Thermometers (A2 level Physics)

1. Temperature
A2 Level Physics
Mukesh N. Tekwani
mukeshtekwani@hotmail.com

2. Thermodynamics
 A branch of physics in which thermal
effects are studied using macroscopic
quantities.
 Macroscopic quantities are:
• Pressure
• Temperature
• Volume
• Internal energy
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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
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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.
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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.
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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
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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.
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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’
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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.
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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
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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
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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
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Degrees C

13. Determining an unknown Temp
 From the st line graph, we get:
P100 - P0
= const
100 - 0
Pt - P0
= const
t-0
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14. Determining an unknown Temp
P100 - P0
= const
100 - 0
Pt - P0
= const
t-0
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15. Determining an unknown Temp
 Equating the LHS of these eqns we get
100 x (Pt – P0)
t =
(P100 – P0)
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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
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17. Thermometric Properties
 Resistance varies linearly with temp
• Give formula
• Used platinum resistance thermometers
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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
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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
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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
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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)
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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
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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
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