Lecture 2 - Laws of Thermodynamics (DOBSON)

Question 1

What is the zeroth law of thermodynamics?

Answer 1

If two systems are (individually) in equilibrium with a thrid system, they are also in thermal equilibrium with each other. DRAW DIAGRAM. Systems in thermal equilibrium do not exchange heat. The temps are equal for all systems in thermal equilibrium.

Question 2

What is the first law of thermodynamics?

Answer 2

(Expresses the principle of conservation of energy). Energy can be transformed (changed from one form to another) but cannot be created or destroyed. Formulated by stating that the change in internal energy of a system (U) is equal to the amount of heat applied to the system (Q), minus the amount of work performed by the system on its surroundings (W). dU = dQ - dW

Question 3

What is the second law of thermodynamics?

Answer 3

After a certain amount of time, isolated physical systems reach equilibrium. In any cyclic process the entropy will either increase or remain the same. dS >/= 0

Question 4

What is the third law of thermodynamics?

Answer 4

This states that the entropy of a perfect crystal approaches zero as the absolute temp approaches zero. In doing so it provides an absolute reference point for the determination of entropy. S=0 when T=0

Question 5

How can heat transfer in fluids be understood?

Answer 5

By considering the kinetic theory of gases.

Question 6

What assumptions does the kinetic theory of gases rely on?

Answer 6

1. The gas consists of very small particles. This small size is such that the total volume of individual gas molecules added up is negligible compared to the volume of the container.
2. These particles have the same mass.
3. The number of molecules is so large that statistical treatment can be applied.
4. These molecules are in constant, random & rapid motion.
5. The rapidly moving particles constantly collide among themselves and with the walls of the container. All of these collisions are perfectly elastic.
6. Except during collisions, the interactions among molecules are negligible.
7. The average kinetic energy of the gas particles depends only on the temp. of the system.

Question 7

What is the expression to describe the kinetic energy of a single particle?

Answer 7

(1/2).m.(v_rms^(2)) = (3/2).K_b.T

Question 8

What happens if there is a spatial variation in temp of the fluid?

Answer 8

This will tend to equilibriate due to successive collisions between the particles. In this way, heat can be conducted through the fluid.

Question 9

How can particles gain and lose KE?

Answer 9

gain (increase temp) by coming into contact with a hot surface. Lose (decrease temp) by colliding with a cold surface. (in this way gas can transfer heat with a solid surface and heat can be transferred between two solid surfaces via an intermediate fluid.

Question 10

Will the rate of heat transfer be faster for a hot fluid or a cold fluid?

Answer 10

Faster for a hot fluid because hot particles move faster than cold ones. (hot fluids have higher thermal conductivity)

Question 11

What is mean free path?

Answer 11

The mean distance that a particle travels before colliding with another particle: 
l = K_b.T/(sqrt(2).pi.d^(2).p) 
l=mean free path
K_b = Boltzmann's constant 
T = Temp
d = atomic (molecular diameter) 
p = pressure

Question 12

How is heat conduction analogous to no. of cars on a motorway?

Answer 12

If an individual particle can travel from one surface to another without any collisions, it will conduct heat very well. However, if we achieve this by decreasing the pressure we also greatly reduce the no. of particles available to conduct the heat.

Question 13

Can you draw the graph of conductivity vs pressure?

Answer 13

YES OR NO

Question 14

What does the thermal conductivity of a gas ultimately depend on?

Answer 14

Temp, pressure and amount of energy each particle is capable of carrying (heat capacity).

Question 15

Equation for calculating the conductivity of an ideal gas?

Answer 15

K = nlambdaCv/3N_a. 
K=conductivity
n = no. of particles per volume
v = mean particle speed
lambda = mean free path
Cv = molar heat capacity
N_a = Avogadro's number

Question 16

Conduction in solids at particle level?

Answer 16

The particles are tethered in place by chemical bonds, making it impossible for them to move in the same way as a liquid or a gas. Heat can be transferred by conduction when adjacent atoms vibrate against one another. This is similar to normal modes in classical mechanics.

Question 17

Why is conduction greater in solids than fluids?

Answer 17

Because the network of relatively fixed spatial relationships between atoms help to transfer energy between them by vibration.

Question 18

What are phonons?

Answer 18

The normal modes have wave and particle-like properties due to quantum mechanics and are called phonons. (analogous to photons of light). Phonons propagate through a solid at the speed of sound in that solid but are scattered by boundaries, defects and other phonons in the solid. This scattering limits the transport of heat through a solid.

Question 19

What is the dominant conduction mechanism in solids that conduct electricity?

Answer 19

Free moving electrons are able to rapidly transfer thermal energy. The electron fluid of a conductive metallic solid conducts nearly all of the heat flux through a solid.

Question 20

General properties of liquid conduction?

Answer 20

In general, there is greater particle to particle interaction than a gas, permitting better conduction. However, bonding between particles is much less rigid than a solid, decreasing phonon propagation.

Question 21

What are the different factors in heat transfer?

Answer 21

1. dT/dx [k/m] - heat flux is proportional to thermal gradient.
2. t [s] - in transient analysis the duration of heat flux needs to be taken into consideration. Longer t brings us closer to steady state.
3. k [W/mK] - quantifies a materials ability to conduct heat
   4, A [m^(2)] - heat flux over a larger area results in a greater transfer of energy.
4. hc [W/m^(2)K] - this is a property indicating the thermal conductivity between two bodies in contact.
5. C [J/K] - the measurable physical quantity that characterises the amount of heat required to changed a substance’s temp by a given amount.

Question 22

What is steady state heat conduction?

Answer 22

A situation where the temp at every point within an object does not change with time. The amount of heat entering any region of an object is equal to the amount of heat coming out plus any work done by the heat.

Question 23

What is transient heat conduction?

Answer 23

Appear after an imposed change in temp at a boundary of an object. When a perturbation of temp of this type happens, temps within the system will change with time toward a new equilibrium with the new conditions.