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

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## Thermodynamics. First Principle of Thermodynamics. Enthalpy.

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Basic thermodynamic terms.

First principle of thermodynamics.

Enthalpy.

Hess’ law.

Work in the human body.

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## 2 Thermodynamics

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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.

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## 3 Subject Of Thermodynamics

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3 Thermodynamics - studies the conversions of

energy exchanged between systems as heat and

work

Biological thermodynamics - studies the general

laws of energy conversion in living systems

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## 4 Thermodynamic Bodies And Systems

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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.

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## 5 Types Of Thermodynamic Systems

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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

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## 6 Thermodynamic Variables

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6 Thermodynamic variables - Properties characterizing the thermodynamic systems

Examples: mass, volume, pressure, etc.

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## 7 Extensive Variables

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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

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## 8 Intensive Variables

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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.

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## 9 Thermodynamic State

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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

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## 10 Types Of Thermodynamic States

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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

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## 11 Equation Of State

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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

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## 12 Thermodynamic States – Graphic Representation

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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.

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## 13 Thermodynamic Process

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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)

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## 14 Thermodynamic Process – Graphic Representation

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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

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## 15 Thermodynamic Potential (State Function)

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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

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## 16 lnternal Energy

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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.

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## 17 Work

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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

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## 18 Heat

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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

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## 19 Energy Changes

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19 Performing work changes the kinetic, potential or internal energy of a thermodynamic system

• Heat exchange alters only the internal energy

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## 20 First Principle Of Thermodynamics

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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.

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