chapter 11 temperature

CAMBRIDGE A LEVEL

PHYSICS

TEMPERATURE

LEARNING OUTCOMES

No. LEARNING OUTCOME

i What is thermal energy? Understand what is meant by thermal

equilibrium.

ii Learn how certain properties vary with temperature and how the

variation in these properties may be used to measure temperature.

iii Compare thermistors with thermocouples as previously calibrated

thermometers.

iv Compare and convert between the Celcius and the Kelvin scales.

THERMAL ENERGYTHERMAL ENERGY

Thermal energy (heat) can be transferred Thermal energy (heat) can be transferredfrom a system (or object) at a highertemperature to a system (or object) at alower temperature, when both objectsare brought into contact.

The transfer of thermal energy occursuntil thermal equilibrium is reached.

Thermal equilibrium is reached whenboth objects have the same temperature.


Example: Place a thermometer into a

beaker that has hot water.Figure 21.10, page

333, Chapter 21:

Thermal Physics;

Cambridge

International AS and

A Level Physics

Coursebook, Sang,

Jones, Chadha and

Woodside, 2nd

edition, Cambridge

University Press,

Cambridge, UK,2014.


Example: Describe what happens

when you place a metal spoon into

a hot cup of coffee.

THERMOMETRYTHERMOMETRY

The term thermometry refers to

measurement of temperature. To measure

temperature, we need two reference points.

Commonly used reference points are the

melting point of pure ice (0 C) and the

boiling point of pure water (100 C).

The range between the two reference points

is then divided into 100 equal intervals.


We may use certain physical We may use certain physicalproperties of materials that changewith temperature and the conceptof thermal equilibrium to measuretemperature.

These materials are then used assensors in thermometers.

THERMOMETRYTHERMOMETRY We must calibrate these thermometers by We must calibrate these thermometers by

using two reference points; usually themelting point of pure ice and boiling point orpure water, and dividing the range by equalintervals.

However, the due to the dependence of thescale on physical properties of materials, wecannot be sure that two calibrated scales willagree with each other.


Among the physical properties that Among the physical properties thatchange due to changes in temperatureare:

I. Changes in volume. Liquids expandwhen there is an increase intemperature, and contract whentemperature decreases e.g. mercury inthe mercury bulb thermometer.


II. Changes in length. Different metals extend

more, when heated, e.g. a bimetallic strip

Figure 17.3, page 553: Chapter 17: TEMPERATURE AND

HEAT; SEARS AND ZEMANSKYS UNIVERSITY PHYSICS WITH

MODERN PHYSICS; Young, Hugh D. and Freedman, Roger

A., Addison Wesley, San Francisco, 2012.


III. Changes in electrical resistance. The

resistance of a metallic conductor

increases with increasing temperature.

However, the resistance of semiconducting

materials decrease with increasing

temperature (e.g. in thermistors)


IV. Potential differences due to different

metals. The thermocouple uses this

principle.

Figure 21.12, page 335,

Chapter 21: Thermal

Physics; Cambridge

International AS and A

Level Physics Coursebook,

Sang, Jones, Chadha and

Woodside, 2nd edition,

Cambridge University

Press, Cambridge,

UK,2014.


When one junction is at a different temperature

relative to the other, there will be an e.m.f.

between the open ends.

The greater the temperature difference, the

larger the e.m.f. produced.

This e.m.f. can be measured by using a voltmeter

as shown on the diagram in the previous slide.

C O M PA R I N G T H E R M O C O U P L E S

W I T H T H E R M I S T O R STable 21.2, page

335, Chapter 21:

Thermal Physics;

Cambridge

International AS

and A Level

Physics

Coursebook,

Sang, Jones,

Chadha and

Woodside, 2nd

edition,

Cambridge

University Press,

Cambridge,

UK,2014.

THE KELVIN SCALETHE KELVIN SCALE

The Kelvin scale is an alternative scale

used in the measurement of

temperature.

This scale is independent of any changes

in physical properties.

The Kelvin scale is also known as the

thermodynamic temperature scale.

THE KELVIN SCALETHE KELVIN SCALEFigure 17.5 (b), page 555:

Chapter 17: TEMPERATURE AND

HEAT; SEARS AND ZEMANSKYS

UNIVERSITY PHYSICS WITH

MODERN PHYSICS; Young, Hugh

D. and Freedman, Roger A.,

Addison Wesley, San Francisco,

2012.


This shifting of axis forms a new scale on

the horizontal axis that is 273.15 C

lower than 0.00 C.

The newly formed scale is known as the

Kelvin scale and has been the standard

thermodynamic temperature scale since

1954.

THE KELVIN SCALETHE KELVIN SCALEFigure 21.11: A

conversion chart relating

temperatures on the

thermodynamic (Kelvin)

and Celcius scales , page

333, Chapter 21: Thermal

Physics; Cambridge

International AS and A

Level Physics

Coursebook, Sang,

Jones, Chadha and

Woodside, 2nd edition,

Cambridge University

Press, Cambridge,

UK,2014.


.

.

Hence: .

This point 0 K or the absolute zero is

equivalent to . .

The absolute zero point is the point where

molecules have the lowest internal energy.

Note that the interval size on the Celcius and

Kelvin scales are equal, i.e. .

chapter 11 temperature

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