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Thermodynamics > Lecture 1 > Flashcards

Lecture 1 Flashcards

1
Q

What does Thermodynamics focus on

A

Energy conservation and state changes (“what” happens)
The amount of energy exchanged and system equilibrium

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2
Q

What does Heat Transfer focus on

A

Rate and mechanism of energy transfer/heat flow (“how” it happens)

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3
Q

Thermodynamics Definition

A

The study of energy, work and heat in systems and their equilibrium states

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4
Q

Heat Transfer definition

A

The study of how heat energy moves between systems due to temperature differences
The process of thermal energy exchange between physical systems from a higher temperature to a lower one.

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5
Q

Mode of analysis for Thermodynamics

A

Examines overall energy changes (e.g. heat and work)

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6
Q

Mode of analysis for Heat Transfer

A

Focuses on conduction, convection and radiation

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7
Q

Thermodynamic System States

A

Concerned with initial and final equilibrium states

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8
Q

Heat Transfer System States

A

Deals with transient (non-equilibrium) processes

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9
Q

Examples of Thermodynamics

A

Efficiency of engines
Entropy Changes
Energy Conservation

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10
Q

Examples of Heat Transfer

A

Cooling of a device
Heat loss through walls

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11
Q

Thermodynamics Mathematical Tools

A

Thermal Equilibrium (0th Law)
Energy Conservation (1st Law)
Entropy (2nd Law)
Absolute Zero (3rd Law)

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12
Q

What is the Zeroth Law?

A

Thermal Equilibrium

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13
Q

What is the First Law?

A

Energy Conservation

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14
Q

What is the Second Law?

A

Entropy

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15
Q

What is the Third Law?

A

Absolute Zero

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16
Q

Heat Transfer Mathematical Tools

A

Fourier’s Law (conduction)
Newton’s Law of Cooling (convection)
Stefan-Boltzmann Law (radiation)

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17
Q

What does Fourier’s Law focus on?

A

Conduction

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18
Q

What does Newton’s Law of Cooling focus on?

A

Convection

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19
Q

What does the Stefan-Boltzmann Law focus on?

A

Radiation

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20
Q

Heat definition

A

The form of energy that can be transferred from one system to another as a result of temperature difference

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21
Q

Specific Heat Capacity (c) definition

A

The energy required to raise the temperature of a unit mass of a substance by one degree Celcius/Kelvin.

Quantifies the material’s ability to store or release thermal energy

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22
Q

Specific Heat Capacity Units

A

J/kg*K

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23
Q

Specific Heat Capacity Equation

A

Q=mc∆T
Q=heat energy (Joules)
m=mass
C=specific heat (J/kg*K)
∆T=change in temperature (K or Celcius)

24
Q

What does it mean to have a Low Specific Heat Capacity

A

The object warms up and cools down quickly as it takes less energy to change its temperature

25
Q

What does it mean to have a High Specific Heat Capacity?

A

The object warms up and cools down slowly as it takes up much more energy to change its temperature

26
Q

Thermal Conductivity (k) definition

A

A material property that measures its ability to conduct heat.
The rate of heat transfer through a unit thickness of the material per unit area per unit temperature difference

27
Q

Thermal Conductivity Units

A

W/m*K

28
Q

Thermal Conductivity Equation

A

k = qL/A(T1-T2)
k: Thermal Conductivity (W/mK)
q: Heat transfer rate (W)
L: Thickness of material (m)
A: Cross-sectional area (m^2)
∆T: Temperature difference (in Kelvin or Celcius

29
Q

What does a higher value of Thermal Conductivity mean?

A

The material is a better heat conductor

30
Q

What does a lower value of Thermal Conductivity mean?

A

The material is a better insulator

31
Q

Thermal Diffusivity (α)

A

A material property that measures how quickly heat spreads through a material.
The ratio of thermal conductivity to the product of the material’s density and specific heat capacity.
m^2/s

32
Q

Thermal Diffusivity Units

A

m^2/s

33
Q

Thermal Diffusivity Equation

A

α = Heat conducted/heat stored = k/ρc
α: Thermal Diffusivity (m^2/s)
k: Thermal Conductivity (W/mK)
ρ: Density (kg/m^3)
c: specific heat capacity (J/kgK)

34
Q

Compare High vs Low Thermal Diffusivity

A

The larger the thermal diffusivity, the faster the propagation of heat into the medium
A large α is the result of a high k or a low c.

35
Q

Conduction Definition

A

The transfer of energy from the more energetic particles of a substance to the adjacent, less energetic ones as a result of interactions between the particles.
Can take place in solids, liquids or gases

36
Q

How does conduction work between solids and fluids?

A

Fluids: Due to collisions and diffusion during random motion.
Solids: Due to vibration of molecules in a lattice and the energy transport by free electrons

37
Q

What affects the rate of heat conduction through a plane layer?

A

Temperature Difference across layer (proportionally)
Heat Transfer area (proportionally)
Thickness of layer (inversely proportional)

38
Q

Rate of Heat Conduction Equation

A

Rate of heat conduction ∝ Area*temperature difference/thickness

Qcond = -kA(dT/dx)

39
Q

Convection Definition

A

The mode of heat transfer between a solid surface and the adjacent liquid or gas that is in motion.
Involves the combined effects of conduction and fluid motion.
The faster the fluid motion, the greater the heat transfer. If motion is zero, heat is transferred by pure conduction.

40
Q

Compare Free vs Forced Convection

A

Forced: if the fluid is forced to flow over the surface by external means (e.g. fan, pump or wind)
Free: fluid motion is caused by buoyancy forces that are induced by density differences due to variation of temperature in the fluid

41
Q

Convection Equation

A

Qconv = hA(𝑻s-𝑻∞)
h: convection heat transfer coefficient (W/m^2)
As: surface area (m^2)
Ts: Surface temperature
𝑻∞: fluid temperature (C)

Proportional to temperature difference

42
Q

Radiation Definition

A

The energy emitted by matter in the form of electromagnetic waves as a result of the changes in the electronic configurations of the atoms or molecules.
All bodies above absolute zero emit thermal radiation

43
Q

How does transfer of energy by radiation differ to conduction/convection?

A

It does not require the presence of an intervening medium

44
Q

Radiation Emitted Equation

A

Qemit = εσ𝑨𝒔𝑻^4𝒔
ε: emissivity of the surface
σ: Stefan-Boltzmann constant
As: surface area (m^2)
Ts: surface temperature

For a blackbody, ε=1

45
Q

Radiation Absorptivity (α)

A

The fraction of the radiation energy incident on a surface that is absorbed by the surface.
A blackbody absorbs the entire radiation incident on it (α=1)

46
Q

Radiation Absorptivity Equation

A

α = Qabsorbed/Qincident

Qincident: the rate at which radiation is incident on the surface
α: the absorptivity of the surface

47
Q

Radiation Equation

A

Qrad = εσA(Ts^4-Tsurr^4)
ε: emissivity of the surface
σ: Stefan-Boltzmann constant
Ts = surface temperature
Tsurr = temperature of surroundings

48
Q

Combined Convection/Radiation Equation

A

Qtotal = (hcombined)(As)((Ts)-T∞)

49
Q

Heat Conduction Definition

A

The transfer of thermal energy from the more energetic particles of a medium to the adjacent less energetic ones.
Can take place in liquids and gases provided that there is no bulk motion involved.

50
Q

How does heat transfer differ from temperature?

A

Heat transfer is a vector quantity - has a direction and magnitude.
It is always normal to an isothermal surface.

51
Q

How are heat transfer problems classified?

A

Heat transfer can be steady or transient.
It can be one-, two- or three-dimensional.

52
Q

What is steady heat transfer?

A

Steady implies that there is no change with time at any point within the medium

53
Q

What is transient heat transfer?

A

Transient implies variation with time, or time dependence

54
Q

Fourier’s Law of Heat Conduction Equation (1D)

A

Qcond = -kA(dT/dx)

55
Q

Fourier’s Law of Heat Conduction Equation (3D)

A

Qx = -kA(∆T/∆x), Qy = -kA(∆T/∆y), Qz=-kA(∆T/∆z)

56
Q
A

Thermodynamics (2 decks)

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