1 / 21

Exploring the 1st Law of Thermodynamics in Simple Systems

Learn about the 1st Law of Thermodynamics and its application in calculating internal energy in simple systems, examining concepts like isobaric, isovolumetric, isothermal, and adiabatic processes with relevant equations and visualizations.

clarke-miranda
Télécharger la présentation

Exploring the 1st Law of Thermodynamics in Simple Systems

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. The 1st Law of Thermodynamics and Simple Systems

  2. Calculating Internal energy • Since the Internal Energy of a gas is the sum of the EK of it’s molecules • Kinetic theory • And the Average EK of a molecule is found by the equation EK Average = (3/2)kT • The total internal energy must be sum of all the molecules energy • U = (3/2)NkT • U = (3/2)nRT

  3. What the 1st law really is • Since thermodynamics is the study on how thermal energy is transferred and transformed into other energies this first law must relate to energy. • We have only one law for energy… • The Conservation of Energy • So the 1st law of thermodynamics must be a restatement of the conservation of energy. Only with a specific focus.

  4. The First Law of Thermodynamics • Because we are interested in the energy transformations within a gas the conservation of energy must be focuses on that. • The 1st Law of Thermodynamics: The change in a gases internal energy must equal the Energy that flows into the gas (Heat) minus the Energy the gas uses to expand (Work) • Change gas’s internal Energy = Heat – Workby gas • DU = Q – Wby gas

  5. Going with the flow • It is important to always remember that both Heat on work are energies that flow into and out of the gas. • They are not properties of the gas itself • Internal energy however is apart of the gas and is one of the gas’s properties. • The change in internal energy then is a change in one of the gas’s properties (related to temperature)

  6. Visualizing the 1st law W Q DU = Q -W

  7. Types of thermal Systems • There are 4 simple systems for a heated gas: • Isobaric • Isovolumetric/Isochoric • Isothermal • Adiabatic

  8. Isobaric systems • Is a system where a gas expands, or contracts, in a way that its pressure remains constant. Pressure Pressure Expanding Compressing Volume Volume

  9. W = F//D D D DV = AD DV W = Area Pressure Volume Calculating Work for an Isobaric system F// = PA W = PAD W = PDV

  10. Is volumetric • This is when a gas is placed in a container that will not change it’s size. • Since the gas will not expand or contract there can be no work done by, or on, the gas • W = 0 J Pressure No Area Volume

  11. Isothermal • This is a very slow process which allows the gas to expand, or contract, so that it’s Temperature is constant. • This means that the gas’s internal energy is a constant (U = [3/2]NRT) • This also means that PV is a constant (PV = nRT)

  12. PV graph for Isothermal P1 Graph function: xy = c Pressure P2 Area = work V2 V1 Volume P1V1 = P2V2

  13. T2 Pressure T1 Volume T2 > T1

  14. Adiabatic Systems • Adiabatic systems when a gas expands, or contracts, very quickly. • Popping a balloon • Ignition of a sparkplug • Firing a single shoot from a gun • There is no time for any heat to flow in or out of the gas. • Q = 0 Joules • In this case any work done must change the internal energy. (PV is not constant.)

  15. P1V1 =P2V2 PV graph for Adiabatic P1 Pressure P2 Area = work V2 V1 Volume

  16. Table of simple systems

  17. Stating the DU in terms of PV • It is sometime easier to solve problems when we think of the change of internal energy in terms of PV rather than T. • Particular when working with a PV graph U = (3/2)NRT PV = NRT U = (3/2)PV DU = (3/2)[PfVf – PiVi]

  18. Viewing a Simple Engine Provides Heat (Energy) for the engine to work Hot Reservoir QH Gas Chamber W Output Receives the Mechanical work done QC W = QH - QC Cold Reservoir Receives all the wasted exhaust of engine

  19. Efficiency • Efficiency = Workout/Workin • Efficiency = Workout/QH • Efficiency = (QH – QC)/QH • Efficiency = 1 – (QC/QH)

  20. Carnot Efficiency • No engine can be 100% Efficient (even if it is an ideal engine). • You need to use some energy to reset you engine for the next cycle. • So this leads to the question:” If the maximum efficiency is not 100%, how can I tell how good is the efficiency of my engine?” • A French scientist name Sadi Carnot found a way of calculating the ideal (maximum) efficieny for a given engine. • No engine can achieve it’s ideal efficiency

  21. Carnot efficiency • Carnot found that the ideal Efficiency depends to the two absolute temperatures (Temperature measured in Kelvin) of the two Reservoir • EfficiencyCarnot = 1 – (TC/TH)

More Related