Topic 10(E): Temperature

10.1 Principles of Thermometry

Why do we need a thermometer?

Who am I?

Nope. Not Jackie Chan.

Who’s this handsome fellow?

Do you know???

Galileo Galilei (1564-1642)

invented the first thermometer in 1596.

Galileo’s Thermoscope.

Galileo’s Thermoscope: Basic Principle

heat is applied at the flask.

air inside flask expands.

colored liquid level drops.

How does it work?

Galileo’s Thermoscope:Explanation with illustrations

At room temperature Allow to cool

Air contracts

Apply heat

When heat is applied

Air expands

Galileo vs Clinical

What differences between the 2 thermometers can you think of?

Galileo’s Thermoscope

Clinical Thermometer

What’s the problem?

No scale on the thermoscope.

So how do we measure the temperature???

Galileo’s Thermoscope

The modern terminology

‘Galileo’s Thermoscope’ is actually what we call a thermometer nowadays.

A thermometer is a device use to measure temperature or temperature changes.

A thermometer works by relying on physical properties that vary linearly with temperature.

2 fixed points: Definitions

Definition of the 2 fixed points on the Celsius Scale.

Ice point: The temperature of pure melting ice at 1 standard atmosphere.

(0c)

Steam point: The temperature of steam from boiling water at 1

standard atmosphere.(100c)

Why???To define a scale on the thermometer, using these 2 reference points, in order to read the temperature.

2 fixed points: Experimental Set-ups

Scale on thermometer: Calibration

How do we come up with a scale or calibrate a thermometer?

0C

100C

100C0C 10 20 30 40 50 60 70 80 90

Scale on thermometer: An exampleQuestion:

An uncalibrated thermometer is attached to a centimeter scale and reads 7.5cm in pure melting ice and 23.5cm in steam at 100C and 12.0cm in a mixture of liquid. What is the temperature of the mixture?

100C0C

23.5cm7.5cm

12.0cm?C

1) change in C for 1cm change in mercury = 100 / (23.5 - 7.5)

= 6.25 C/cm

2) length of mercury from 0 C to ? C = 12.0 - 7.5 = 4.5cm

3) temperature of mixture = 4.5 x 6.25 = 28.125 28.1 C

Relationship between height of mercury column and the Celsius

temperature

100

x 100oo

o

x xc

x x

Types of temperature scales

Conversion Kelvin (K) (S.I. Unit)

Fahrenheit (F) Celsius (C)

Kelvin (K) (S.I. Unit) #[K] = ([°F] + 459.67) · 5/9 [K] = [°C] + 273

Fahrenheit (F) #[°C] = ([°F] − 31.73) · 5/9

Celsius (C)

Example 10.1.3.1

• The diagram shows the positions of the mercury threads in a thermometer. What is the distance between each 1 Oc mark on the thermometer?A) 0.22 cmB) 0.24 cmC) 2.0 cmD) 2.2 cm

Example 10.1.3.2

• J99 P1 Q14– The lengths of mercury thread in the stem of a mercury

thermometer are given in three situations– Length in melting ice = 20 mm– Length in steam above boiling water = 170 mm– Length in liquid X = 50 mm

– What is the temperature of liquid X?• A) 20 Oc• B) 25 Oc• C) 30 Oc• D) 33.3 Oc

10.2 Liquid-in-glass Thermometers

Temperature-Dependent Properties

Definition:

Physical properties that changes with temperature.

In the case of a liquid/mercury thermometer, what do you think is the physical properties that changes with temperature?

Why is mercury the liquid of choice??

Can you think of other temperature-dependent properties used in some thermometers???

Table of common types of thermometers

Types of thermometer Physical property #Classification Comments

Liquid-in-glass thermometers

* Mercury thermometer

Volume of fixed mass of mercury

Mechanical

good for infreque

nt readings

* Alcohol thermometer Volume of fixed mass of alcohol

* Clinical thermometer Volume of fixed mass of mercury

* Thermocouple (thermopile – thermocouples

connected in series)

Voltage of 2 metals with different conductivity

Thermojunctive good for rapid

readings

Temperature-Dependent Properties

Do you know how it is used to measure temperature???

Gauge

When there is a temperature difference between the probe junction and reference junction, an e.m.f. is generated between the wires which gives a reading to the voltmeter.

Thermocouple (Simplified diagram)

V

Reference Junction

VoltmeterWire metal of

electrical conductivity A

Wire metal of electrical

conductivity B

Probe Junction

Example 10.2.1

• J2001 P1 Q14– Which instrument is most suitable for measuring a

rapidly changing temperature?

A) Alcohol-in-glass thermometerB) Clinical thermometerC) Mercury-in-glass thermometerD) Thermocouple

Features of a mercury-in-glass thermometer

• In pairs, discuss the features of a mercury-in-glass thermometer

• Why is the thermometer built with such features?

Sensitivity

Range

Responsiveness

Structures in a thermometer that affect sensitivity, responsiveness and range

Sensitivity Responsiveness Range

Bulb sizeLarge High Low Low

Small Low High High

Bulb wall thickness

Thick Low

Thin High

Bore diameter

Wide Low High

Narrow High Low

Example 10.2.2.1

• How can the sensitivity of a liquid-in-glass thermometer be increased?

A) Use a liquid which is a better conductor of heat

B) Use a thinner-walled bulb

C) Use a longer tube

D) Use a tube with a narrower bore

Example 10.2.2.2

• To enable a mercury-in-glass thermometer to respond to changes in temperature

A) the bore should be narrow

B) the stem wall should be thick

C) the bulb should contain a lot of mercury

D) the bulb wall should be thin

Example 10.2.2.3

• J92 P1 Q14– Which of the following determines the range of a

mercury thermometer?

A) the length of the stemB) the thickness of the bulbC) the volume of the bulbD) the volume of the bulb and stem

Example 10.2.2.4

• J2000 P1 Q14– What is reduced when a mercury thermometer is made

using a bigger bulb, but with a capillary tube of the same diameter and length?

A) AccuracyB) LinearityC) RangeD) Sensitivity

Example 10.2.2.5

• J88 P1 Q4– The diagram (not drawn to scale) shows two mercury thermometers, A and B,

identical in every respect except that the bulb of A is much larger than that of B.

State, giving a reason for your answer in each case, which thermometer

(a) will cover the larger range of temperature(b) will indicate more quickly a small rise in temperature

The End