Post on 04-Jan-2016
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PSC 151Laboratory Activity 6
Temperature and the Thermometer
What is temperature?
Temperature is what we measure with a thermometer.
What is a thermometer?
A thermometer is any devise that has an observable and measurable property that changes in response to a change in the temperature of the device… Thermometric Property
How does a thermometer measure temperature?
The operation of a thermometer is based on the Zeroth Law of Thermodynamics
A thermometer can only measure its own temperature.
Zeroth Law of Thermodynamics
“If two objects initially at different temperatures are placed in thermal contact, their temperatures will change until they
are at the same equilibrium temperature”This condition is called thermal equilibrium.”
TH > TC
thermal equilibrium reached
TH TC
TeqTeq
TH TC↓ ↑
To measure the temperature of an object a thermometer is placed in thermal contact with the object. Once the thermometer and object have reached thermal equilibrium it can be assumed that the temperature of the thermometer is equal to the temperature of the object.
How do we measure the temperature of the thermometer?
It must be calibrated with systems whose temperatures are known or defined.
Two systems that are commonly used are:
1. A mixture of ice and water.
2. Boiling water
These systems are used because it has been observed that as long as ice and water are present together the temperature remains constant, and as long as water is boiling its temperature remains constant.
The temperature of a mixture of ice and water is defined to be:0 degrees Celsius, 0°C or 32 degrees Fahrenheit, 32°F.
The temperature of boiling water is defined to be:100 degrees Celsius, 100°C or 212 degrees Fahrenheit, 212°F.
Consider the calibration of a liquid-in-glass thermometer.
Liquid-in-Glass Thermometer
glass housing
capillary
liquid reservoir
The thermometric property is the height of the liquid in the capillary
As the temperature of the thermometer changes, the liquid in the reservoir expands or contracts causing the level of the liquid in the capillary to rise or fall.
Once the temperature of the thermometer becomes constant the level of the liquid will be constant.
What temperature corresponds to this level of liquid?
T =?
Calibration of a Liquid-in-Glass Thermometer
Insert the thermometer into a container holding a mixture of ice and water and wait for thermal equilibrium to be reached.
Height of liquid has
fallen.
0°C32°F
Place a “mark” on the thermometer where the liquid stopped moving and label with the defined temperature of ice/water (0°C or 32°F)
ThermalEquilibrium
reached
Insert the thermometer into a container holding boiling water and wait for thermal equilibrium to be reached.
Height of liquid has risen.
Source of heatTo keep water
boiling
0°C32°F
Place a “mark” on the thermometer where the liquid stopped moving and label with the defined temperature of boiling water (100°C or 212°F)
100°C212°F
ThermalEquilibrium
reached
0°C32°F
100°C212°F
Determine how the height of the liquid relates to the temperature.
Temperature, °C
Height, cm
The slope of the graph = ΔhΔT =constant
The space between the two marks can be divided into equally spaced divisions:
100 divisions for Celsius (each division = 1C°) or
180 divisions for Fahrenheit (each division = 1F°) .
Constant Volume Gas Thermometer
Pressure Gauge
lbin2 ,psi
Pressure is the thermodynamic property.
10
0
5
1520
25
30
10
Pressure
lb
in2
Rigid (constant volume) metal bulb containing gas
Pressure at Room Temperature
Troom = ?
10
0
5
1520
25
30
10
Pressure
lb
in2
Proom =15.8 lbin2
Pressure at the Ice Point
Tice = 0°C
10
0
5
1520
25
30
10
Pressure
lb
in2
Pice =14.4 lbin2
Pressure at the Steam Point
Tsteam = 100°C
10
0
5
1520
25
30
10
Pressure
lb
in2
Psteam =19.4 lbin2
System Pressure, lb/in2 (psi) Temperature, °C
Room
Ice Point
Steam Point
15.8 ?
14.4 0
19.4 100
Graph of Temperature versus Pressure
T =mP+bCalibration Equation
Pressure, psi
Ice Point (14.4psi,0°C)
Steam Point(19.4psi,100°C)
m =ΔTΔP =100°C −0°C
Psteam−Pice
Choose one of the two calibration points.i.e., ice point.
Determining the intercept, b
Determining the slope, m
T =20 C°psi⋅P +b
b =T −20 C°psi⋅P
b =0°C −20 C°psi⋅14.4psi( )
Solve the partial equation for “b”.
m =ΔTΔP =100°C −0°C
Psteam−Pice
m = 100°C −0°C19.4psi−14.4psi
m =100C°5psi
m =20 C°psi
b =0°C −20 C°psi⋅14.4psi( )
b =−288°C
Calibration Equation
T =20 C°psi⋅P−288°C
T =20 C°psi⋅P−288°C
What is the room temperature? P=15.8psi
T =20 C°psi 15.8psi( )−288°C
T =316°C −288°C
T =28°C
What is the room temperature in °F?
TF =1.8TC +32
TF =1.8 28( )+ 32
TF =50.4 + 32
TF =82.4°F
0
5
1015
20
25
30psi0
25
50
75100
125
150
200
175
kPa Pressure GaugekPa
Pressure Gaugepsi
0
5
10
15
20
25
30psi
0
25
50
75
100125
150
200
175
kPa
P=112kPa