Thermal Physics

Question 1

What is an Open system in thermodynamics?

Answer 1

A system is classed as open if it can exchange physical properties (energy and matter) with its environment.

Question 2

What is a Closed system in thermodynamics?

Answer 2

This is a system that can exchange energy, but cannot exchange matter with it surroundings.

Question 3

What is an Isolated system in thermodynamics?

Answer 3

This is a system that cannot exchange anything with its environment.

Question 4

What is the definition of a system in thermodynamics?

Answer 4

• A system is an object, objects or areas of interest.
• The environment/surroundings are everything else in the universe surrounding the system.

Question 5

What is heat?

Answer 5

Energy transported down a temperature gradient by microscopic processes is called heat.

Question 6

What is thermodynamic equilibrium?

Answer 6

This is where there are no net macroscopic flows into or out of a particular system.

Question 7

What is thermal contact?

Answer 7

This is when two systems are in contact and heat can flow between them.

Question 8

What is meant when a system is thermally isolated?

Answer 8

This is when system is prevented from transferring heat with another system.

Question 9

What is a conductor?

Answer 9

This is a material that can transfer heat/heat can travel through.

Question 10

What is the Zeroth law of thermodynamics

Answer 10

If two systems are each in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other.

This states that if 3 of more systems, say A, B and C are connected in such a way that B is connected to A and C, but A and C are not connected to each other, then if A-B and B-C are both in equilibrium it can be inferred that A and C are also in equilibrium with each other.

Question 11

What is a thermodynamic coordinate?

Answer 11

A thermodynamic coordinate is a quantity that takes a single value when a system is in a particular equilibrium state. Therefore the state of the equilibrium system can be unambiguously specified by fixing the values of a certain number of thermodynamic coordinates.

Question 12

The Ideal gas law

Answer 12

This law only stands true if the gas is an ideal gas

PV=Nk_BT

Where P is pressure, V is volume, N is the number of gas particles, k_B is 1.380649 x 10^-23 J K^-1 (Boltzmann constant) and T is the absolute temperature in kelvin.

Question 13

What are the two categories that thermodynamic coordinates can be separated into?

Answer 13

• Extensive quantities do change.
• Intensive quantities do not change.

Question 14

What is the Internal energy of a thermal dynamic system?

Answer 14

The internal energy, U, is a thermodynamic coordinate representing its total energy.

Question 15

The first law of thermodynamics

Answer 15

The change in internal energy of a system is equal to the sum of the work done o the system and the heat transferred to the system.

ΔU = W + Q

Question 16

What is an Adiabatic process?

Answer 16

This is a process in which there is no heat transfer, and only work is done such as extending a spring.

Question 17

What is the heat capacity of a system?

Answer 17

If a small amount of heat is added to a system, and this changes the temperature, then the heat capacity is the ratio.

C = ΔQ / ΔT

The heat capacity, C, can be followed by a subscript that denotes which coordinates are kept constant as heat is delivered.

Question 18

What is specific heat capacity?

Answer 18

This is an intensive quantity given by dividing the heat capacity by the mass of the system.

Question 19

What is isothermal compressibility?

Answer 19

This gives a fractional change in volume of a substance as the pressure is increased as the temperature is kept constant.

Question 20

What is isothermal bulk modulus?

Answer 20

This is the reciprocal of compressibility and gives the change in pressure for a (fractional) change in volume.

Question 21

What is an isobaric process?

Answer 21

This is when the pressure of a system remains constant during a process.

Question 22

What is an isothermal process?

Answer 22

This is a process where the temperature of a system remains constant.

Question 23

What is an isochoric/isovolumetric process?

Answer 23

This is a process where the volume remains constant.

Therefore no work is done and ΔU = Q

Question 24

What is an adiabatic process?

Answer 24

This is a process where the heat (Q) of a system remains constant.

i.e. Q = 0 and ΔU = W

Question 25

What is a **quasistatic** process?

Answer 25

This is a process that happens so slowly that it is essentially at equilibrium states at all times.

Question 26

What is a **reversible process**?

Answer 26

This is a process in which the change to the system can be reversed by a making a small adjustment to the system. The process must be **quasistatic** and the current state of the system does not depend on its previous state. (A pump increasing pressure in a system can be reversed to lover pressure again in ver small increments. changes would be almost imperceivable.)

Question 27

What is an **irreversible process**?

Answer 27

This a process where changes happen quickly and the changes are large. If a pump pumped lots of air into a system rapidly, air currents could be felt and equilibrium would be lost. No process could be done to reverse these effects.

Question 28

**The Clausius version of the second law of thermodynamics**

Answer 28

No process is possible that *only* has the effect of transferring heat from a cooler system to a hotter system.

Question 29

**The Kelvin version of the second law of thermodynamics**

Answer 29

No process is possible that *only* has the effect of converting heat entirely into work.

Question 30

What is a **cycle**?

Answer 30

This is a process in which all systems involved return to their initial states in the end.

Question 31

Formula for **thermal efficiency**

Answer 31

The thermal efficiency of a heat engine is: η = W_out/Q₁ = (Q₁ - Q₂)/Q₁ = 1 - Q₂/Q₁ According to the kelvin version of the second law η < 1.

Question 32

What is **Carnot's theorem**?

Answer 32

This states that any heat engine operating between two reservoirs cannot be more efficient than a Carnots engine working between the same two reservoirs.

Question 33

What is the efficiency of all reversible heat engines between the same two temperature reservoirs?

Answer 33

For any two reversible heat engines between two heat reservoirs, their efficiency will always be the same. The efficiency of any reversible heat engine is given by: η_rev = 1 - Q₂/Q₁ = 1 - T₂/T₁

Question 34

What is the **coefficient of performance, K_R**?

Answer 34

This is a measure of the efficiency of a refrigerator, and is given by the amount of heat removed from the cooler object relative to the work needed to do so. calculated: *k*_R = Q₂/*W* = Q₂/(Q₁ - Q₂) (To calculate the coefficient of performance for a heat pump, *k*_H, the process is reversed and so replace both numerators with Q₁)

Question 35

What is **Entropy**?

Answer 35

This is a thermodynamic coordinate that stated how ordered/disordered a selection of atoms and molecules are. A solid has a low entropy as the particles form an organised lattice structure that can be predicted, whereas in a gas or liquid the atoms ae free moving, and therefore have a higher entropy.

Question 36

What is the **thermodynamic expression for entropy**?

Answer 36

For an infinitesimal exchange of heat, d*Q*, at temperature, *T*, the infinitesimal change in entropy, d*S*, obeys d*S* >= 1/*T* d*Q* where equality occurs for a reversible process only.

Question 37

What is **latent heat** and its formula?

Answer 37

Latent heat, *L*, is the energy associated with a finite change of entropy, Δ*S*, in a system undergoing a change of state at a constant temperature, *T*. *L* = Δ*ST* Specific latent heat, *l*, is the latent heat per unit mass of a substance.

Question 38

What is the **third law of thermodynamics**?

Answer 38

As the temperature of a system tends towards absolute 0, its entropy tends to a constant, *S*₀.

Question 39

What is the **fundamental postulate of statistical mechanics**?

Answer 39

All accessible microstates of an isolated system are equally likely (i.e. they have the same probability).

Question 40

What is **Boltzmann's entropy formula**?

Answer 40

The entropy of a system, *S*, is related to the number of microstates, Ω, by *S* = *k*_Bln(Ω)

Question 41

What is the **Maxwell speed distribution**?

Answer 41

The probability *p(V) dv* that an atom of mass *m* in an ideal gas has a speed between *v* and *v+dv*. Speed is defined in the range 0 =< *v* =< ∞

Question 42

What is the **root -mean-square speed**?

Answer 42

This is the square root of the mean of the square of speed. *v_rms* = sqrt(<*v ²*>) For an ideal gas in three dimensions this is given by *v_rms* = sqrt(3*k_BT/m*)

Question 43

Calculating the **internal energy of a monatomic ideal gas**

Answer 43

An ideal gas of *N* identical atoms of mass *m*, in thermal equilibrium at temperature T, has an internal energy *U* given by *U = N 3/2 k_BT*

Question 44

What is the **equipartition theorem**?

Answer 44

consider a classical system which has a degree of freedom *f* that has an energy *E_f ⍺ f ^a*, where *a*is a positive real constant. The equipartition theorem states that in thermodynamic equilibrium at temperature *T*, this degree of freedom has a corresponding contribution *U_f = f>* to the internal energy given by *U_f = 1/a k_B T* The total internal energy *U* of the system is given by summing up the contributions from all degrees of freedom.