Thermodynamics First Law

Question 1

What is a system?

Answer 1

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

Question 2

What are thermal surroundings?

Answer 2

The rest of the universe besides the system we study

Question 3

What is the universe made of?

Answer 3

A system and it’s surroundings

Question 4

What is an isolated system?

Answer 4

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

Question 5

What are the two *types* of property?

Answer 5

Intensive and extensive properties

Question 6

What are the four thermodynamic variables?

Answer 6

Pressure, temperature, volume and number of moles

Question 7

What is an intensive property?

Answer 7

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

Question 8

What is an extensive property?

Answer 8

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

Question 9

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

Answer 9

They are uniform or homogenous

Question 10

What equation of state links the four thermodynamic variables?

Answer 10

pV = nRT

Question 11

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

Answer 11

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

Question 12

What types of wall allow or prohibit thermal contact?

Answer 12

Diathermal walls allow thermal contact; adiabatic walls prohibit it

Question 13

What type of energy is transferred through a diathermal wall?

Answer 13

Heat energy

Question 14

What is the flow of heat at thermal equilibrium?

Answer 14

At thermal equilibrium, all heat flow ceases

Question 15

What does the property of temperature allow you to determine?

Answer 15

Temperature determines if systems are in thermal equilibrium with each other

Question 16

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

Answer 16

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

Question 17

What type of walls allow or prohibit mechanical contact?

Answer 17

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

Question 18

What type of energy is transferred through a diathermal wall?

Answer 18

Work (i.e. force x displacement)

Question 19

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

Answer 19

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

Question 20

What type of walls allow or prohibit mechanical contact?

Answer 20

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

Question 21

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

Answer 21

The mass density of the two systems are equal

Question 22

How can you express most thermodynamic problems?

Answer 22

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

Question 23

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

Answer 23

Question 24

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

Answer 24

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

Question 25

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

Answer 25

Heat added to a system is positive

Question 26

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

Answer 26

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

Question 27

How would you define a thermal reservoir?

Answer 27

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

Question 28

How would you define the latent heat of a material?

Answer 28

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

Question 29

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

Answer 29

Freezes at 273.15 K

Question 30

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

Answer 30

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

Question 31

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

Answer 31

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

Question 32

What did Joule's Paddle-Wheel Experiment prove?

Answer 32

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

Question 33

What is a thermodynamic process?

Answer 33

The passage between two equilibrium states A and B

Question 34

What is a quasistatic process?

Answer 34

A process involving a succession of states

Question 35

What is a reversible process?

Answer 35

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

Question 36

What is the first law of thermodynamics?

Answer 36

∆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

Question 37

Define the internal energy of a system U

Answer 37

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

Question 38

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

Answer 38

The temperature of the system T

Question 39

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

Answer 39

dU = ƌQ + ƌW

Question 40

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

Answer 40

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

Question 41

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

Answer 41

Cv = ƌQv/dT Qv is the heat added at constant volume Cv is also the partial derivative of U wrt temperature

Question 42

Define H, the enthalpy of a system

Answer 42

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

Question 43

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

Answer 43

Cp = ƌQp/dT Qp is the heat added at constant pressure Cp is also the partial derivative of H wrt temperature

Question 44

What is the internal energy U of an ideal gas?

Answer 44

U = 3/2 nRT

Question 45

What equation links Cp and Cv?

Answer 45

Cp = Cv + nR

Question 46

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

Answer 46

Cp = Cv + nR Cv = ∂U/∂T = 3/2 nR Cp = 3/2 nR + nR = 5/2 nR

Question 47

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

Answer 47

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

Question 48

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

Answer 48

Reversible processes are full lines; irreversible processes are dashed lines

Question 49

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

Answer 49

∆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)

Question 50

Define isobaric, isovolumetric and isothermal processes

Answer 50

Isobaric - constant pressure Isovolumetric - constant volume Isothermal - constant temperature

Question 51

Define the work done in terms of pressure and volume

Answer 51

Question 52

Define the work done in an isobaric process

Answer 52

Question 53

Define the work done in an isovolumetric process

Answer 53

Question 54

Define the work done in an isothermal process

Answer 54

Question 55

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

Answer 55

Question 56

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

Answer 56

U=U(T) so ∆U=0 Q=-W

Question 57

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

Answer 57

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

Question 58

Define gamma, Ɣ

Answer 58

Ɣ=Cp/Cv p V^Ɣ is constant for n moles of gas

Question 59

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

Answer 59

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

Question 60

Contrast the energy changes for isothermal and adiabatic expansions

Answer 60

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

Question 61

Contrast the energy changes for isothermal and adiabatic compressions

Answer 61

Isothermal compression: external work is turned into heat in the surroundings Adiabatic compression: work done puts energy into the gas