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Flashcards in 2 First principle of TD Deck (21):

Thermodynamics. First Principle of Thermodynamics. Enthalpy.

Basic thermodynamic terms.
First principle of thermodynamics.
Hess’ law.
Work in the human body.


2 Thermodynamics

2 Developed during the 1800’s to explain how
engines (particularly steam engines)
converted heat into work.
• The first steam engines were very, very
inefficient, converting perhaps 2 % of their
fuel into useful work.
• The word “Thermodynamics” was coined by
Lord Kelvin in 1849.


3 Subject Of Thermodynamics

3 Thermodynamics - studies the conversions of
energy exchanged between systems as heat and
Biological thermodynamics - studies the general
laws of energy conversion in living systems


4 Thermodynamic Bodies And Systems

4 • Thermodynamic body - a very large ensemble of particles
• Thermodynamic system - a set of thermodynamic bodies exchanging energy
Characteristics of thermodynamic bodies and systems:
volume, pressure, temperature, energy, etc.


5 Types Of Thermodynamic Systems

5 • lsolated - do not exchange matter and energy with the environment
• Closed - exchange energy but do not exchange matter
• Open - exchange both matter and energy with the environment


6 Thermodynamic Variables

6 Thermodynamic variables - Properties characterizing the thermodynamic systems
Examples: mass, volume, pressure, etc.


7 Extensive Variables

7 Chraracterize the system as a whole (directly proportional to the amount of substance in the system)
• An extensive variable can be calculated as the sum of the values of the variable for the separate subsystems that compose the system
Examples: mass, volume, energy, enthalpy, entropy, etc


8 Intensive Variables

8 May have different values at different points in the system
• Do not depend on the amount of substance in the system
• Can not be summed
Examples: temperature, density, etc.


9 Thermodynamic State

9 • Thermodynamic state - the set of instantaneous values of the thermodynamic variables of a system
• The state changes if any of the variables changes its value


10 Types Of Thermodynamic States

10 • Equilibrium state – all the thermodynamic variables of the system remain constant over time and have the
same values at all points in the system
• Non-equilibrium state - the variables have different values at different points in the system and change over time


11 Equation Of State

11 • An equilibrium state is determined by the pressure (p), volume (V), temperature (T) and amount of substance of the system (n).
• Equation of state - a relationship between the variables of the equilibrium state, describing the properties of the system: f(p, V, T, n) = 0


12 Thermodynamic States – Graphic Representation

12 [graph]
Equilibrium states can be represented by a point
on a pressure-volume diagram (pV-diagram).
Non-equilibrium states can not be represented
on a pV-diagram.


13 Thermodynamic Process

13 • Thermodynamic Process:
The transition of a thermodynamic system from one state to another
• Types of thermodynamic processes:
—Equilibrium (quasistatic) process – the system is always in equilibrium state (the process runs infinitely slow)
—Non-equilibrium process – the system passes through non-equilibrium states (the process runs with finite speed)


14 Thermodynamic Process – Graphic Representation

14 [graph]
Equilibrium processes can be represented by a curve
on a pV-diagram.
Non-equilibrium processes can not be represented on
a pV-diagram


15 Thermodynamic Potential (State Function)

15 Variable of a thermodynamic system whose changes
depend on the initial and final states only, and do not
depend on the path taken by the process


16 lnternal Energy

16 lnternal energy is a thermodynamic potential.
• The total potential energy associated with the inter-atomic and inter-molecular forces and the kinetic energy of the thermal motion of the atoms and molecules in the
thermodynamic system.
• Does not depend on the position and movement of the system as a whole.


17 Work

17 Work is a form of energy transfer which can described by changes of macroscopic system variables
Example: a gas can perform work when expanding (volume change) against an external force


18 Heat

18 Heat is a form of energy transfer by changing the
microscopic thermal motions of particles
Example: when a hot and cold body are in contact,
their molecules exchange energy


19 Energy Changes

19 Performing work changes the kinetic, potential or internal energy of a thermodynamic system
• Heat exchange alters only the internal energy


20 First Principle Of Thermodynamics

20 The amount of heat δQ transferred to a thermodynamic system is used to increase the internal energy of the system by dU and to perform work δA on the environment:
δQ = dU + δA
This is essentially a law about energy conservation in thermodynamics.


21 Enthalpy

21 When a system changes its volume by dV at constant pressure p, the work performed by the system is:
δA = p.dV
When a system changes its volume at constant pressure, the quantity Q is a thermodynamic
potential - enthalpy (H):
δQ = dU + p.dV = dH