08 Temperature.pdf

Lesson Objectives:

At the end of the lesson you should be able to:

• Explain how a physical property which varies with temperature may be used to define a temperature scale and state examples of such properties.

• Explain the need for fixed points and state what is meant by ice point and steam point.

Lesson Overview

•

Temperature and Its Measurement

•

Common Temperature Scales

Trigger Question:

What is heat?

• Heat is a measure of the quantity of heat energy present in a body.

• Heat flows from a higher temperature to a lower temperature (Eg: Hotter to colder object)

Higher

Temp. Heat Lower _Temp.

TEMPERATURE (PRIOR KNOWLEDGE)

• Temperature is the measure of the degree or

intensity of hotness of a body.

• Is measured using a thermometer

Temperature

Temperature is the measure of the degree of

hotness

of a body.

≈ 15 million ºC

Temperature of the centre of the Sun

- 196 ºC

DNA samples storage in liquid nitrogen

1064 ºC

Melting point of gold

- 89.2 ºC

All time coldest point on earth

1. Which thermometer?

The choice of thermometers to use depends

on :

•

the range of temperatures to be measured;

•

the accuracy required;

1. Which thermometer?

In order for thermometer to be useful, it

must have a

physical property

that

changes with temperature. The substances

used

are

called

thermometric

1. Which thermometer?

Some examples of physical properties are:

•

the expansion of a column of liquid in

a capillary tube

;

• the electrical resistance of a platinum wire; • the voltage of a thermocouple;

• the expansion of a bimetallic strip;

Examples of thermometers

Platinum thermometer

(-50° to 400°C)

Mercury-in-glass thermometer

Thermocouple Thermometer

(-200° to 1500°C)

Thermocouple Thermometer

Bimetallic

Thermometer _ThermometerPressure

Desirable features of thermometers

•

an easy-to-read scale

•

safe to use

•

responsive to temperature changes

•

sensitive to small temperature changes

•

ability to measure a wide range of

2. Temperature scales

To construct a thermometer, we need to have a

standard temperature scale on a thermometer.

1. Choose a thermometric substance and its

physical property e.g volume of mercury.

2. Choose two fixed points which are standard degree of hotness or coldness. They should be

2. Temperature scales

3. Divide the temperature range between the two fixed points into a fixed number equal parts

e.g. 100.

We assume the physical property varies

3. Fixed points

Ice point

: The temperature of

pure melting

ice

at

standard atmospheric pressure

and

is

assigned

a value of

0 °C

.

Steam point

: The temperature at which

boiling water changes into steam

at

standard atmospheric pressure

and is

4. How to calibrate a thermometer?

• Place the bulb and stem of the

thermometer in a funnel

containing pure melting ice.

• When mercury level is steady, marked it as ice point, l₀, at that

particular part of the

thermometer stem. The ice point is assigned a value of 0°C.

Precaution: Crushed ice should be used in order to ensure good contact between the thermometer and the ice.

STEP 2: • Place thermometer as shown in

set up. The thermometer must be placed above the boiling water.

• When mercury level is steady, marked it as steam point, _l100, at

that particular part of the

thermometer stem. The steam point is assigned a value of 100°C.

4. How to calibrate a thermometer

?

Precaution: A manometer should be included to ensure that the pressure inside the apparatus is the same as the atmospheric pressure outside. (Shown in Textbook but not in diagram above)

STEP 3:

Divide the interval between the fixed points

by 100 equal divisions. Each division is 1

degree Celsius (°C)

4. How to calibrate a thermometer

?

In the Celsius scale, the interval between

the fixed points is divided into

100

equal

divisions for easy reading. Each division is

5. The Kelvin or Absolute Scale

•The zero of the Kelvin scale is the absolute

zero of temperature, which in theory is the

lowest possible temperature that any substance can reach.

5. The Kelvin or Absolute Scale

• Absolute zero (0K) corresponds with -273°C on the Celsius scale.

• The magnitude of a unit in Kelvin scale is the same as that on the Celsius scale. (Eg: 1K = 1°C)

5. The Kelvin or Absolute Scale

0 K



-273 °C

0 °C



273 K

From Celsius scale to Kelvin scale, add 273 K.

or

Let’s Try !!!

(a) 30ºC = _______________ K

(b) - 45ºC = ________________ K

(c) 15 K = _________________ ºC

(d) 283 K = _________________ ºC

303

228

-258

6. Liquid-in-glass Thermometer

This type of thermometer uses the property that liquid expands on heating. Most liquids expand at different rates at different temperature ranges.

One exception is mercury which expands

uniformly over a good range of temperature.

We can say that mercury expands linearly

Clinical Thermometer

• Short range of temperature: typically from 35 °C to 42 °C.

• Constriction in the capillary tube just above the bulb prevents mercury from falling back to the bulb.

6. Calculation of temperature based on ice and

steam points

x₀ 0 C

x₀: length of mercury at 0 °C.

100 C

x₁₀₀ x_{at 100 °C.}100: length of mercury

100 C

x₁₀₀ – x₀



x_ x_{at temperature} : length of mercury _



0 100

0

100

x

x

x

x











C

x

x

x

x











100

0 100 0 



Where

x

_θ : length at unknown temp,

θ

x

₀ : length at ice point

x

₁₀₀ : length at steam point

Example 1

C

x

x

x

x











100

0 100 0 



Example 1 [Solution]

General Equation

for celsius scale

C

X

X

X

X











100

0 100 0 



X

can be any physical property that

changes with temperature e.g.

Resistance of Platinum Wire.

wire A wire B wire A sensitive voltmeter hot junction cold junction

7. Thermocouples

▪ If the junctions are at different temperatures,

a voltage is produced.

▪ The larger the temperature difference, the larger the voltage produced.

A thermocouple

consists of two wires of different metals

wire B

wire A

Where ε : e.m.f. produced

Δθ : temperature difference between reference junction and

probe









7. Thermocouples

7. Thermocouples

It is suitable for measuring wide temperature

differences, which vary rapidly due to its quick response and the temperature at a point as the wire junctions are very small

It can operate over a very wide range of temperatures from –200°C to 1500°C.

A thermocouple does not

measure absolute

Example 2

In a certain thermocouple, a voltage reading of

+ 4.00 mV was obtained with one junction in pure melting ice and the other in boiling water at standard atmospheric temperature. When the hot junction was taken out of the boiling water and placed in boiling methane, the voltage registered was – 6.46 mV.

Example 2

-6.46 mV

Example 2 [Solution]

For a thermocouple thermometer, e.m.f, ε is

proportional to temperature difference, Δθ

Therefore,

4.00 mV α (100-0) °C (For water)

-6.46 mV α (θ - 0)°C (For boiling methane)

** When solving problems involving proportionality, remember to place the reference temperature on the same side in each equation (Eg: in this example, 0 °C

Example 2 [Solution]

Solving by cross multiplication,

Temperature

Thermometer

Physical properties that change with temperature

measured by

Fixed points: ice point steam point requires

Volume of fixed mass of liquid

Thermocouple Scales: Celsius scale Kelvin scale like Mercury-in-glass thermometer Structure Sensitivity Range Linearity Responsiveness

has the advantages of

e.m.f. (voltage) used in