands implies a tolerance of +/- 1% hence the resistance R will
be anywhere between 8.118M to 82.82M  ( 8200000 – 82000 and 8200000 + 82000)
1st digit
2nd digit
3rd digit
zeros to be added
tolerance

19. Resistor Label Codes
Numeric Labeling
• uses three digits to indicate the resistance value
• first two digits give the first two digits of the resistance
value
• third digit gives the multiplier or number of zeros that
follow the first two digits
• code is limited to values of 100 or greater

20. Resistor Label Codes
Alphanumeric Labeling
• three- or four-character label that uses both digits and
letters
• label typically consists of only three digits or two or three
digits and one of the letters R, K, or M
• letter is used to indicate the multiplier, and the position of
the letter indicates the decimal point placement
• letter R indicates a multiplier of 1 (no zeros after the digits)
• K indicates a multiplier of 1000 (three zeros after the digits)
• M indicates a multiplier of 1,000,000 (six zeros after the digits).

21. Conductance
• a measure of the ease with which current is established
• the reciprocal of resistance and is symbolized by G
• unit is siemens, abbreviated as S
• in equation form,
Relationship between
Resistance (R) and
Conductance (G)
Where:
ρ - (Greek letter rho) resistivity of the material
l - length of the material
A - cross-sectional area of the material
EQN. 3
EQN. 4

22. Sample Problems (English)
1. What is the resistance of a 100-ft length of copper
wire with a diameter of 0.020 in. at 20°C?
2. An undetermined number of feet of wire have been
used from a carton of copper wire. Find the length of the
remaining copper wire if it has a diameter of 1/16 in. and a
resistance of 0.5 .
3. What is the resistance of a
copper bus-bar, as used in the
power distribution panel of a
high-rise office building, with
the dimensions indicated?

23. Sample Problems (AWG)
4. Find the resistance of 650 ft of #8 copper wire (T =
20°C).
5. What is the diameter, in inches, of a #12 copper wire?
6. For the system of the figure below, the total resistance of
each power line cannot exceed 0.025 , and the maximum
current to be drawn by the load is 95 A. What gage wire
should be used?

24. Sample Problems (Metric)
7. Determine the resistance of 100 ft of #28 copper telephone
wire if the diameter is 0.0126 in.
8. Determine the resistance of the thin-film resistor of
the figure shown below if the sheet resistance RS (defined by
Rs = /d) is 100 .

25. Sample Problems (Metric)
9. A kilometer of wire having a diameter of 11.7 mm and a
resistance of 0.031 ohm is drawn down so that its diameter is
5.0 mm. What does its resistance become?
10. A one-meter rod of 2-cm diameter is drawn until its
resistance is 100 times the initial resistance. Its length
afterwards is?

26. Assignment #1
1. The electrical resistance of a wire made of a certain material varies as its
length and inversely as the square of its diameter. If a wire of 100 m long
and 1.25 mm diameter has a resistance of 30 , find the resistance of
the wire of the same material whose length and diameter are 30 m and
0.75 mm respectively.
2. Design an electric unit heater to take 2000 W for its coil on 200 V dc
circuit (equivalent to having a 20  resistance). Using a nichrome wire
with resistivity of 660 -CM/ft and a cross-section (diameter) of 40 mils,
determine the length of the wire needed.

27. Assignment #1
3. A hallow rectangular aluminum conductor has outside
dimensions of 1 in by 1.5 in, the walls are 1/4 in thick. The
resistivity of aluminum at 20°C is 17.02 -CM/ft. Compute
the resistance of 105 ft of this conductor for a temperature
of 20°C. It is required that the computation of the area in CM
be shown.
4. A 1000 ft length of #8 AWG copper wire has a resistance of
0.628  at 20°C. Determine the resistance of this same mass
of copper if it is drawn down to one-half the cross section.

28. Assignment #2
(Research about the composition, construction, manufacture, use and application of the following types of
resistors)
I. Fixed Resistors
1.Carbon Composition
2.Metal Film Chip
3.Chip Resistor Array
4.Resistor Network
II. Variable Resistors
1.Potentiometer
2.Rheostat
3.Thermistor
4.Photoconductive Cells