12. THERMODYNAMICS II Flashcards
1
Q
- What are the three physical states that matter exists in?
A
- solid
- liquid
- vapor
2
Q
- What can change about a material when it absorbs heat?
A
- its physical state
3
Q
- What is Melting?
Give an example.
A
- it is when a solid changes from the solid state
- to the liquid state
- the process requires energy
- this is because the molecules of a solid are found in a rigid structure
- they are not free to move without added energy
- EG: melting an ice cube to form liquid water
4
Q
- What is Freezing?
A
- it is when a liquid is frozen to form a solid
- it is the reverse of melting
- it requires energy to be removed from the liquid
- this is so that the molecule can settle into a rigid structure
5
Q
- What is Vaporizing?
Give an example.
A
- it is changing a substance from its liquid state to its vapour (gas) state
- this process requires energy
- this is because the molecules must be freed from their clusters
- boiling liquid water
- this transforms it to water vapour (or steam)
6
Q
- What is steam?
A
- it is the gas of individual water molecules
7
Q
- What is Condensing?
A
- it is when a gas is condensed to form a liquid
- it is the reverse of vaporizing
- it requires energy to be removed from the gas
- this allows the molecules of the gas to cluster instead of flying away from one another
8
Q
- What is a material’s heat of transformation?
What is another word for the heat of the transformation?
A
- it is the amount of energy required per unit mass
- to change the state of a particular material
- it does NOT change the temperature
- the Latent Heat
9
Q
- What is the symbol for heat of transformation?
A
- L
10
Q
- What is the formula to work out the heat of the transformation?
A
Q= L x m
- Q= the measure of the heat energy (J/kg)
- L= constant
= it is the latent heat dependent on the material
= it is measured in joules - m = mass (kg)
11
Q
- What is the Heat of Vaporization (Lv)?
A
- it is the amount of energy per unit mass
- that must be added to vaporize a liquid
- OR that must be removed to condense a gas
12
Q
- What is the Heat of Fusion (Lf)?
A
- it is the amount of energy per unit mass
- that must be added to melt a solid
- OR that must be removed to freeze a liquid
13
Q
- Why is an effective mode of body heat loss important?
A
- all our metabolism goes into making heat
- very little of our metabolism goes into useful mechanical work
- our body temperature would rise very quickly if we did not have effective modes of heat loss
14
Q
- What are the five modes of heat loss?
A
- Radiation
- Convection
- Conduction
- Evaporation of sweat
- Evaporation of water through breathing
15
Q
- What is radiation loss?
What percentage of heat is lost through this?
A
- it is the loss of heat by the emission of electromagnetic radiation
- 54%-60% heat loss
16
Q
- What is convection?
What percentage of heat is lost through this?
A
- the loss of radiated heat
- it is removed by moving air
- around 25% of heat is lost
17
Q
- What is conduction?
What percentage of heat is lost through this?
A
- it is the direct transfer of heat through contact
- around 25% of heat is lost
18
Q
- What is the Evaporation of sweat?
What percentage of heat is lost through this?
A
- it is the loss of heat by the evaporation of water
- around 7% of heat is lost
19
Q
- What is the Evaporation of water through breathing?
What percentage of heat is lost through this?
A
- it is the loss of heat by the evaporation of water
- around 14% of heat is lost
20
Q
- What do the magnitude and importance of the four modes of heat loss depend on?
A
- clothing
- environment
- surroundings
21
Q
- What is thermal radiation?
A
- it is the exchange of energy as heat between an object and its environment
- this is done via electromagnetic waves
22
Q
- What happens when you stand near a fire?
A
- you are warmed
- you absorb the thermal radiation from the fire
- your thermal energy increases as the fire’s thermal energy decreases
- there is no medium required for the heat transfer via radiation
23
Q
- Why is there no medium required for heat transfer that is accomplished through radiation?
A
- radiation can travel through a vacuum
24
Q
- According to Planck’s Law, what do all objects at a temperature (T) emit?
A
- they emit Thermal radiation
25
25. What is essential in Planck's law?
- the presence of a black body
26
26. What is a Black Body?
List three characteristics.
1. It is a body that absorbs all the thermal radiation falling onto it
2. At thermal equilibrium, it emits as much energy as it absorbs
3. It is a good absorber of radiation
and a good emitter of radiation
27
27. What is Radiation incident?
- the unintended exposure of an individual to radiation
28
28. What are the three types/components of Radiation Incident?
- reflection of the radiation by the object
- absorption of the radiation into the object
- transmission of radiation through the object
29
29. What is Emissivity?
- it is the fraction of the energy incident on the object
that is absorbed
30
30. What can be said about the Emissivity of a shiny, metallic, reflective surface?
- it is very low
- less than 0.1
NB: emissivity exists on a scale from 0 to 1
31
31. What can be said about the Emissivity of dull, black surfaces?
- they have the highest emissivity
- between 0.9 and 1
32
32. What is the unit for Emissivity?
- ε
33
33. What is the formula to calculate emissivity?
P = 𝑒. σ . A . T⁴
- P= the radiated power (Watts)
- 𝑒 = emissivity
= it is a ratio
= it has no units
- σ= the Stefan - Boltzman Constant
= 5.67 x 10 ⎺⁸ (W/m².K⁴)
- A = the surface area (m²)
- T = the temperature (Kelvins)
34
34. What does the Stefan Boltzman Law represent?
How can this be expressed in a formula?
- the energy per second (power) radiated by the surface
- ▵Q / ▵t
35
35. Where does the body receive radiant energy from?
- the surrounding objects
36
36. What unit of measurement can we find to calculate the approximate difference between the energy radiated by the body AND the energy absorbed from the radiation from the surroundings?
- we can look for Hr
- this is the rate of energy loss or gain
- it is another way of writing power
- it is measured in watts
37
37. What is the formula for Hr?
- Hr = Power (watts)
- Kr = constant
= it is a parameter
= it is about 2.1 x 10⁴ (J/m²hr°C)
= it can also be written as 5.0 kcal/m²hr°C
- Ar = the effective body surface area (m²)
= it is what is emitting the radiation
- 𝑒 = surface emissivity
- (Ts - Tw) = difference in temperature
= this can be in any unit
- Ts = the skin temperature (°C)
- Tw = the surrounding walls temperature (°C)
38
38. When does Conduction occur?
- when the thermal energy moves through a material as a result of collisions
- these collisions happen between free electrons, ions, atoms and molecules of the material
39
39. What can be said about the average kinetic energy of a hotter substance?
- it is higher
40
40. What happens with regards to energy between materials in contact with one another?
- the higher energy atoms in the warmer substances transfer energy to the lower energy atoms in the cooler substances
- this is a result of atomic collisions between the two
- this allows heat to flow from hot to cold
41
41. How would we calculate the temperature difference?
▵T = T1 - T 2
42
42. What is the temperature gradient?
- the ▵T / ▵ x
- ▵T = the change in temperature
= the unit is not important
- ▵x = the thickness of the body
- this is the rate of change of temperature with distance
43
43. How would we work out the quantity of heat (▵Q) that is transmitted from Face 1 to Face 2 of the body for a given time (▵t)?
▵Q / ▵t = kT x A x ▵T/▵x
kT= this is the thermal conductivity of the material
= this depends on the material of the body
= it has a unit of W/m.K
A= the cross sectional area (m²)
▵T = the change in temperature
= the unit is not important
▵x = thickness
= change in distance (m)
44
44. What is another way of working out ▵Q / ▵t?
▵Q / ▵t = h x A x ▵T
- h = the coefficient of the conduction heat transfer
- A = the cross sectional area (m²)
- ▵T = change in temperature
= the unit is not important
45
45. How else can we calculate the coefficient of conduction heat transfer (h)?
h = kT / ▵x
- kT= the thermal conductivity
= W/m.K.
- ▵x= thickness of the body
= the distance (m)
46
46. What is the flame of a candle classified as?
- it is convection
- it is this convection that is transporting the thermal energy upwards
- this energy transfer occurs when:
- a fluid (air)
- encounters an object (match)
- with a higher temperature than that of the fluid
-the temperature of the part of the fluid that is in contact with the hot object increases
- the fluid expands
- it becomes less dense
- it is now lighter than the surrounding cooler fluid
- buoyant forces cause it to then rise
47
47. How can the heat loss due to Convection/Conduction be worked out?
- we must find the heat loss due to convection (Hc)
- this is the rate of energy transfer
- it is another way of working out power
- it has a unit of Watts
48
48. What is the formula for working out Hc?
Hc = Kc x Ac x (Ts - Ta)
- Hc = the heat loss due to convection
= power (in watts)
- Kc = a parameter
= it depends on the movement of the air
= when the speed of the air is less than 0.2 m/s
= it then has a value of 3.1 W/m²K
- Ac = the effective body surface area (m²)
- Ts = the skin temperature in Kelvins
- Ta = the air temperature in Kelvins
49
49. When is the Convective heat loss greater?
- when the air is moving
- the Convective heat loss is much smaller when the air is still
50
50. What is Convection within the body called?
- the flow of blood
51
51. What does the flow of blood change?
What does it not affect?
- it changes the distribution of the temperature within the body
- it does not affect the average temperature of the body
52
52. What is the normal core body temperature?
- 36.5° - 37.5°
53
53. How much does the core temperature generally change during the waking hours?
- it increases by 0.5 ° C
54
54. What is an example of how well the body controls its temperature?
- the nude body can maintain its core temperature (between 36°C and 38°C) for several hours
- it can do this within environmental temperature changes that range from 21°C and 54°C in dry air
55
55. What is the correlation between temperature deviation from the normal body temperature, and exposure time?
- the larger the temperature deviation from the normal body temperature
- the shorter the maximum possible exposure time until death
56
56. What is the Convection Temperature for a naked adult man as compared to a naked baby?
(if we have minimal air movement)
- naked adult man = 26°C
- naked baby = 35°C
57
57. Where is the Thermoneutral Zone located?
(the zone of minimal metabolism)
- it is between the Tc (Convection Temperature)
AND
- the Th (Hypothermia Temperature)
58
58. What happens below the Tc (convection temperature)?
- the heat loss increases
- this is due to convection and radiation
59
59. What increases metabolism?
- increases in muscle tension
- shivering
60
60. What increases muscle tension and shivering?
- increases in the difference between the body temperature and the environmental temperatures
61
61. What happens to the core body temperature at low temperatures?
- it cannot be maintained
- there is cooling
- hypothermia develops
62
62. How does heat loss happen above Th
(Hypothermia Temperature)?
- it is dominated by evaporation
63
63. Give the core temperature ranges for each of the following:
63.1: Mild Hypothermia
63.2: Moderate Hypothermia
63.3: Severe Hypothermia
63.1: 32°C- 35°C
63.2: 28°C - 32°C
63.3: less than 28°C
64
64. Which temperature is usually lower than the Core
temperature (37°C)?
- the temperature of the body skin (Tskin)
- this usually sits at 34°C
65
65. What is Thermoregulation?
- it is the maintenance of a constant core body temperature in an organism
- both humans and other warm-blooded organisms thermoregulate
66
66. What is the term given to warm-blooded organisms?
- homeotherms
67
67. What aspects of the human body allow it to thermoregulate?
- the human body has temperature-sensitive receptors
- these are located in the skin, hypothalamus, midbrain, spinal cord and the abdominal cavity
- these thermoreceptors sense either hot and cold
- most areas of the human body have both kinds of thermoreceptors
- the sensors send information to the hypothalamus in the brain
- this controls the body's response to the stimuli
68
68. How do the blood vessels in the body react when the internal body temperature or the skin temperature gets too low?
- these blood vessels are found adjacent to the skin
- the blood vessel becomes smaller
- it constricts
- it reduces the blood flow from the core to the surface
- this process is called Vasoconstriction
69
69. What are the effects of Vasoconstriction?
- it reduces the rate of heat loss from the skin surface
- it allows the skin to get colder
70
70. What is Vasodilation and when is it used?
- it is when the blood vessel increases in size
- it expands
- this happens when there is a need to increase the rate of heat loss
- this happens when the body is too hot
- the blood supply to the skin is increased by Vasodilation
- this increases the blood flow in the skin
- this reduces the blood pressure
71
71. What is Piloerection?
- it is when the fine body hairs stand on end
- this is an attempt to reduce convective heat loss from the skin
- the raising of the air is intended to create a stationary layer of air between the skin and the surroundings
72
72. What is Shivering?
- it is a physiological response to cold temperatures
- cold signals are transmitted to the hypothalamus
- they trigger the reflex
- the body muscles begin to make small movements to use up some metabolic energy
- this generates thermal energy
73
73. What is Perspiration/ Sweating?
- it is a very effective cooling mechanism
- this is because the Latent Heat (L) of evaporation of water is high
- heat loss that is caused by the evaporation of the liquid from the skin is the only way we can lose heat when the air is above 37°C
- the human body has sweat glands in the skin
- these excrete fluid
- it is mainly water
- it has some dissolved salts in it
74
74. Read through this summary.
Does everything make sense?
- yes
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75. Read through this summary.
Does everything make sense?
- yes
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76. Read through this summary.
Does everything make sense?
- yes
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77. Read through this summary.
Does everything make sense?
- yes
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78. Read through this summary.
Does everything make sense?
- yes
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79. Read through this summary?
Does everything make sense?
- yes
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80. Read through this summary.
Does everything make sense?
- yes
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81. Read through this summary.
Does everything make sense?
- yes
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82. Read through this summary.
Does everything make sense?
- yes
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83. Read through this summary.
Does everything make sense?
- yes
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84. Read through this summary.
Does everything make sense?
- yes
