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# Thermometers

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Thermometers (A2 level Physics)

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### Thermometers

1. 1. Temperature A2 Level Physics Mukesh N. Tekwani mukeshtekwani@hotmail.com
2. 2. Thermodynamics  A branch of physics in which thermal effects are studied using macroscopic quantities.  Macroscopic quantities are: • Pressure • Temperature • Volume • Internal energy 2
3. 3. Kinetic Theory  Kinetic theory is also concerned with thermal effects.  But here we assume the existence of atoms and molecules.  The laws of mechanics and statistics are applied to a large number of these molecules.  Microscopic level 3
4. 4. Thermal Equilibrium  If a body (X) at a higher temperature is in contact with a body (Y) at a lower temperature, then X will transfer heat energy to the lower temp object (Y)  This process will continue till the objects attain the same temperature.  The temp of both objects will become same.  This thermal state in which both bodies are at the same temp is called thermal equilibrium. 4
5. 5. Using Physical Properties to Measure Temperature  Many physical properties change with temperature • Change in volume (e.g. expansion of a liquid) • Change in length of a mercury column • Change in resistance of a wire • Change in pressure of a gas at constant volume  All these properties can be used in different types of thermometers  A thermometer is an instrument for measurement of temperature. 5
6. 6. Imp Definitions  The physical property on which a particular thermometer is based is called the thermometric property.  Thermometric substance is the material used in the thermometer, whose property varies with temperature  Thermometric property should vary linearly with temp over a reasonable range of temp  The range of linearity of the thermometric property is called the thermometric range.  E.g., mercury is a thermometric substance 6
7. 7. Temperature Scales  Each type of thermometer can be used to establish its own temp scale.  How is this done? • Every substance changes state at a fixed temperature. E.g., solid-to-liquid OR liquid- to-gas • These fixed temperatures can be used to define reference temperatures. These reference temperatures are called fixed points. 7
8. 8. Temperature Scales  We take the value of the thermometric property at the two fixed points and divide the range into a number of EQUAL steps or degrees.  This way we set up an “empirical scale” of thermometer.  Empirical => ‘derived from experiment’ 8
9. 9. Temperature Scales  E.g., suppose we consider the fixed points as the melting point of ice (00) and boiling point of water (1000)  Divide the range from 0 to 100 into 100 equal degrees. This gives the empirical scale of temperature for that thermometer. 9
10. 10. Disagreement between Thermometers  The choice of thermometric susbstance and thermometric property and the assumed relation between the property and temperature leads to an individual temp scale.  Measurements made with such a thermometer may not agree with measurements made by any other temp scale 10
11. 11. Disagreement between Thermometers  This disagreement is removed by the following: • Using a particular thermometric substance • Using a particular thermometric property • A particular relation between that property and the temp scale. E.g., a linear relation between length of a mercury column and temp 11
12. 12. Determining an unknown Temp  Consider the following graph which shows the variation of a property P with temp. Thermometric range P100 = t2 – t1 Pt P0 t1 0 t 100 t2 12 Degrees C
13. 13. Determining an unknown Temp  From the st line graph, we get: P100 - P0 = const 100 - 0 Pt - P0 = const t-0 13
14. 14. Determining an unknown Temp P100 - P0 = const 100 - 0 Pt - P0 = const t-0 14
15. 15. Determining an unknown Temp  Equating the LHS of these eqns we get 100 x (Pt – P0) t = (P100 – P0) 15
16. 16. Thermometric Properties  Length varies linearly with temperature • Give formula • Used in liquid-in-glass thermometers  Pressure varies linearly with temperature • Give formula • Used in constant volume gas thermometers  Volume varies linearly with temperature • Give formula • Used in constant pressure gas thermometers 16
17. 17. Thermometric Properties  Resistance varies linearly with temp • Give formula • Used platinum resistance thermometers 17
18. 18. Thermometers  Factors to consider while choosing a thermometer • Accuracy • Sensitivity (distance between divisions on the scale) • Range of temps it can measure • Speed of response – measure rapidly varying temperatures • Sensitive part of the thermometer should be small so that it does not absorb much heat from the object • It shuld be easy to read – no complicated calibrations / settings 18
19. 19. Platinum Wire Thermometers  These are based on the variation of electrical resistance due to temperature  Resistance of a metal wire increases with increase in temp. Rt = R0 ( 1 + α t)  Range of temp they can measure is from -260 C to 1700 C  Temp sensor or material is a coil of fine platinum wire 19
20. 20. Platinum Wire Thermometers  Advantages of platinum resistance thermometers: • High accuracy • L ow drift • Wide operating range  Disadvantages of Platinum Thermometers • Not very sensitive to small changes in temp • Slow response time 20
21. 21. Thermistor  A thermistor is a type of resistor whose resisatnce changes with temperature  Therm (heat / temp) + Resistor = Thermistor  Semiconductors are used in these devices  The electrical resistance decreases very rapidly with increasing temp.  Thus, we say that thermistors have a negative temp coeff. of resistance (NTC) 21
22. 22. Thermistors  Advantages of thermistors: • Sensitivity is high • Their size is very small – hence can be used to measure temp of small objects  Disadvantages of Thermistors • Their scale is non-linear 22
23. 23. Thermocouple  This makes use of the thermoelectric effect  Thermoelectric device - A thermoelectric device creates a voltage when there is a different temperature on each side  When the junctions of two different conductors, such as copper and constantan wire are at different temperatures, an emf (voltage) is developed.  The relation between emf and temp diff is not linear.  The emf generated is small – typically about 5 mV for a temp diff of 100 C  Advantages: • Can be used to measure rapidly changing temps. • Can be used to measure temp of small objects 23