1st Law of Thermodynamics
The First Law of Thermodynamics simply states that energy can be neither created nor destroyed (conservation of energy). Thus power generation processes and energy sources actually involve conversion of energy from one form to another, rather than creation of energy from nothing.
The 1st Law of Thermodynamics tells us that energy is neither created nor destroyed, thus the energy of the universe is a constant. However, energy can be transferred from one part of the universe to another. To work out thermodynamic problems we will need to isolate a certain portion of the universe, the system, from the remainder of the universe, the surroundings.
The energy transfer between different systems can be expressed as:
E1 = E2 (1)
where
E1 = initial energy
E2 = final energy
The internal energy encompasses:
- The kinetic energy associated with the motions of the atoms
- The potential energy stored in the chemical bonds of the molecules
- The gravitational energy of the system
The first law is the starting point for the science of thermodynamics and for engineering analysis.
Based on the types of exchange that can take place we will define three types of systems:
- isolated systems: no exchange of matter or energy
- closed systems: no exchange of matter but some exchange of energy
- open systems: exchange of both matter and energy
The first law makes use of the key concepts of internal energy, heat, and system work. It is used extensively in the discussion of heat engines.
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Internal Energy Internal energy is defined as the energy associated with the random, disordered motion of molecules. It is separated in scale from the macroscopic ordered energy associated with moving objects; it refers to the invisible microscopic energy on the atomic and molecular scale. For example, a room temperature glass of water sitting on a table has no apparent energy, either potential or kinetic . But on the microscopic scale it is a seething mass of high speed molecules. If the water were tossed across the room, this microscopic energy would not necessarily be changed when we superimpose an ordered large scale motion on the water as a whole. |
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Heat Heat may be defined as energy in transit from a high temperature object to a lower temperature object. An object does not possess "heat"; the appropriate term for the microscopic energy in an object is internal energy. The internal energy may be increased by transferring energy to the object from a higher temperature (hotter) object - this is called heating. |
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Work When work is done by a thermodynamic system, it is usually a gas that is doing the work. The work done by a gas at constant pressure is W = p dV, where W is work, p is pressure and dV is change in volume. |
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Heat Engines Refrigerators, Heat pumps, Carnot cycle, Otto cycle |
The change in internal energy of a system is equal to the head added to the system minus the work done by the system:
dE = Q - W (2)
where
dE = change in internal energy
Q = heat added to the system
W = work done by the system
1st law does not provide the information of direction of processes and does not determine the final equilibrium state. Intuitively, we know that energy flows from high temperature to low temperature. Thus, the 2nd law is needed to determine the direction of processes.
Enthalpy is the "thermodynamic potential" useful in the chemical thermodynamics of reactions and non-cyclic processes. Enthalpy is defined by
H = U + PV (3)
where
H = enthalpy
U = internal energy
P = pressure
V = volume
Enthalpy is then a precisely measurable state variable, since it is defined in terms of three other precisely definable state variables.
Go to Thermodyamics key values internationally agreed, Standard state and enthalpy of formation, Gibbs free energy of formation, entropy and heat capacity and Standard enthalpy of formation, Gibbs energy of formation, entropy and molar heat capacity of organic substances for listing of values for a lot of inorganic and organic substances.
Entropy is used to define the unavailable energy in a system. Entropy defines the relative ability of one system to act to an other. As things moves toward a lower energy level, where one is less able to act upon the surroundings, the entropy is said to increase. Entropy is connected to the Second Law of Thermodynamics.
For the universe as a whole the entropy is increasing.
Related Topics
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Thermodynamics
Work, heat and energy systems.
Related Documents
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2nd Law of Thermodynamics
Entropy and disorder. -
Carnot Efficiency
The efficiency of the Carnot cycle. -
Combustion Heat
Heat of combustion (energy content) for som common substances - with examples how to calculate heat of combustion. -
Energy
Energy is the capacity to do work. -
Energy Balance and Energy Ratio
Energy balance and energy payback ratio. -
Energy Transfer Equation
Fluid energy transfer. -
Equilibrium
The mechanical, thermal, electrostatic, phase or chemical states of equilibrium. -
Heat, Work and Energy
Heat vs. work vs. energy. -
Mechanical Energy Equation vs. Bernoulli Equation
The Mechanical Energy Equation compared to the Extended Bernoulli Equation. -
Standard State and Enthalpy of Formation, Gibbs Free Energy of Formation, Entropy and Heat Capacity
Definition and explanation of the terms standard state and standard enthalpy of formation, with listing of values for standard enthalpy and Gibbs free energy of formation, as well as standard entropy and molar heat capacity, of 370 inorganic compounds. -
Standardized Enthalpies and Entropies
Standardized enthalpies and entropies for some common substances. -
Symbols Used to Denote a Chemical Reactions and Process or Condition
Explanation of symbols used as subscripts or superscripts to tell more about the type of chemical reaction, process or condition. -
Thermodynamic Key Values Internationally Agreed
Internationally agreed, internally consistent, values for the thermodynamic properties (standard enthalpy of formation, entropy and [H°(298)-H°(0)]) of key chemical substances. -
Thermodynamic Terms - Functions and Relations
Common thermodynamic terms and functions - potential energy, kinetic energy, thermal or internal energy, chemical energy, nuclear energy and more. -
Third Law of Thermodynamics
The entropy of a substance is zero if the absolute temperature is zero. -
WABT - Weighted Average Bed Temperature
Definition and examples of calculation of weighted average bed temperature in adiabatic reactors. -
Water-Steam Mollier Diagram
Enthalpy-entropy diagram for water and steam. -
Work done by Force
Work done by a force acting on an object. -
Zeroth Law of Temperature
The direction of heat flow.
