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Thermal Physics > Thermodynamics First Law > Flashcards

Thermodynamics First Law Flashcards

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1
Q

What is a system?

A

The part of the universe we are interested in, e.g. Earth, a human, a box

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2
Q

What are thermal surroundings?

A

The rest of the universe besides the system we study

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3
Q

What is the universe made of?

A

A system and it’s surroundings

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4
Q

What is an isolated system?

A

A system that has no interaction (no exchange of energy, matter, momentum etc) with other systems or the surroundings

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5
Q

What are the two *types* of property?

A

Intensive and extensive properties

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6
Q

What are the four thermodynamic variables?

A

Pressure, temperature, volume and number of moles

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7
Q

What is an intensive property?

A

A property of part of the system that is equal to that of the whole system, e.g. pressure or temperature

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8
Q

What is an extensive property?

A

A property of the system which is different to that of the whole system, e.g. volume or number of moles

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9
Q

How would you describe the intensive properties of a system at equilibrium?

A

They are uniform or homogenous

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10
Q

What equation of state links the four thermodynamic variables?

A

pV = nRT

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11
Q

What does it mean if two systems, a and b, are in thermal contact?

A

At equilibrium, Ta = Tb, and a and b are in thermal equilibrium

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12
Q

What types of wall allow or prohibit thermal contact?

A

Diathermal walls allow thermal contact; adiabatic walls prohibit it

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13
Q

What type of energy is transferred through a diathermal wall?

A

Heat energy

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14
Q

What is the flow of heat at thermal equilibrium?

A

At thermal equilibrium, all heat flow ceases

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15
Q

What does the property of temperature allow you to determine?

A

Temperature determines if systems are in thermal equilibrium with each other

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16
Q

What does it mean if two systems, a and b, are in mechanical contact?

A

At equilibrium, Pa = Pb, and a and b are in mechanical equilibrium

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17
Q

What type of walls allow or prohibit mechanical contact?

A

Moveable or flexible walls allow mechanical contact; immovable or inflexible walls disallow thermal contact

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18
Q

What type of energy is transferred through a diathermal wall?

A

Work (i.e. force x displacement)

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19
Q

What does it mean if two systems, a and b, are in mechanical contact?

A

Mass can be exchanged between a and b, which come to ‘mass equilibrium’

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20
Q

What type of walls allow or prohibit mechanical contact?

A

Walls permeable to matter allow mass contact; walls impermeable to matter prohibit it

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21
Q

What can you say about the gas densities of two systems in mass equilibrium?

A

The mass density of the two systems are equal

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22
Q

How can you express most thermodynamic problems?

A

What new equilibrium state is reached after the removal of internal constraints in an isolated composite system?

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23
Q

How would you define the heat capacity C of a system?

A
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24
Q

How would you define the specific heat c of a system?

A

The heat capacity per unit mass, i.e. Q=mc∆T (for gases, need to be specified for constant pressure or volume)

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25
Q

What is the sign convention for the heat, Q, added to a system?

A

Heat added to a system is positive

26
Q

What would the final temperature be if you replaced an adiabatic wall between two systems at Ta and Tb with a diathermal wall?

A

This can be considered as a “weighted average” of the two initial temperatures, weighted by heat capacity

27
Q

How would you define a thermal reservoir?

A

An object A (with heat capacity Ca) such that an object B (with heat capacity Cb) placed in contact with it will adopt the temperature of A, since Cb/Ca

28
Q

How would you define the latent heat of a material?

A

The energy required to cause a phase change (e.g. melting, boiling) in 1kg of the material with no change in temperature

29
Q

Describe the phase diagram of water. Give the temperature, in Kelvin, at which water freezes under 1 atm of pressure

A

Freezes at 273.15 K

30
Q

What is the rate of flux (flow of energy) between Thot and Tcold for a rectangular parallelepiped?

A

k is the thermal conductivity, units W/m/K

31
Q

What is the rate of cooling for an object of heat capacity C at temperature T?

A

k is the thermal conductivity, units W/m/K

32
Q

What did Joule’s Paddle-Wheel Experiment prove?

A

The work done by a weight (mgh) is equivalent to the heat gained by water (C∆T), so heat and work are equivalent

33
Q

What is a thermodynamic process?

A

The passage between two equilibrium states A and B

34
Q

What is a quasistatic process?

A

A process involving a succession of states

35
Q

What is a reversible process?

A

A process in which no dissipative forces (e.g. friction) act

36
Q

What is the first law of thermodynamics?

A

∆U = Q + W

Q is the heat added TO the system

W is the work done ON the system

∆U is the change in the system’s internal energy

37
Q

Define the internal energy of a system U

A

U is an extensive property, and a state function of the thermodynamic variables: U=U(p,T,n)

38
Q

What is thermodynamic variable does the internal energy of an ideal gas depend on?

A

The temperature of the system T

39
Q

What is the infinitesimal form of the first law of thermodynamics?

A

dU = ƌQ + ƌW

40
Q

Express the “pV” work done by an expanding gas, qualifying the assumptions you make

A

ƌW = -p dV

Assume the process is quasistatic, so at all points F=F’

Assume the process is reversible, so all the work goes into the gas

41
Q

Define Cv, the heat capacity of a gas at constant volume

A

Cv = ƌQv/dT

Qv is the heat added at constant volume

Cv is also the partial derivative of U wrt temperature

42
Q

Define H, the enthalpy of a system

A

H = U + pV = U + nRT

H is an extensive property of the system, and is a state function where H=H(n,p,V)

43
Q

Define Cp, the heat capacity of a gas at constant pressure

A

Cp = ƌQp/dT

Qp is the heat added at constant pressure

Cp is also the partial derivative of H wrt temperature

44
Q

What is the internal energy U of an ideal gas?

A

U = 3/2 nRT

45
Q

What equation links Cp and Cv?

A

Cp = Cv + nR

46
Q

If the internal energy of an ideal gas is 3/2 nRT, what are the values of Cp and Cv?

A

Cp = Cv + nR

Cv = ∂U/∂T = 3/2 nR

Cp = 3/2 nR + nR = 5/2 nR

47
Q

Why is U a state function, but Q and W are not?

A

Changes in U depend only on the start and end point of the variables, not the path or process

Both W and Q depend on the path followed, as they are forms of energy transfer

48
Q

How are reversible and irreversible process represented on a P/V digram?

A

Reversible processes are full lines; irreversible processes are dashed lines

49
Q

What is the change in internal energy for the adiabatic free expansion of an ideal gas? Is this process reversible?

A

∆U = Q + W

Q is zero, as the expansion is adiabatic

W is zero as there is no opposing force

So ∆U is zero, as U=U(T) and T is constant

The process is irreversible, because the intermediate states are not equilibrium states (non-uniform density, pressure etc)

50
Q

Define isobaric, isovolumetric and isothermal processes

A

Isobaric - constant pressure

Isovolumetric - constant volume

Isothermal - constant temperature

51
Q

Define the work done in terms of pressure and volume

A
52
Q

Define the work done in an isobaric process

A
53
Q

Define the work done in an isovolumetric process

A
54
Q

Define the work done in an isothermal process

A
55
Q

What is the change in internal energy of a system wrt pressure and volume?

A
56
Q

What is the change in heat energy for an isothermal process?

A

U=U(T) so ∆U=0 Q=-W

57
Q

Describe expansion and compression of a gas in terms of heat and work

A

In expansion, a gas absorbs heat and converts it all to work In compression, work is entirely converted to heat

58
Q

Define gamma, Ɣ

A

Ɣ=Cp/Cv

p V^Ɣ is constant for n moles of gas

59
Q

Describe and contrast p/V graphs for adiabatic and isothermal expansion

A

In both cases, as volume increases pressure decreases, and pressure is larger when temperature is larger. The gradient is steeper for adiabats than isotherms

60
Q

Contrast the energy changes for isothermal and adiabatic expansions

A

Isothermal expansion: heat is absorbed from the surroundings, and is converted to works done by the gas

Adiabatic: the energy of the gas in turned into work

61
Q

Contrast the energy changes for isothermal and adiabatic compressions

A

Isothermal compression: external work is turned into heat in the surroundings

Adiabatic compression: work done puts energy into the gas

Thermal Physics (2 decks)

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