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Department Of Technical Education Andhra Pradesh

Name : P.S.R.Muralidhar Designation : Sr.Lecturer in ECE Branch : ECE Institution : GMR. Polytechnic, Gajwel, Year/Semester : I Year Subject : Basic Electronics Subject Code : AEI-105 Topic : Resistors Sub Topic : Property of resistance &

Ohm’s law Duration : 50 min Teaching aids : PPT, visuals Revised by : N.Venkateswerlu Lecturer in ECE

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Objectives

On completion of this class you would be able to

understand the

• Property of Resistance

• Ohm’s law

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• Bulb

• Fan

• Refrigerator

• Telephone

• Computer

Name Some Electrical Gadgets

Known to Unknown

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Some Electrical Gadgets

Fig.1 Electrical Gadgets

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1. What they require for working?

Electricity or Current.

2. What is Current?

Flow of electrons constitute Current.

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1. What do we do when somebody says

‘Sun rises in the West?’

We oppose it.

2. What do we do when somebody tries to steal

something from us?

We oppose or Resist it.

3. How do we generally react to a change in a system?

We oppose or Resist it.

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Property of Resistance

• The property of a material to oppose flow of current is

known as resistance.

• The unit of resistance is ohm and is denoted by the

symbol Ω .

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Ohm’s Law

• Ohm’s law gives the relationship between the applied

potential difference across the conductor and the

resulting current through it.

Statement

• “The temperature remains constant, the current (I)

passing through a conductor is directly proportional to

the applied voltage (V) across the same conductor”

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Ohm’s Law (Contd…)

V IαV R I=

Where

V = Voltage across the conductor in volts

I = Current passing through the conductor in amperes

R = Resistance of the conductor in ohms

VR

I=

OtherwiseV

IR

=Or

Mathematically,

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Limitations of Ohm’s Law

• Ohm’s law can not be applied to non-linear devices

such as vaccum & gas filled devices and

semiconductor devices

• It can not be applied to arcing devices.

• It can not be applied to those electrolytes in which

gases are liberated on the electrodes.

• It can not be applied to those conductors whose

temperature changes due to the flow of current through

them.

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Resistor

• A passive component that offers resistance is known as

Resistor.

Fig.2 Resistor

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Resistor is represented by the letter ‘R’

R

1 2

Symbol of Resistor

Fig.3 Circuit Symbol of Resistor

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Unit of Resistance• The unit of resistance is ohm and is denoted by the

symbol Ω .

• when a voltage ‘V’ is applied across a resistor and ‘I’ is

the current passing through it, then

• According to ohm’s law

Where

• V = Voltage across the resistor in volts• I = Current passing through the conductor in

amperes• R = Resistance of the conductor in ohms

VR

I=

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If V=1 Volt, I = 1 ampere , then

11

1

voltR ohm

ampere= =

Unit of Resistance (contd..)

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• To limit current in a circuit

• To get required voltage drop

• As a load element

We use Resistors commonly

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Discussion

• What is the difference between Resistor and

Resistance?

• Resistor is an electronic component which

opposes the flow of current.

• Resistance is the property of a resistor to oppose

the flow of current through it.

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Problem

• Calculate the resistance of a resistor when there is a current of 2A flowing through it due to the application of 40V.

• Solution

Given data

I = 2A

V=40V

To find R=?V

RI

=

4020

2= = Ω

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Summary

• Every material in the Nature offer opposition to the flow of Current.

• This opposition is called Resistance.

• Higher the opposition, lower the current and vice-versa .

• The unit of Resistance is Ohm (Ω).

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Quiz

1. Unit of resistance is

(a) ohms

(b) volt

(c) ampere

(d) Henry

19

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2. A resistor _______ the flow of current ?

(a) opposes

(b) aids

(c) opposes & aids

(d) all of the answers

Quiz (Contd…)

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3. Current is _______ proportional to resistance ?

(a) inversely

(b) directly

(c) inversely & directly

(d) none

Quiz (Contd…)

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Frequently Asked Questions

1.What is Resistance ?

2.What are its uses ?

3. State ohms law and write its limitations.