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TEMPERATURE &THERMOMETERS Definition of Temperature Temperature is defined as a measure of average kinetic energy of a substance. It is also defined as a measure of how hot or cold a substance is.

How to measure Temperature? We can measure temperature with a thermometer. There are different types of thermometers. They are made by using a physical quantity that changes with temperature such as expansion or volume of a liquid. When a liquid is heated it expands. This property of a liquid to expand is used to make thermometers.

Types of thermometers PHYSICAL PROPERTY THAT CHANGES WITH TEMPERATURE

T YPE OF THERMOMETER

1 Volume of a fixed mass of liquid

Liquid – in – Glass thermometers e.g. mercury – in - glass and alcohol – in - glass

2

Thermocouple Electromotive force (e.m.f)

3

4

Resistance of metal

Pressure of a fixed mass of gas at constant volume

Resistance thermometer

Constant volume gas thermometer

The earliest devices used to measure the temperature were called thermoscopes.


How to make a scale of a thermometer? To make a scale on a thermometer, two fixed points are taken. 1. Lower fixed point 2. Upper fixed point.

Then the distance between the two fixed points is divided into equal divisions or DEGREES. This gives a scale on the thermometer. e.g. The distance between 0 oC and 100 oC is 240 mm. We want to make a scale with 100 equal divisions or 100 DEGREES. So we divide 240/100 = 2.4mm. So there will be 100 divisions each 2.4 mm apart. Each mark on the thermometer is a measure of 1 oC. This is a DEGREE CELSIUS scale. Such a scale with 100 divisions is known as a Centigrade Scale. This is known as calibrating a thermometer.

1.

Lower fixed point : this is the temperature of pure melting ice, i.e. 0 oC. Also known as ICE POINT.

How to find the lower fixed point?

To find the lower fixed point put the thermometer in a funnel containing crushed, melting ice. You will see the mercury thread in the thermometer fall. When the mercury thread stops shrinking any further and becomes static put a mark on the thermometer and label it as 0oC. This is the lower fixed point.

We use a funnel not a beaker so that the water produced escapes out otherwise the thermometer will be dipped into it and will give the temperature of water not ice.

o

0C

2.

Upper fixed point:

this is the temperature of steam, i.e. 100 oC. Also known as STEAM POINT.

How to find the upper fixed point? steam

100 oC

To find the upper fixed point put the thermometer in a bottle containing pure distilled boiling water. Keep the thermometer above the water in steam. This is to make sure that the thermometer records a temperature of 100oC because impurities may raise the temperature of boiling water whereas steam is sure to be at 100 oC. You will see the mercury thread inside the thermometer to rise. When the mercury thread stops rising and becomes static then put a mark at this point on the thermometer and label it as 100oC. This is the upper fixed point.


How to find an unknown temperature on thermometer? Use the formula:

θ=

l θ – l0 l100 – l0

100 oC

l 100

θ

where

= unknown temperature.

θ

lθ = length of the unknown temperature above the bulb l0 = length of the 0oC mark above the bulb l100 = length of the 100oC mark above the bulb

lθ 0oC l0

LIQUID – IN – GLASS THERMOMETERS

bulb

There are two types of liquid – in- glass thermometers. 1. Laboratory thermometer, 2. Clinical thermometer. The thermometers can contain either mercury of alcohol. Mercury is used in thermometers most commonly because of its advantages over the alcohol.

Differences between mercury and alcohol. MERCURY 1

2

Expands uniformly Responds quickly to temperature changes

ALCOHOL Does not expand uniformly Responds slowly to temperatures changes

3

Does not stick to glass

Sticks to glass

4

High boiling point ( 357oC)

Low boiling point ( 78oC)

5

High freezing point ( -39 oC)

Low freezing point ( -115 oC)

6

Meniscus is visible

Needs to be coloured

7

Poisonous

Safe

8

Expensive

cheap


Common features of lab thermometer and clinical thermometer. Design Feature

Purpose/ working principle

1

The bulb has a thin glass wall

It allows quick conduction of heat through the glass to the liquid (glass is a poor conductor of heat so we need to make it as thin as possible).

2

The size of the bulb is small

3

The bore of the capillary tube is fine

Makes the thermometer more responsive to heat. Small bulb will contain a small amount of liquid which will absorb heat more quickly and so respond more quickly. This allows a noticeable movement of the mercury thread for a small change in temperature. If the bore is wider then a small change in temperature will go unnoticed as there will be less cubical expansion of mercury inside the bore.

4

The bore of the capillary tube is uniform. Small in size

It ensures even expansion of the liquid. The liquid will expand evenly when its temperature rises throughout the entire length of the thermometer. Makes it portable and cheap to produce

5

Distinctive features of the Lab Thermometer DESIGN FEATURE 1

The glass walls of the thermometer are made thick

PURPOSE/ WORKING PRINCIPLE It makes the reading of the mercury thread easy by magnifying it.

Distinctive features of the Clinical Thermometer DESIGN FEATURE

PURPOSE/ WORKING PRINCIPLE

The scale is limited to a small range from 35 oC to 42 oC.

It allows a greater accuracy and the stem can be made short.

2

Constriction

It stops the back flow of the mercury once the thermometer records the temperature by breaking the mercury thread at the constriction and therefore allows maximum temperature to be taken with accuracy.

3

The stem is pear – shaped ( oval)

It acts as a magnifying glass and makes it easy to take reading from the thermometer.

1


Sensitivity:

The change in length of the mercury thread for a small change in temperature.

Sensitivity can be calculated by the formula: Length of scale in mm / number of degrees. Unit of sensitivity: mm / oC or mm / K.

How to increase sensitivity? 1. make the bore of the capillary tube narrow 2. make the bulb larger.

Low sensitivity: when the mercury thread does not show a noticeable change in length for a small change in temperature. High sensitivity: when the mercury thread shows a noticeable change in length for a small change in temperature.

Range:

It is the span of temperature which the thermometer can measure.

Lab thermometer: -10oC to 110 oC. Clinical thermometer: 35oC to 42oC.

How to increase range? 1. depends on the material used in the thermometer. 2. make the stem longer ( increase volume of stem) 3. make the bulb smaller ( decrease volume of bulb) 4. REMEMBER: A larger bulb will increase the sensitivity but decrease the range of the thermometer.

Responsiveness: it is the ability of a thermometer to give a reading in a short time. How to increase responsiveness? 1. make the bulb from thin glass 2. make the bulb smaller (a small bulb will hold less mercury so it will absorb heat quickly and respond in less time).

Linearity: It is the even expansion of the liquid with an increase in temperature. The length of the mercury thread must increase by the same amount for the same increase in temperature.

Thermocouple A thermocouple is made by joining two wires of different metals as shown.


One end of the thermocouple is placed in the reference junction (ice) and the other end is placed in the measuring junction (hot). This causes an e.m.f. to be induced which makes a current flow in the wires. The galvanometer measures small currents and thus gives a reading when a current flows through it. The galvanometer is calibrated to give readings in degrees or Kelvin. A thermocouple has the following advantages over liquid –in-glass thermometers. 1. 2. 3.

It can measure high temperatures up to 1500 oC. It can measure rapidly changing temperatures i.e. it is more responsive. It can go into small places so it is easy to take the temperature of a furnace by making a small hole in it.

Some common thermocouples.

Some common types of thermometers

By Shafaq Hafeez shafaq@physics.com.pk


Thermometers