1. Temperature 
We associate the concept of temperature with how hot or cold an object feels
Our senses provide us with a qualitative indication of temperature
Our senses are unreliable for this purpose
We need a reliable and reproducible method for measuring the relative hotness or coldness of objects
We need a technical definition of temperature

2. Thermal Contact 
Two objects are in thermal contact with each other if energy can be exchanged between them
The exchanges we will focus on will be in the form of heat or electromagnetic radiation
The energy is exchanged due to a temperature difference

3. Thermal Equilibrium 
Thermal equilibrium is a situation in which two objects would not exchange energy by heat or electromagnetic radiation if they were placed in thermal contact
The thermal contact does not have to also be physical contact

4. Zeroth Law of Thermodynamics
If objects A and B are separately in thermal equilibrium with a third object C, then A and B are in thermal equilibrium with each other
Let object C be the thermometer
Since they are in thermal equilibrium with each other, there is no energy exchanged among them

5. Zeroth Law of Thermodynamics,
Example 
Object C (thermometer) is placed in contact with A until they achieve thermal equilibrium ;The reading on C is recorded
Object C is then placed in contact with object B until they achieve thermal equilibriumThe reading on C is recorded again. If the two readings are the same, A and B are also in thermal equilibrium

6. Temperature – Definition
Temperature can be thought of as the property that determines whether an object is in thermal equilibrium with other objects
Two objects in thermal equilibrium with each other are at the same temperature
If two objects have different temperatures, they are not in thermal equilibrium with each other

7. Thermometers 
A thermometer is a device that is used to measure the temperature of a system
Thermometers are based on the principle that some physical property of a system changes as the system’s temperature changes

8. Thermometers, cont These properties include: The volume of a liquid
The dimensions of a solid
The pressure of a gas at a constant volume
The volume of a gas at a constant pressure
The electric resistance of a conductor
The color of an object

9. A temperature scale can be established on the basis of any of these physical properties

10. Thermometer, Liquid in Glass
A common type of thermometer is a liquid-in- glass
The material in the capillary tube expands as it is heated
The liquid is usually mercury or alcohol

11. Calibrating a Thermometer
A thermometer can be calibrated by placing it in contact with some natural systems that remain at constant temperature
Common systems involve water
A mixture of ice and water at atmospheric pressure
Called the ice point of water

12. A mixture of water and steam in equilibrium
Called the steam point of water
Once these points are established, the length between them can be divided into a number of segments

13. Celsius Scale The ice point of water is defined to be 0o C
The steam point of water is defined to be 100o C
The length of the column between these two points is divided into 100 increments, called degrees

14. Problems with Liquid-in-Glass Thermometers
An alcohol thermometer and a mercury thermometer may agree only at the calibration points
The discrepancies between thermometers are especially large when the temperatures being measured are far from the calibration points

15. The thermometers also have a limited range of values that can be measured
Mercury cannot be used under –39o C
Alcohol cannot be used above 85o C

19. Physics 1 Thermal Physics
HEAT
EXPANSION & CONTRACTION
What changes in dimensions occur when heat is extracted or added to a system ?

20. How does a change in temperature affect the dimensions of a system?
Give examples where you have to consider the changes in the dimensions of a system when heat is added or extracted

22. A iron disc with a hole in it is heated.
Will the diameter of the hole (a) increase, (b) decrease or (c) not change? Q

23. Holes get bigger

24. T < T2 Q
As metal expands, the distance between any two points increases. A hole expands just as if it’s made of the same material as the hole.

25. A nut is very tight on a screw
A nut is very tight on a screw. Which of the following is most likely to free it?
(a) Cooling it
(b) Heating it
(c) Either
(d) Neither

26. Bimetallic strips
Two strips of different metals welded together at one temperature become more or less curved at other temperatures because the metals have different values for their coefficient of linear expansion .
They are often used as thermometers and thermostats Q
lower metal expands more than upper metal when heated

27. Most solids and liquids expand when heated. Why?
Average distance between atoms
Inter-atomic forces 
“springs”
Internal Energy U is associated with the amplitude of the oscillation of the atoms

28. Collisions of thermally oscillating atoms make them shift further apart
Repulsive force PE
Attractive force
Solid heated  increased vibration of atoms  increase max displacement either side of equilibrium position  vibration is asymmetric  mean distance increases with increasing temperature E3 E2 E1
Separation of atoms
THERMAL EXPANSION
average distance between atoms

29. LINEAR THERMAL EXPANSION
Ceramics (deep PE troughs) low expansion coefficients
 ~10-6 K-1
Polymers high expansion coefficients
 ~ 10-4 K-1
Metals
 ~ 10-5 K-1
 coefficient of linear expansion

30. Ao Lo Vo DL L A V Linear Area Volume
* Simple model: assume  and  are independent of temperature, T < 100 oC
* Wood expands differently in different directions

31. Volume expansion – solid cube
Vo = Lo3
V = L3 = (Lo +  Lo T)3 = Lo3(1 +  T)3
V = Lo3 (1 + 3  T + 3 2 T2 + 3 T3)
V = Lo3 (1 + 3  T) (ignoring higher order terms)
V - Vo = V = 3  Lo3T =  Vo T
 
V0
coefficient of linear expansion
 coefficient of volume expansion

32. Water has an anomalous coefficient of volume expansion,  is negative between 0 °C and 4 °C.
Liquid water is one of the few substances with a negative coefficient of volume expansion at some temperatures (glass bottles filled with water explode in a freezer) – it does not behave like other liquids
T > 4 °C water expands as temperature increases
0 < T < 4 °C water expands as temperature drops
from 4 °C to 0 °C
T = 3.98 °C water has its maximum density

33.  kg.m-3

34. BUOYANCY - FLOATING AND SINKING
Why do ice cubes float on water?

35. Lakes freeze from top down rather from bottom up
Water on surface cools towards 0 °C due to surrounding environment. Water as it cools and becomes more dense, it sinks carrying oxygen with it (it is most dense at about 4 °C). Warmer water moves up from below. This mixing continues until the temperature reaches 4 °C. Water then freezes first at the surface and the ice remains on the surface since ice is less dense than water (0.917 g/mL). The water at the bottom remains at 4 °C until almost the whole body of water is frozen. Without this peculiar but wonderful property of water, life on this planet may not have been possible because the body of water would have frozen from bottom up destroying all animal and plant life.

36. When should you buy your petrol?
2 am
2 pm

37. When should you buy your petrol?
2 am
2 pm

38. Problem
A surveyor uses a steel measuring tape that is exactly m at a temperature of 20 oC. (a) What is the length on a hot summer day when the temperature is 35 oC? (b) On the hot day the surveyor measures a distance off the tape as m. What is the actual distance?
Y & F Examples 17.2 /3.
steel = 1.210-5 K-1

39. Part (b) is “tricky” expansion by a factor 2 Solution
L0 = m T = 15 oC  = 1.210-5 K-1
L = L0(1 +  T) = m
Part (b) is “tricky”
expansion by a factor 2
The actual distance is larger than the distance read off the tape by a factor
L / L0
true distance = (35.794) ( ) / (50.000) m = m