Thermodynamics 1st Law Physics Lesson 20 by Owen Borville 12.17.2025
A thermodynamic system, its boundary, and its surroundings must be defined with all the roles of the components fully explained before analysis.
Thermal equilibrium is reached with two objects if a third object is in thermal equilibrium with the other two separately. A general equation of state for a closed system has the form ƒ(p, V, T) = 0, with an ideal gas as an illustrative example.
Positive (negative) work is done by a thermodynamic system when it expands (contracts) under an external pressure. Heat is the energy transferred between two objects (or two parts of a system) because of a temperature difference. Internal energy of a thermodynamic system is its total mechanical energy.
Net work for a finite change in volume can be found by integrating the equation pdV from V1 to V2 to find the net work: W = ∫(V2-V1) pdV
The internal energy of a thermodynamic system is a function of state and this is unique for every equilibrium state of the system. The internal energy of a system or the average total energy is the sum of the average mechanical energies of all entities, including kinetic and potential energies. Eint = ∑Ki (avg) + Ui (avg) The internal energy of an ideal gas is Eint = 3/2nRT.
The first law of thermodynamics requires an increase in internal energy of the thermodynamic system given by the heat added to the system less the work done by the system in any thermodynamic process.
The thermal behavior of a system is described in terms of thermodynamic variables. For an ideal gas, these variables are pressure, volume, temperature, and number of molecules of moles of the gas.
For systems in thermodynamic equilibrium, the thermodynamic variables are related by an equation of state.
A heat reservoir is so large that when it exchanges heat with other systems, its temperature does not change.
A quasi-static process takes place so slowly that the system involved is always in thermodynamic equilibrium.
A reversible process is one that can be made to retrace its path and both the temperature and pressure are uniform throughout the system.
There are several types of thermodynamic processes, including (1) isothermal: the system's temperature is constant (2) adiabatic: no heat is exchanged by the system (3) isobaric: the system's pressure is constant (4) isochoric: the system's volume is constant.
First Law of Thermodynamics processes include: (1) isothermal: ΔEint = 0, Q = W (2) adiabatic: Q = 0, ΔEint = -W (3) isobaric: ΔEint = Q-W (4) isochoric: W = 0, ΔEint = Q
Heat Capacities of an Ideal Gas: the molar capacity at constant pressure Cp is given by Cp = Cv + R = dR/2 + R, where d is the number of degrees of freedom of each molecule or entity in the system. A real gas has a specific heat close to but higher than that of the corresponding ideal gas with Cp ≅ Cv + R. The ratio of molar heat capacities is = γ = Cp/CV
Quasi-static adiabatic expansion of an ideal gas produces a steeper pV curve than that of the corresponding isotherm. Pressure changes more rapidly with volume than in an isothermal process, as temperature changes. A realistic expansion can be adiabatic but rarely quasi-static (Therefore, the condition for an ideal gas in a quasi-static adiabatic process is pVγ = constant).
The first law of thermodynamics is ΔEint = Q-W, where ΔEint is the change in internal energy of a system, Q is the net heat transfer (the sum of all heat transfer into and out of the system), and W is the net work done (the sum of all work done on or by the system).
Both Q and W are energy in transit. Only ΔEint represents an independent quantity capable of being stored. Internal energy Eint of a system depends only on the state of the system and not on how it reached that state. Metabolism of living organisms, and photosynthesis of plants, are specialized types of heat transfer, doing work, and internal energy of systems.
The first law of thermodynamics implies that machines can be harnessed to do work that humans previously did by hand or by external energy supplies such as running water or the heat of the Sun. A machine that uses heat transfer to do work is known as a heat engine.
Processes used by heat engines that flow from the first law of thermodynamics are the isobaric, isochoric, isothermal, and adiabatic processes. These processes differ from each other based on how they affect pressure, volume, temperature, and heat transfer.
If the work done is performed outside the environment, work (W) will be a positive value. If the work done is done to the heat engine system, work (W) will be a negative value.
Some thermodynamic processes, including isothermal and adiabatic processes, are reversible in theory, so that both the thermodynamic system and the environment can be returned to their initial states. However, because of loss of energy owing to the second law of thermodynamics, complete reversibility does not work in practice.
A thermodynamic system, its boundary, and its surroundings must be defined with all the roles of the components fully explained before analysis.
Thermal equilibrium is reached with two objects if a third object is in thermal equilibrium with the other two separately. A general equation of state for a closed system has the form ƒ(p, V, T) = 0, with an ideal gas as an illustrative example.
Positive (negative) work is done by a thermodynamic system when it expands (contracts) under an external pressure. Heat is the energy transferred between two objects (or two parts of a system) because of a temperature difference. Internal energy of a thermodynamic system is its total mechanical energy.
Net work for a finite change in volume can be found by integrating the equation pdV from V1 to V2 to find the net work: W = ∫(V2-V1) pdV
The internal energy of a thermodynamic system is a function of state and this is unique for every equilibrium state of the system. The internal energy of a system or the average total energy is the sum of the average mechanical energies of all entities, including kinetic and potential energies. Eint = ∑Ki (avg) + Ui (avg) The internal energy of an ideal gas is Eint = 3/2nRT.
The first law of thermodynamics requires an increase in internal energy of the thermodynamic system given by the heat added to the system less the work done by the system in any thermodynamic process.
The thermal behavior of a system is described in terms of thermodynamic variables. For an ideal gas, these variables are pressure, volume, temperature, and number of molecules of moles of the gas.
For systems in thermodynamic equilibrium, the thermodynamic variables are related by an equation of state.
A heat reservoir is so large that when it exchanges heat with other systems, its temperature does not change.
A quasi-static process takes place so slowly that the system involved is always in thermodynamic equilibrium.
A reversible process is one that can be made to retrace its path and both the temperature and pressure are uniform throughout the system.
There are several types of thermodynamic processes, including (1) isothermal: the system's temperature is constant (2) adiabatic: no heat is exchanged by the system (3) isobaric: the system's pressure is constant (4) isochoric: the system's volume is constant.
First Law of Thermodynamics processes include: (1) isothermal: ΔEint = 0, Q = W (2) adiabatic: Q = 0, ΔEint = -W (3) isobaric: ΔEint = Q-W (4) isochoric: W = 0, ΔEint = Q
Heat Capacities of an Ideal Gas: the molar capacity at constant pressure Cp is given by Cp = Cv + R = dR/2 + R, where d is the number of degrees of freedom of each molecule or entity in the system. A real gas has a specific heat close to but higher than that of the corresponding ideal gas with Cp ≅ Cv + R. The ratio of molar heat capacities is = γ = Cp/CV
Quasi-static adiabatic expansion of an ideal gas produces a steeper pV curve than that of the corresponding isotherm. Pressure changes more rapidly with volume than in an isothermal process, as temperature changes. A realistic expansion can be adiabatic but rarely quasi-static (Therefore, the condition for an ideal gas in a quasi-static adiabatic process is pVγ = constant).
The first law of thermodynamics is ΔEint = Q-W, where ΔEint is the change in internal energy of a system, Q is the net heat transfer (the sum of all heat transfer into and out of the system), and W is the net work done (the sum of all work done on or by the system).
Both Q and W are energy in transit. Only ΔEint represents an independent quantity capable of being stored. Internal energy Eint of a system depends only on the state of the system and not on how it reached that state. Metabolism of living organisms, and photosynthesis of plants, are specialized types of heat transfer, doing work, and internal energy of systems.
The first law of thermodynamics implies that machines can be harnessed to do work that humans previously did by hand or by external energy supplies such as running water or the heat of the Sun. A machine that uses heat transfer to do work is known as a heat engine.
Processes used by heat engines that flow from the first law of thermodynamics are the isobaric, isochoric, isothermal, and adiabatic processes. These processes differ from each other based on how they affect pressure, volume, temperature, and heat transfer.
If the work done is performed outside the environment, work (W) will be a positive value. If the work done is done to the heat engine system, work (W) will be a negative value.
Some thermodynamic processes, including isothermal and adiabatic processes, are reversible in theory, so that both the thermodynamic system and the environment can be returned to their initial states. However, because of loss of energy owing to the second law of thermodynamics, complete reversibility does not work in practice.