physics 2 mt-j 1.2-echo

1. TEMPERATURE AND HEAT
Thermodynamics part 1

2. Temperature and Thermal
Equilibrium

3. What is Temperature?
 The degree of hotness or coldness of a body or
environment
 A measure of the ability of a substance, or more
generally of any physical system, to transfer heat
energy to another physical system.
 A measure of the average kinetic energy of the
particles in a sample of matter, expressed in terms of
units or degrees designated on a standard scale.

4. How to measure hotness and coldness?
 We can use materials that has measurable
properties that varies with hotness and coldness
 Example:
mercury or ethanol (expands when hot,
contracts when cold)
This material can be used as thermometer.

5. How to use thermometers?
For example:
Measuring the temperature of a hot coffee:
The thermometer interacts with the
coffee. The thermometer becomes hotter
while the coffee becomes a little colder
The reading will stabilized. The
interaction does not further cause
the system to change.
Thermal Equilibrium has
been reach!

6. What is Thermal Equilibrium?
 Two system is said to be in thermal equilibrium if
and only if they have the same temperature.

8. Temperature Scales

9. Celsius and Fahrenheit Scale
 They are based on the boiling point and freezing
point of water
TF = (9/5) TC + 32
TC = (5/9)(TF- 32)

10. Kelvin Scale
 Base on the relationship of temperature and
pressure at constant volume with ideal gases.
 The absolute zero temperature is the temperature
when the absolute zero pressure is attained.
-273.15 oC = 0 K
TK = TC + 273.15

11. Seat Work 3
Convert the following to desired temperature scale:
1. 500 oC to oF
2. 212 oF to oC
3. 500 K to oF
4. 100 oF to oC
5. 1000 K to oC
6. 150 oC to K
7. 50 oF to K

12. Thermal Expansion

13. Thermal Expansion
 Most materials expand
when its temperature
increases and contract
when its temperature
decreases.
Example:
- Opening a jar.
Railways bend because of thermal
expansion

14. Linear Expansion
∆L = Lf – Li
∆T = Tf – Ti
∆L = Li∆T
= coefficient of linear expansion
Material ( K-1)
Coefficients of linear Aluminum 2.4 x 10-5
expansion Brass 2.0 x 10-5
Copper 1.7 x 10-5
Glass 0.4-0.9 x 10-5
Invar (Nickel-Iron alloy 0.09 x 10-5
Quartz 0.04 x 10-5
Steel 1.2 x 10-5

15. Example:
1. A surveyor uses a steel measuring tape that is
exactly 50,000 m long at a temperature of 20oC.
What is its length on a hot summer day when the
temperature is 35oC?
2. In example 1, the surveyor uses the measuring tape
to measure a distance when the temperature is
35oC; the value that she reads off the tape is
35.794 m. What is the actual distance? Assume that
the tape measure is calibrated for use at 20oC.

16. Volume Expansion
∆V = Vf – Vi
∆V = Vi∆T
= 3 = coefficient of volume expansion
Coefficients of Volume expansion
Solids ( K-1) Liquids ( K-1)
Aluminum 7.2 x 10-5 Ethanol 75 x 10-5
Brass 6.0 x 10-5
Copper 5.1 x 10-5 Carbon 115 x 10-5
disulfide
Glass 1.2-1.7x 10-5
Invar (Nickel-Iron alloy 0.27 x 10-5 Glycerin 49 x 10-5
Quartz 0.12 x 10-5 Mercury 18 x 10-5
Steel 3.6 x 10-5

17. Example
1. A glass flask with volume 200 cm3 is filled to the
brim with mercury at 20oC. How much mercury
overflows when the temperature of the system is
raised to 100oC? The coefficient of linear
expansion of glass is 0.40 x 10 -5.
2. A metal rod is 40.125 cm long at 20.0oC and
40.148 cm long at 45oC. Calculate for the
average coefficient of linear expansion of the rod
on this temperature range.

18. Heat

19. What is Heat?
 Energy transfer that takes place solely because of
temperature difference is called heat flow or heat
transfer. This is usually called heat.
 Since heat is a transfer of energy, the standard unit
for heat is joules.
 Heat and temperature is not the same!

20. Heat vs Temperature
 Heat is in Joules while temperature is in Kelvin
 Temperature is a quantitative description of hotness
and coldness while heat is the energy transferred
due to difference in temperature.
 Temperature can change by adding or taking away
heat or energy through mechanical work.

21. Units of Heat
 Standard unit Joule.
Other units of Heat
1calorie (cal) = 4.186 J
1 kilocalorie (kcal) = 4186 J

22. Specific Heat
How much heat is needed?

23. Specific Heat
Q = mc T
Q – heat required to raise or to lower the
temperature of an object
m – mass
T – change in temperature
c – specific heat – the amount of heat required to
raise 1 kg of a substance by 1K or 1oC.

24. Specific Heat
- The greater the specific heat of a material, the
more heat must be transferred to it or taken from it
to change the temperature of a given mass of it.
Example
- Sand and water in beach (water has high specific
heat)
- At day water is cold, sand is hot
- At night water is hot, sand is cold

25. Specific Heat
Substance Specific Heat (J/kg*oC or K)
Air 1050
Alcohol, ethyl 2430
Aluminum 920
Copper 390
Iron or Steel 460
Lead 130
Mercury 140
Water 4186
Wood 1680

26. Example
1. During a bout with the flu an 80-kg man ran a
fever of 39.0oC instead of the normal body
temperature of 37.0oC. Assuming that the human
body is mostly water, how much heat is required to
raise his temperature by that amount? Specific
heat of water is 4186 J/kgK.

27. Example
2. A half-liter of water at 350 K is cooled by
removing 63 kJ of heat. What is its final
temperature?
3. A 0.250-kg cup at 20oC is filled with 0.250 kg of
boiling coffee. The cup and the coffee come to
thermal equilibrium at 80oC. If no heat is lost, what
is the specific heat of the cup material? Consider
coffee as water.

28. Phase Change and Latent Heat

29. Phase Change
 Matter usually comes in three phases, solid, liquid
and gas.
 Matter changes phase due to temperature change
Example: Water
- solid, below 0oC
- liquid, 0oC to 100oC
- gas, above100oC
 Additional heat is required when changing phase

30. Phase Change
 More heat is taken away when water is converted
to ice at 0oC compared to bringing water in liquid
form to 0oC.
 More heat is put in when water is converted into
water vapor at 100oC compared to bringing water
in liquid form to 100oC
 Additional energy is needed to break or make
intermolecular bonds between molecules.
 The additional energy is accounted for by Latent
Heat

31. Latent Heat
 Latent Heat of Fusion (Lf)
- heat required to change the phase of 1 kg of
material from liquid to solid.
 Latent Heat of Vaporization (Lv)
- heat required to change the phase of 1 kg of
material from liquid to gas.
Q = +/- mLf
Q = +/- mLv

32. Latent Heat
 Some Latent heats of materials:
Substance Normal Melting Point Lf Normal Boiling Point Lv
(K) (J/kg) (K) (J/kg)
Hydrogen 13.84 58.6 x 103 20.26 452 x 103
Nitrogen 63.18 25.5 x 103 77.34 201 x 103
Oxygen 54.36 13.3 x 103 90.18 213 x 103
Mercury 234 11.8 x 103 630 854 x 103
Water 273.15 334 x 103 373.15 2256 x 103
Sulfur 392 38.1 x 103 717.75 326 x 103
Gold 1336.15 64.5 x 103 2933 1578 x 103

33. Example
1. A physics student wants to cool 0.25 kg of Diet Omni-
Cola (mostly water), initially at 25oC, by adding ice at
-20oC. How much ice should she add so that the final
temperature will be 0oC with all the ice melted if the
specific heat of the container may be neglected?
2. A heavy copper pot of mass 2.0 kg (including the
copper lid) is at a temperature of 150oC. You pour
0.10 kg of water at 25oC into the pot, then quickly
close the lid of the pot so that no steam can escape.
Find the final temperature of the pot and its contents,
and determine the phase (liquid or gas) of the water.
Assume that no heat is lost to the surroundings.

34. Example
3. A 0.10-kg of piece of ice at 0oC is placed in a liter
of water at room temperature (20oC) in an
insulated container. Assuming that no heat is lost to
the container, what is the final temperature of
water?
4. A 20kg block of ice at -10oC, is put inside a
cylinder containing water at 50oC. All the ice is
melted and their final temperature is 10oC. How
much water is present initially inside the cylinder?

35. SW
1. A cube of aluminum 10cm on each side is cooled
from 100oC to 20oC. If the heat removed from the
aluminum cube were added to a copper cube of
the same size at 20oC, what would be the final
temperature of the copper cube?( (Al) =
2.7g/cm3 , (Cu) = 8.9g/cm3)
2. How much ice (0oC) must be added to 1.0 kg of
water(liquid) at 100oC so as to end up with all
liquid at 20oC?

36. Methods of Heat Transfer
Conduction, Convection and Radiation

37. Methods of Heat Transfer
 Conduction
- use of thermal conductor (ex. Metals)
 Convection
- use of fluids (liquids or gas)
 Radiation
- no medium, uses EM wave to transfer heat

38. Conduction
 Modern theory views that thermal conductions
are due to electrons that are free to move.
 Metals have many free electrons. They are
good heat conductors.
 Non-metals such as wood or cloth have few
free electrons. They are poor heat conductors
or thermal insulator

39. Conduction
 In general, ability to conduct heat depends on
phase.
 Gases are poor conductors, molecules are relatively
far apart.
 Solids are better conductors, molecules are closer.
 Heat conduction can be quantitatively described as
the time rate of heat flow in a material for a given
∆T.
 H = ∆Q/ ∆t, change of heat/change in time.
 H= heat current

40. Conduction
A = total surface area
d = distance, thickness of slab
k = thermal conductivity constant
∆T/d = heat gradient
Good conductors have high thermal conductivity
constant while poor conductors have low thermal
conductivity constants.

41. Conduction
Substance Thermal Conductivity (k) (J/(m*s*oC)
Aluminum 205
Copper 385
Iron and Steel 50.2
Silver 406
Transformer Oil 0.18
Water 0.57
Air 0.024
Brick 0.71
Concrete 0.8
Styrofoam 0.01
Wood, oak 0.15
Vacuum 0

42. Examples 1
 A Styrofoam box used to keep drinks cold at a
picnic has a total area of 0.80 m2 and wall
thickness of 2.0 cm. it is filled with ice, water, and
cans of Omni-Cola at 0oC. What is the rate of
heat flow into the box if the temperature of the
outside wall is 30oC?

43. Example 2
 A silver bar with length of 200 cm with a cross
sectional area of 4 cm2 is put in contact with steam
at 100oC at one end and with water at 20oC on the
other end. Compute for the heat current if the silver
bar is perfectly insulated.

44. Example 3
 A steel bar 10.0 cm long is welded end to end to a
copper bar 20.0 cm long. Both bars are insulated
perfectly on their sides. Each bar has a square
cross-section, 2.00 cm on a side. The free end of the
steel bar is maintained at 100oC by placing it in
contact with steam, and the free end of the copper
bar is maintained at 0oC by placing it in contact
with ice. Find the temperature at the junction of the
two bars and the total rate of heat flow.

45. Convection
 Transfer of heat by mass motion of a fluid from one
region of space to another.
Example
- house cooling and heating system
- cooling system of automobile

46. Convection
 Forced convection – if the fluid moves by using a
pump.
Example:
- blood circulation (heart-pump)
 Natural convection or free convection – if the flow is
caused by difference in density.
Example:
- daily weather

47. Radiation
 Transfer of heat by electromagnetic waves such as
visible light, infrared and ultraviolet radiation.
 Most heat are transferred through radiation
Example:
- heat from the sun
- heat from charcoal grill

48. Radiation
 Heat current due radiation is;
Stefan-Boltzmann Law
H- heat current
T – Temperature of the body, must be in Kelvin
A- surface area
e – emissivity, between 0 to 1
- Stefan – Boltzmann constant
= 5.670400 x 10-8 W/m2 K4

49. Example 1
 A thin square steel plate, 10 cm on a side, is heated
in a blacksmith’s forge to a temperature of 800oC.
The emissivity is 0.60, what is the total rate of
radiation energy (heat current)?

50. Radiation
 Net rate of radiation from a body to the
surrounding.
Ts – temperature of the surrounding

51. Example 2
 If the total surface area of the human body is 1.20
m2 and the surface temperature is 303 K, find the
total rate of radiation of energy from the body if
the surroundings are at a temperature of 293.15 K.
assume that the emissivity is 0.6.