Lecture # 5 by

Saad Zahid

Introduction to Resistance The opposition in the flow of current is called resistance.

The unit of measurement of resistance is the ohm(Ω).

Introduction to Resistance At a fixed temperature of 25°C (room temperature),

the resistance is related to the other three factor by

Types of Resistors – Fixed Resistors Resistors are made in many forms, but all belong in either

of two groups:

fixed or variable. The most common of the low-wattage,

fixed-type resistors is the molded carbon composition resistor.

The relative sizes of all fixed and variable resistors change with the power rating, increasing in size for increased power ratings in order to withstand the higher currents and dissipation losses.

Variable resistors have resistance that can be varied by turning a dial, knob, screw, or whatever seems appropriate for the application.

Color Coding and Standard Resistor Values

A whole variety of resistors are large enough to have their resistance in ohms printed on the casing. However, some are too small to have numbers printed on them, so a system of color coding is used.

•The first and second bands represent the first & second digits, respectively. • The third band determines the power-often multiplier for the first two digits. • The fourth band is the manufacture’s tolerance. • The fifth band is a reliability factor, which gives the percentage of failure per 1000 hours of use.

Conductance The quantity of how well the material will conduct

electricity is called conductance (S).

Indicating that increasing the area or decreasing either the length or the resistivity will increase the Conductance.

(Siemens, S)

Example Calculate the resistance for the following conduction

values

1. 0.05 S

2. 0.1 S

Solution:

1. R = 1 / G => 1 / 0.05 = 20 Ω

2. R = 1 / G => 1 / 0.1 S = 10 Ω

Self Task Calculate the conductance for the following resistance

values

1. 1K Ω

2. 5K Ω

Solution:

1. G = 1 / R => 1 / 1000 = 0.001 S

2. G = 1 / R => 1 / 5000 = 0.0002 S

Active or Passive Elements Active Elements: Circuit elements capable of generating electrical energy are called active elements.

For example, voltage and current sources.

Passive Elements: Circuit elements that cannot generate electrical energy are called passive elements.

For example, resistors, inductors and capacitors.

Ideal Source Ideal Voltage Source:

Ideal voltage source is a circuit element that maintains a prescribed voltage across its terminal regardless of the current flowing in those terminal.

Ideal Current Source:

Ideal current source is a circuit element that maintains a prescribed current through its terminals regardless of the voltage across those terminals.

Independent Sources Independent sources:

These sources maintain a voltage or current in a circuit without depending on any other voltage and current source.

(a) Independent Voltage Source (b) Independent Current Source

Dependent Sources Dependent sources:

These sources depend upon other voltage and current sources to maintain voltage or current in a circuit.

Dependent sources are also called controlled sources.

(b) Dependent Current Source (a) Dependent Voltage Source

Ohm’s Law

Introduction to Ohm’s Law Current I is directly proportional to the voltage V, and

inversely proportional to the resistance R.

V=IR

I=V/R

R=V/I

Example Determine the current resulting from the application

of a 9-V battery across a network with a resistance of 2.2 Ω.

I = V / R =

Self Task Calculate the resistance of a 60-W bulb if a current of

500 mA results from an applied voltage of 120 V.

R = V / I=

Self Task Calculate the current through the 2-kΩ resistor if the

voltage drop across it is 16 V.

I = V / R =

Self Task Calculate the voltage that must be applied across

the soldering iron to establish a current of 1.5 A through the iron if its internal resistance is 80 Ω.

V

Plotting the Ohm’s law Resistance is kept constant.

Power Power is an indication of how much work can be done

in a specified amount of time, that is, a rate of doing work.

p = dw/dt where p = power in watts

w = energy in joules

t = time in seconds

The electrical unit of measurement for power is the watt (W)

Power Watt: One watt of power equals to work done in 1 sec by 1

volt of Potential difference in moving 1 coulomb of charge.

Power

Eq. 1

Eq. 2

Eq. 3

Example Find the power delivered to the dc motor.

Self Task What is the power dissipated by a 5-Ω resistor if the

current is 4 A?

Power Sometimes the power is given and the current or

voltage must be determined.

Example Determine the current through a 5-kΩ resistor when

the power dissipated by the element is 20 mW.