lecture 1 thermodynamics

Lecture no.1
Fundamental concepts of Thermodynamics TD
The System is the region where we focus our attention. Surrounding is the rest of the universe.
Universe = System + Surrounding
Temperature
Refers to the degree of hotness and coldness of the body usually expressed by numbers
? The absolute temperature scale is also called the thermodynamic scale or the kelvin scale and tempertures on this scale are denoted (K).
? Absolute or Kelvin Scale
The lowest possible temperature on the Celsius Scale is -273?C. The Kelvin Scale just takes this value and calls it 0K, or absolute zero.
To convert from C to K just add 273.
K=C+273 OR we can write
T= t + 273.15
T: absolute temp., t: celsius temp
Note : T=0 appears to be an unattainable condition
? At low temperatures the lower energy levels are expected to be populated more, as compared to higher energy levels. As we heat the system, more and more molecules will be promoted to higher energy levels.*
? Conversion between Fahrenheit and Celsius

Units of heat
Heat is a form of energy so we can always use Joules. More common in thermodynamics is the calorie: By definition 1 calorie is the amount of heat required to change the temperature of 1 gram of water 1?C.
Pressure:
Pressure is the force per unit area. It is the momentum transferred per unit area, per unit time.* H.W
P = momentum transferred/area/time.
Pressure is related to momentum, while temperature is related to kinetic energy. (H.W: How? explain??)
Processes in TD:
Here is a brief listing of a few kinds of processes, which we will encounter in TD:
Isothermal process: the process takes place at constant temperature (e.g. freezing of water to ice at –10?C)
Isobaric process: the process takes place at constant pressure (e.g. heating of water in open air? under atmospheric pressure)
Isochoric process: the process takes place at constant volume(e.g. heating of gas in a sealed metal container).
Reversible process: the process takes place when the system is close to equilibrium at all times (and infinitesimal alteration of the conditions can restore the universe (system + surrounding) to the original state.
Adiabatic process: the process at which (no heat is added/removed to/from the system) that is mean (dq is zero)
Heat and Work
Work (W) in mechanics is displacement (d) against a resisting force (F).
W = F ? d
Work has units of energy (Joule, J). Work can be expansion work (P?V) in gases, electrical work [eV], magnetic work etc. The transfer of energy as a result of a temperature difference is called heat.
Work Done by a Gas : Work=(Force)x(distance) =F?y
? Force=(Presssure)x(Area)
? W=P(A?y)
=P?V
State functions in TD
A property which depends only on the current state of the system (as defined by T, P, V etc.) is called a state function. This does not depend on the path used to reach a particular state.
Analogy: one is climbing a hill- the potential energy of the person is measured by the height of his CG(=0.58h) from the ground level. If the person is at a height of h, then his potential energy will be mgh, irrespective of the path used by the person to reach the height (paths will give the same increase in potential energy of mgh).
In TD this state function is the internal energy (U or E). (Every state of the system can be described to a unique U). Hence, the work needed to move a system from a state of lower internal energy (=UL) to a state of higher internal energy (UH) is (UH) ? (UL).
W = (UH) ? (UL)
Heat capacity:
Heat capacity is the amount of heat (measured in Joules or Calories) needed to raise an unit amount of substance (measured in grams or moles) by a unit in temperature (measured in ?C or K). This ‘heating’ (addition of energy) can be carried out at constant volume or constant pressure.
Heat capacity at constant Volume (CV), Heat capacity at constant Pressure (CP).
Units: Joules/Kelvin/mole, J/K/mole, J/?C/mole, J/?C/g.
Heat capacity is an extensive property (depends on ‘amount of matter’). The amount of heat (Q) added into a body of mass m to change its temperature an amount ?T is given by
Q=m C ?T

Compare the amount of heat energy required to raise the temperature of 1 kg of water and 1 kg of iron 20 ?C?

C is called the specific heat and depends on the material and the units used. Note that by definition, the specific heat of water is 1 cal/g?C.
The Laws of Thermodynamics
1. Zeroth law of thermodynamics
If two bodies are each in thermal equilibirum with a third body they will be in thermal equilibirum with each other.
2. The First Law
The internal energy of an isolated system is constant. A closed system may exchange energy as heat or work. Let us consider a close system at rest without external fields. There exists a state function U such that for any process in a closed system:
?U = Q + W.....................[1]
Q?heat flow in to the system, W ? work done on the system (work done by the system is negative)
Q & W are not state functions ? i.e. they depend on the path of a process.
U is the internal energy. Being a state function for a process ?U depends only of the final and initial state of the system.
?U = Ufinal – Uinitial.
In contrast to U, Q & W are NOT state functions (i.e. depend on the path followed). For an infinitesimal process eq. [1] can be written as: dU = dQ + dW.
The medical application of the first law is the energy of the human body is controlled by the first law of thermodynamics for a closed system.
?U: changed in stored energy
?Q: heat lost or gained
?W: work done by the body.
It s an important to determine the rates for the energy changes in a small time ?t:

Where ?U/?t: catabolic rate
?Q/?t: heat loss or absorption rate
?W/?t: mechanical power
Basal Metabolic Rate (BMR): The amount of energy needed to perform a minimal body function (breathing, pumping blood through the arteries under resting conditions). BMR depends on thyroid function. A person of an overactive thyroid has a higher BMR than with normal thyroid function.
Since the energy used for metabolism becomes heat and dissipated from the skin so it related to surface area or the mass of the body. BMR depends on the temperature of the body, if a patient has temp. 40° C or 3 above normal, BMR is about 30% greater than normal.
3. The second law
The second law comes in many equivalent forms: It is impossible to build a cyclic machine that converts heat into work with 100% efficiency ? Kelvin’s statement of the second law. Another way of viewing the same: it is impossible to construct a cyclic machine that completely (with 100% efficiency) converts heat.
4. The Third Law
For substances at internal equilibrium, undergoing an isothermal process, the entropy change goes to zero as T (in K) goes to zero.

The law is valid for pure substances and mixtures. Close to Zero Kelvin, the molecular motions have to be treated using quantum mechanics ? still it is found that quantum ideal gases obey the third law (H.W: what are the ideal gas properties?).
Ideal and Perfect Gases
In TD one such system is the ideal gas. In an ideal gas the molecules do not interact with each other (Noble gases come close to this at normal temperatures). An ideal gas obeys the equation of state:

As the molecules of an ideal gas do not interact with each other, the internal energy of the system is expected to be ‘NOT dependent’ on the volume of the system. I.e.:

A gas which obeys both the above equations is called a perfect gas.
Internal energy (a state function) is normally a function of T &V: U =U(T,V). For a perfect gas: U = U (T) only.*
Heat Transfer Mechanisms
Conduction: (solids--mostly) Heat transfer without mass transfer. Convection: Typically very complicated, Very efficient way to transfer energy. liquid convection, vortex formation, Sunspot, solar simulation, (liquids/gas) Heat transfer with mass transfer

KC const. depend on the movement of air
AC surface area
Ta air temp.
Radiation: Everything that has a temperature radiates energy. Takes place even in a vacuum.

? Note: if we double the temperature, the power radiated goes up by 24 =16.
? If we triple the temperature, the radiated power goes up by 34=81.
? A lot more about radiation when we get to light.