Unit 5: Temperature

Unit 5: TemperatureSec 2 Science - Physics

Learning OutcomesStudents should be able to:1. Explain how the volume of a fixed mass of liquidmay be used to define temperature scale and stateexamples of other such properties.2. Explain the need for fixed points and state what ismeant by ice point and steam point.3. Explain how a thermocouple thermometer works.Sec 2 Science - Physics

How do we measure temperature?1.Stand with one foot on a tiled floor andone foot on a wooden floor. The floor is allat the same temperature, but does it feelso?Sec 2 Science - Physics

How do we measure temperature?2. Try putting your left and right hands into2 cans of water of different temperatureas shown in the diagram below.Then place both hands into the centrepot of water. What would you feel?Sec 2 Science - Physics

How do we measure temperature? Is our skin a good gauge of temperature? How can we obtain accuratemeasurements of temperature?Sec 2 Science - Physics

Temperature and Heat Temperature is a measure of the degree of‘hotness’ or ‘coldness’ of a body Heat is the amount of internal thermal energyin an objectSec 2 Science - Physics

SI unit for Temperature SI unit for temp is Kelvin, K (not degree celsius!!!) To convert degree Celsius to Kelvin:T (Kelvin) (Celsius) 273Example,17 C in K: T (17 273) 290 K360K in C: (360 – 273) 87 CIce Point0 C273 KSteamPoint100 C373 KAbsolute-273 ahrenheit Scale: F C 1.8 32Sec 2 Science - Physics0K

ExampleConvert a) 45 C into Kelvin,b) 823 K into degrees Celsius Sec 2 Science - Physicsa) 45 C 45 273 318 Kb) 823 K 823 – 273 550 C

How do we measure temperature? The thermometer is an instrument used tomeasure temperature accurately. It makes use of the physical properties ofsubstances such as mercury that varycontinuously and linearly with temperature. They are called thermometric substances.Sec 2 Science - Physics

How do we measure temperature?Thermometric substancesPhysical PropertyThermometerVolume of a fixed mass ofliquidMercury-in-glass thermometer,Alcohol-in-glass thermometerElectromotive forceThermocoupleResistance of a piece ofmetalResistance thermometerPressure of fixed mass ofgas at constant volumeConstant-volume gasthermometerSec 2 Science - Physics

Types of ThermometersResistance ThermometerMaximum MinimumThermometerGalileoThermometerInfrared ThermometerBimetallicThermometerSec 2 Science - PhysicsThermocoupleThermometer

What makes a good thermometer? An easy-to-read scaleLinearitySafe to useResponsive to temperature changesSensitive to small temperature changesAble to measure a wide range of temperaturesSec 2 Science - Physics

How do we measure temperature?A home-madethermometer using a jarand a strawPhysical property used ?:volume of a fixed mass of liquidCan you suggest how to makeit read temperature?Water (? C)Sec 2 Science - Physics

Constructing a temperature scale(Calibrating a thermometer)Step 1: Choose an appropriate substanceChoose a suitable thermometric substance and itsphysical propertiese.g. Volume of mercury which variescontinuously with changes in temperatureSec 2 Science - Physics

Constructing a temperature scaleStep 2: Choose two fixed points Choose two standard degrees of hotness or coldnesswhich are easily obtainable and reproducible. These arecalled fixed points. The two fixed points for the Celsius Scale are:– Temperature of pure melting ice at one atmosphericpressure – ice point (Lower fixed point)– Temperature at which boiling water changes intosteam at one atmospheric pressure – steam point(Upper fixed point) Record the values of the physical property (e.g. lengthof mercury column) of substance at these two fixedpoints.Sec 2 Science - Physics

Constructing a temperature scalesteampointice inding the ice pointfinding the steam pointWhy are ice point and steam points chosen? Easily obtainable and reproducibleFor purpose of standardisationSec 2 Science - Physics

Constructing a temperature scaleStep 3: Set up the scaleWhen two fixed points are marked, the temperature between themare divided into equal divisions so that temperatures can be readeasily and preciselyUsing the Celsius Scale, the interval between the fixed points is dividedinto 100 equal divisions.Each division is 1 degree Celsius ( C)Sec 2 Science - Physics

Calculating the Temperature on an Unmarked ThermometerAt 0 C:At 100 C:At C:x100xx0x x0 100 Cx100 x0where xoSec 2 Science - Physics length of liquid thread at ice pointx100 length of liquid thread at steam pointx length of liquid thread at C

ExampleAt 0 C, the length of the mercury thread, l0 was8cm and at 100 C, the length of the mercurythread, l100 was 33cm. What is the temperaturewhen the mercury thread, l is a) 26cm, b) 3cm?At 0 C:At 100 C:33 cm8 cmSec 2 Science - Physics

Example (a)When l is 26cm,By calculation,l l0 x 100l100 l026 8 x 100 72 ºC33 8Graphically,l100 33cml/cml l0 8cm0 ?100l l0 l100 l0 0 100 0Sec 2 Science - Physics / C

Example (b)When l is 3cm,l l0 x 100 ºCl100 l03 8 x 100 -20 ºC33 8Sec 2 Science - Physics

liquid-in-glass thermometersglass stem withthickened wallbulb with thinstrengthened wallcapillarytubemercuryice pointsteampointvacuumin spaceLaboratory ThermometerSec 2 Science - Physicssafetychamber

Liquid-in-glass thermometersCharacteristics: RangeSensitivityResponsivenessLinearitySec 2 Science - Physics

RangeThe range of thermometer is the interval between the minimumand maximum temperatures that it can measure.MinimumMaximumRange of clinical thermometer is 35 C –42 CSec 2 Science - Physics

SensitivityThe sensitivity of a mercury thermometer is defined as the changein length of mercury column per unit degree change intemperature ( mm/ ºC)Which thermometer is moresensitive? A or BB is more sensitive5ºC change results in a largerlength of mercury column in Bcompared to ASec 2 Science - Physics

SensitivitySensitivity depends on:1. Size of the glass bulb Larger glass bulb increases the sensitivityA larger bulb has more mercury for expansion to cause a big change inthe length of the mercury thread than a smaller bulb for the sametemperature2. Size of the capillary tube Narrower capillary tube increases the sensitivityA small expansion of the mercury in the bulb will cause a bigger changein the length of the mercury thread of a narrower tube than a widertubeSec 2 Science - Physics

ResponsivenessThe responsiveness of a thermometer is the speed in which areading can be obtained.It depends on the thickness of the glass bulb.A thin glass bulb is more responsive because heat takes lesstime to pass through a thin glass wall.Sec 2 Science - Physics

LinearityLinearity measures the proportionality of change of the physicalproperty with the temperatureA thermometer is linear if its temperature readings changeproportionally with the length of the mercury columnMercury is used in thermometer because it expands linearly withtemperatureSec 2 Science - Physics

liquid-in-glass thermometersFeaturePurposeMercuryExpands uniformly over a widerange of temperature.Narrow bore ofcapillary tubeMakes the thermometer moresensitiveBulb is made of thinglassHeat can be conducted quicklyto the mercury (responsive)Round glass stemActs as a magnifying lens tomake temperature readingeasy.Sec 2 Science - Physics

Clinical thermometers A thermometer specially designed for measuring thetemperature of the human body. It is an ordinary mercury thermometer with twomodifications: short temperature range from about 35 C to 42 C constriction in capillary tube just above the bulb of the thermometer The constriction prevents mercury from flowing back intothe bulb by itselfcurvedglass stemconstrictionbulbSec 2 Science - Physicsnormal bodytemperaturespace holds mercuryif thermometer isoverheated

Clinical thermometersSec 2 Science - Physics

CheckpointWrite down key characteristics of an ideal thermometer.1. Larger glass bulb increases the sensitivity2. Narrower capillary tube increases the sensitivity3. Thin glass bulb increases responsiveness4. Mercury gives linearitySec 2 Science - Physics

Why water cannot be used in liquid-in-glass thermometers Water doesn’t expand or contract uniformly From 0 C to 4 C, water actually contracts instead ofexpanding. From 4 C to 100 C, water expandsSec 2 Science - Physics

Thermocouple Thermometer Consists of two types of wires made of differentmetals such as copper and ironSec 2 Science - Physics

Thermocouple Thermometer The ends of the wires are joined together toform two junctionsSec 2 Science - Physics

Thermocouple Thermometer Temperature is calculated using the readings ofthe voltmeterSec 2 Science - Physics

How does a thermocouple thermometer work? One junction is kept at a known temperature(e.g. 0 C). This junction is known as the reference junction.Reference junction,usually at 0 C.Sec 2 Science - Physics

How does a thermocouple thermometer work? The other junction, the probe tip, is used tomeasure the unknown temperature of othersubstances.Probe tip tomeasure unknowntemperature.Sec 2 Science - Physics

How does a thermocouple thermometer work? When the two junctions are at differenttemperatures, a small voltage (also known aselectromotive force e.m.f.) is produced.Sec 2 Science - Physics

How does a thermocouple thermometer work? When the two junctions are at differenttemperatures, a small voltage (also known aselectromotive force e.m.f.) is produced. Combining the information of the knownreference temperature and the difference oftemperature between the probe tip and thereference junction, the absolute temperaturereading can be obtained.Sec 2 Science - Physics

Defining Equation of Thermocouple Thermometer where e.m.f. produced temperature difference between the referencejunction and the probeBy comparing the e.m.f. readings with a standardcalibration table, we can then determine the unknowntemperature.Sec 2 Science - Physics

ExampleIn a certain thermocouple thermometer, a voltage readingof 4.00 mV was obtained with one junction in melting iceand the other in boiling water at one standard atmosphere.When the hot junction was taken out of the boiling waterand placed in boiling propane, the voltage registered was-1.50 mV. Find the temperature of the boiling propane onthe Celsius scale of the thermocouple.Sec 2 Science - Physics

ExampleUsing ,4.00 mV (100 - 0) Cand-1.50 mV ( - 0) C,Sec 2 Science - Physicswhere is the temperature of boiling propane.

ExampleUsing ,4.00 mV (100 - 0) Cand -1.50 mV ( - 0) C,of boiling propane. 0 100 0 where is the temperature 1.504.00 1.50 100 37.5 C4.00The temperature of the boiling propane is -37.5 C.Sec 2 Science - Physics

Advantages of a Thermocouple Thermometer Robust, compact and fairly accurate Able to measure a very large temperature range of -200 C to 1500 C by choosing suitable types of metals forwires A and B. Very responsive to rapidly changing temperatures due toits small mass and because metals are good conductorsof heat. As the output is an electrical signal (e.m.f. reading), itcan be connected to suitable electrical equipment forchecking rapid or sudden temperature changes.Sec 2 Science - Physics

Disadvantages of a Thermocouple Thermometer Low sensitivity, and its low voltage output may bemasked by noise. Accuracy, usually no better than 0.5 C, may not be highenough for some applications. Requires a known temperature reference, usually 0 Cice water. Non-linearity could be leSec 2 Science - Physics

Thermocouple ThermometerA thermocouple probe is inserted into the lava flow tomeasure the temperatureSec 2 Science - Physics

Convert a) 45 C into Kelvin, b) 823 K into degrees Celsius a) 45 C 45 273 318 K b) 823 K 823 – 273 550 C . Sec 2 Science - Physics The thermometer is an instrument used to measure temperature accurately. It makes use of the physica