4. Thermometry. Temperature and Heat Flow Temperature Scales Thermometers

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1 4. Thermometry Measuring temperature by sensation is very imprecise. That is why we need a temperature scale and a thermometer to measure temperature more accurately. Temperature and Heat Flow Temperature Scales Thermometers

2 temperature temperature and heat flow A measure of the degree of hotness of a body. is measured using a thermometer heat The energy that is transferred from one region to another region due to a difference in temperature. higher temperature lower temperature hot heat flow cold

3 Thermometry is the science and practice of of measuring temperature. thermometers The choice of which thermometer to use depends on range of temperatures to be measured accuracy required physical characteristics of the substances being examined

4 thermometers A thermometer must have a physical property that changes with temperature. Examples include: the volume of a column of liquid in a capillary tube the electrical resistance of a platinum wire the e.m.f. (electromotive force or voltage) of a thermocouple the curvature of a bimetallic strip the pressure of a gas at constant volume thermometers

5 thermometers desirable features of a thermometer an easy-to-read scale safe to use sensitive to temperature changes ability to measure a wide range of temperatures

6 thermometers 1. linearity The temperature reading changes linearly or proportionately with the length of the mercury column. temperature against length graph a straight line graph 2. responsiveness The speed with which a reading can be obtained.

7 thermometers 3. sensitivity The change in length of the liquid column per unit degree change in temperature, e.g. 2 cm/ C 4. range The interval between the minimum and the maximum temperatures that it can measure.

8 temperature scales To obtain a standard scale on a thermometer, two fixed points to be marked are chosen. for the purpose of standardisation always the same under given conditions These two fixed points are Ice point and Steam point. temperature scales

9 the Celsius scale The two fixed points for the celsius scale: temperature scales Ice Point Temperature of pure melting ice at standard atmospheric pressure Steam Point Temperature at which boiling water changes into steam at standard atmospheric pressure Assigned value of 0 C Assigned value of 100 C In Celsius scale (centigrade scale), the interval between the fixed points is divided into 100 equal divisions for easy reading. Each reading is 1 degree Celsius ( C)

10 the Celsius scale temperature scales ice point steam point melting ice funnel flask steam boiling water finding the ice point finding the steam point

11 the Celsius scale 1. Note the position of the mercury in the tube at the ice point and the steam point 2. For any height x θ of the mercury, the corresponding Celsius temperature θ is given by x 100 x 0 height of column x 100 x θ corresponding Celsius temperature 100 C steam point θ x θ -x 0 θ = x 100 C x 100 x 0 x θ -x 0 x 0 ice point 0 C

12 the Kelvin or absolute scale The Kelvin scale has its zero at absolute zero, which is the lowest temperature that any substance can reach. has a SI unit of kelvin (K) absolute zero corresponds with 273 C on the Celsius scale T (kelvin) = θ (celsius) The magnitude of a unit on both scales is equal Celsius scale C Kelvin scale 473 K C 373 K C 100 K 0 0 C 273 K C C C 173 K 73 K 0 K

13 thermocouple consists of two wires of different metals joined together at the end to form two junctions if the junctions are at different temperatures, a voltage is produced wire A wire B wire A cold junction hot junction sensitive voltmeter the larger the temperature difference, the larger the voltage produced

14 thermocouple the temperature range depends on the two metals used for the wires can operate over a very wide range of temperatures from 200 C to 1700 C Advantages: suitable for measuring 1. wide temperature differences, 2. temperature which vary rapidly due to its quick response and 3. the temperature at a point, as the wire junctions are very small