Topic 2 Resistance

1
Topic 2 Resistance

• Familiar with the parameters that determine the
  resistance of an element
• Be able to calculate the resistance from the
  given dimensions and material characteristics
• Understand the effects of temperature on the
  resistance of a material
• Develop some understanding of superconductors

2
2.1 Introduction

• Conductors are those materials that permit a
  generous flow of electrons with very little
  external force (voltage) applied
• The resistance to the flow of electrons of any
  material with a uniform cross-sectional area is
  determined by the following factors
• Material
• Length
• Cross-sectional Area
• Temperature

3
Introduction

• Material and its unique molecular structure will
  react differently to pressures to establish
  current through its core.
• Conductors Permit generous flow of charge
• Insulators Have high resistance
• As the temperature of most conductors increases,
  the increased motion of particles within the
  molecular structure makes it increasingly
  difficult for the free carriers to pass
  through, and the resistance level increases.

4
2.2 Resistance Circular Wires

• The higher the resistivity of a conductor, the
  higher its resistance (R).
• The longer the length of a conductor, the higher
  its resistance.
• The lower the cross-sectional area of a
  conductor, the higher its resistance.
• The higher the temperature of a conductor, the
  higher its resistance.

R
5
Resistance Circular Wires

• Fig. 3.2 in the text Why R2 gt R1 in each case?

6
Resistance Circular Wires

• Area of a conductor is measured in circular mils
  (CM).
• The mil is a unit of measurement for length and
  is related to the inch by
• A wire with a diameter of 1 mil has an area of 1
  circular mil (CM).

Fig. 3.4 and Fig. 3.5
7
Table 3.1
8
Resistance Circular Wires

• The resistance of a circular wire is determined
  by
• (ohms) (3.1)
• R ohms (W), r CM-W/ft at T 20oC,
• l feet, A area in circular mils (CM)
• Example 3.1 in the text What is the resistance
  of 100 ft length of copper wire with diameter of
  0.020 in. at 20oC?

9
Resistance Circular Wires

• Example 3.2 in the text An undetermined number
  of feet of wire have been used from the carton in
  Fig. 3.6. Find the length of the remaining copper
  wire if it has a diameter of 1/16 in. and a
  resistance of 0.5 W.

• Example 3.3
• Convert the regular area
• to CM before applying equation (3.1) to
• calculate the
• resistance!!

10
Resistance Circular Wires

• Resistivity is not the only factor used in
  determining the best conductor. Other factors
  are
• Malleability ability of a material to be
  shaped
• Ductility ability of a material to be drawn
  into long, thin wires
• Temperature sensitivity
• Resistance to abuse
• Cost

11
Resistance Circular Wires

• Copper is the most widely used material because
  it is quite malleable, ductile and available.
• Aluminum was tried for general wiring but
  because of its thermal characteristics created
  difficulties.
• Silver and gold are used but because of cost,
  they have been limited to places that justify the
  cost.
• Tungsten has a resistivity three times that of
  copper but there are occasions when its physical
  characteristics (durability and hardness) are the
  overriding considerations.

12
2.3 Wire Tables

• Designed to standardize the size of wire
  produced by manufacturers, it contains the
  following information
• Cross-sectional area in circular mils
• Diameter in mils
• Ohms per 1000 feet at 20C
• Weight per 1000 feet
• Maximum allowable current in amperes, as
  determined by the National Fire Protection
  Association

13
AWG Wire Table

• The American Wire Gage (AWG) indicates cable size
  in numbers - AWG Copper Wire Table
• http//www.interfacebus.com/Copper_Wire_AWG_SIze.
  html

(Table 3.2 is for W/1000 ft _at_20oC!!)
14
2.4 Resistance Metric Units

• Metric units are used in the design of resistive
  elements including thin-film resistors and
  integrated circuits.
• Generally the meter is too large of a unit of
  measure for most applications, so the centimeter
  (cm) is usually employed.

• The resistivity of material r is actually the
  resistance of a sample block. Use equation
• with Table 3.3 and A in cm2

when calculate R!!
15
Table 3.3 for A in cm2 and l in cm
16
Resistance Metric Units

• Example 3.7 in the text Determine the resistance
  of 100 ft of 28 copper telephone wire if the
  diameter is 0.0126 in.

• Example 3.3
• Calculate the regular area
• in cm2, and convert length to cm before
  applying equation (3.1) with the r from Table
  3.3!!

17
2.5 Temperature Effects

• Temperature has a significant effect on the
  resistance of conductors, semiconductors and
  insulators.
• For good conductors, an increase in temperature
  will result in an increase in the resistance
  level. Consequently, conductors have positive
  temperature coefficients.
• For semiconductor materials, an increase in
  temperature will result in a decrease in the
  resistance level. Consequently, semiconductors
  have negative temperature coefficients.
• As with semiconductors, an increase in
  temperature will result in a decrease in the
  resistance of an insulator. The result is a
  negative temperature coefficient.

18
2.6 Superconductors

• Superconductors are special conductors of
  electric charge that, for all practical purposes,
  have zero resistance.
• Superconductors allow electrons to pass through
  with very high speed. (the speed of light 186000
  mi/s)
• The relatively low speed (1000 mi/s) of
  electrons through conventional conductors is due
  to collisions with atoms and repulsive forces
  from other electrons.
• Cooper effect Electrons travel in pairs and
  help each other maintain a significantly higher
  velocity through the medium.

19
Superconductors

• Currently, superconductivity exists only under
  very low temperature
• Before 1986
• Superconductivity could only be established at
  temperatures colder than 23 K ( - 250C)
• After 1986
• Physicists Alex Muller and George Bednorz of the
  IBM Zurich Research Center found a ceramic
  material, lanthanum barium copper oxide that
  exhibited superconductivity at 30 K.
• Professors Paul Chu and Man Kven Wu raised the
  temperature to 95 K using a superconductor of
  yttrium barium copper oxide, enabling liquid
  nitrogen (boiling point 77 K) to be used for
  cooling.

20
2.7 Resistors

• In electrical/electronics circuits, one of the
  most importance elements/devices is resistor.
• Resistors are used to create resistances in a
  circuit for various purposes. You will learn
  various resistors in the labs and in future
  courses.
• Resistors are made in many forms but all belong
  in either of two groups
• Fixed resistors are made of metal films,
  high-resistance wire or carbon composition
• Variable resistors have a terminal resistance
  that can be varied by turning a dial, knob,
  screw, or anything else appropriate for the
  application

21
2.8 Color Coding and Standard Resistor Values

• You will learn these in the labs.
• Search the Internet to find out how to remember
  the color coding and standard values.

22
Highlight of the Topic

• The ohm (symbol used O) is the unit of
  electrical resistance, named after Georg SImon
  Ohm.
• The graphic symbol of a resistor in a circuit
  diagram is
• Reading Chapter 3
• Problems of Chapter 3 1-10, 18-21