Thermodynamics First Law Flashcards
What is a system?
The part of the universe we are interested in, e.g. Earth, a human, a box
What are thermal surroundings?
The rest of the universe besides the system we study
What is the universe made of?
A system and it’s surroundings
What is an isolated system?
A system that has no interaction (no exchange of energy, matter, momentum etc) with other systems or the surroundings
What are the two *types* of property?
Intensive and extensive properties
What are the four thermodynamic variables?
Pressure, temperature, volume and number of moles
What is an intensive property?
A property of part of the system that is equal to that of the whole system, e.g. pressure or temperature
What is an extensive property?
A property of the system which is different to that of the whole system, e.g. volume or number of moles
How would you describe the intensive properties of a system at equilibrium?
They are uniform or homogenous
What equation of state links the four thermodynamic variables?
pV = nRT
What does it mean if two systems, a and b, are in thermal contact?
At equilibrium, Ta = Tb, and a and b are in thermal equilibrium
What types of wall allow or prohibit thermal contact?
Diathermal walls allow thermal contact; adiabatic walls prohibit it
What type of energy is transferred through a diathermal wall?
Heat energy
What is the flow of heat at thermal equilibrium?
At thermal equilibrium, all heat flow ceases
What does the property of temperature allow you to determine?
Temperature determines if systems are in thermal equilibrium with each other
What does it mean if two systems, a and b, are in mechanical contact?
At equilibrium, Pa = Pb, and a and b are in mechanical equilibrium
What type of walls allow or prohibit mechanical contact?
Moveable or flexible walls allow mechanical contact; immovable or inflexible walls disallow thermal contact
What type of energy is transferred through a diathermal wall?
Work (i.e. force x displacement)
What does it mean if two systems, a and b, are in mechanical contact?
Mass can be exchanged between a and b, which come to ‘mass equilibrium’
What type of walls allow or prohibit mechanical contact?
Walls permeable to matter allow mass contact; walls impermeable to matter prohibit it
What can you say about the gas densities of two systems in mass equilibrium?
The mass density of the two systems are equal
How can you express most thermodynamic problems?
What new equilibrium state is reached after the removal of internal constraints in an isolated composite system?
How would you define the heat capacity C of a system?
How would you define the specific heat c of a system?
The heat capacity per unit mass, i.e. Q=mc∆T (for gases, need to be specified for constant pressure or volume)
What is the sign convention for the heat, Q, added to a system?
Heat added to a system is positive
What would the final temperature be if you replaced an adiabatic wall between two systems at Ta and Tb with a diathermal wall?
This can be considered as a “weighted average” of the two initial temperatures, weighted by heat capacity
How would you define a thermal reservoir?
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
How would you define the latent heat of a material?
The energy required to cause a phase change (e.g. melting, boiling) in 1kg of the material with no change in temperature
Describe the phase diagram of water. Give the temperature, in Kelvin, at which water freezes under 1 atm of pressure
Freezes at 273.15 K
What is the rate of flux (flow of energy) between Thot and Tcold for a rectangular parallelepiped?
k is the thermal conductivity, units W/m/K
What is the rate of cooling for an object of heat capacity C at temperature T?
k is the thermal conductivity, units W/m/K
What did Joule’s Paddle-Wheel Experiment prove?
The work done by a weight (mgh) is equivalent to the heat gained by water (C∆T), so heat and work are equivalent
What is a thermodynamic process?
The passage between two equilibrium states A and B
What is a quasistatic process?
A process involving a succession of states
What is a reversible process?
A process in which no dissipative forces (e.g. friction) act
What is the first law of thermodynamics?
∆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
Define the internal energy of a system U
U is an extensive property, and a state function of the thermodynamic variables: U=U(p,T,n)
What is thermodynamic variable does the internal energy of an ideal gas depend on?
The temperature of the system T
What is the infinitesimal form of the first law of thermodynamics?
dU = ƌQ + ƌW
Express the “pV” work done by an expanding gas, qualifying the assumptions you make
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
Define Cv, the heat capacity of a gas at constant volume
Cv = ƌQv/dT
Qv is the heat added at constant volume
Cv is also the partial derivative of U wrt temperature
Define H, the enthalpy of a system
H = U + pV = U + nRT
H is an extensive property of the system, and is a state function where H=H(n,p,V)
Define Cp, the heat capacity of a gas at constant pressure
Cp = ƌQp/dT
Qp is the heat added at constant pressure
Cp is also the partial derivative of H wrt temperature
What is the internal energy U of an ideal gas?
U = 3/2 nRT
What equation links Cp and Cv?
Cp = Cv + nR
If the internal energy of an ideal gas is 3/2 nRT, what are the values of Cp and Cv?
Cp = Cv + nR
Cv = ∂U/∂T = 3/2 nR
Cp = 3/2 nR + nR = 5/2 nR
Why is U a state function, but Q and W are not?
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
How are reversible and irreversible process represented on a P/V digram?
Reversible processes are full lines; irreversible processes are dashed lines
What is the change in internal energy for the adiabatic free expansion of an ideal gas? Is this process reversible?
∆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)
Define isobaric, isovolumetric and isothermal processes
Isobaric - constant pressure
Isovolumetric - constant volume
Isothermal - constant temperature
Define the work done in terms of pressure and volume
Define the work done in an isobaric process
Define the work done in an isovolumetric process
Define the work done in an isothermal process
What is the change in internal energy of a system wrt pressure and volume?
What is the change in heat energy for an isothermal process?
U=U(T) so ∆U=0 Q=-W
Describe expansion and compression of a gas in terms of heat and work
In expansion, a gas absorbs heat and converts it all to work In compression, work is entirely converted to heat
Define gamma, Ɣ
Ɣ=Cp/Cv
p V^Ɣ is constant for n moles of gas
Describe and contrast p/V graphs for adiabatic and isothermal expansion
In both cases, as volume increases pressure decreases, and pressure is larger when temperature is larger. The gradient is steeper for adiabats than isotherms
Contrast the energy changes for isothermal and adiabatic expansions
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
Contrast the energy changes for isothermal and adiabatic compressions
Isothermal compression: external work is turned into heat in the surroundings
Adiabatic compression: work done puts energy into the gas
