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Department of Technical Education Andhra Pradesh

Name : N. Ranga Prasad.

Designation : Lecturer in ECE.

Branch : Electronics & Communication Engg.

Institute : GMR Polytechnic, Gajwel.

Semester : 1 st year.

Subject : Electronics Components and Devices.

Subject Code : AEI-105.

Topic : Resistors.

Duration : 100 min.

Sub Topic : Effect of temperature on Resistors.

Teaching Aids : PPT.

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RECAP

• In previous class about

.potentiometers

.rheostats

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OBJECTIVES

Upon completion of this topic the student will be able to understand.

• The behavior of a resistor with respect to temperature.

• The temperature coefficient of resistance.

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EFFECT OF TEMPERATURE ON RESISTANCE

The effect of temperature on resistance is different

in different materials.

• The resistance of pure metals like al,cu increases with increases in temperature.

• It means that all metals exihibit positive temperature

coefficient of resistance.

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• In alloys the increase in resistance with temperature is very small

• The resistance of semiconductors and insulators decreases with increase in temperature.

• These semiconductors exihibit negative temperature coefficient

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TEMPERRATURE COEFFICIENT OF RESISTANCE α

For metallic conductors the resistance increases with

increase with in temperature.

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This increase in resistance depends directly on

• ITS INITIAL RESISTANCE.

• THE RISE IN TEMPERATURE.

• THE NATURE OF THE MATERIAL.

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DERIVATION OF TEMPERATURE COEFFICIENT OF RESISTANCE

• Let us consider a conductor whose resistance is R at 0˚C and Rt at t° C.

• The increase in resistance R = Rt – Ro.

• Rt – Ro α Ro x t i.e., the increase in resistance depends on its initial resistance and rise in temperature.

• Rt – Ro = α Rot or Rt = Ro ( 1 + α t).

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‘α’ is constant & is known as temperature co-efficient of resistance of the conductor

hence α = Rt – Ro = ΔR .

Rot Rot

Hence temperature coefficient of resistance of a material may be defined as the change in resistance per ohm per degree centigrade.

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Summary

We have discussed about

• Behavior of a resistor with temperature.

• Temperature co-efficient of resistance.

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Quiz

1. α is _________________ coefficient of resistance.

a. temperature.

b. voltage.

c. current.

d. all of the above.

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Quiz

2. Increase in resistance value depends on ______________.

a. initial resistance.

b. initial voltage.

c. initial current.

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Frequently Asked Questions

1. Write the factors on which resistance value varies with temperature.

2. Define temperature coefficient of resistance.

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