1 / 21

First Law of Thermodynamics

First Law of Thermodynamics. Physics 313 Professor Lee Carkner Lecture 8. Exercise #6 Spring. Work done on spring W =  F dx =  kx dx = ½kx 2 = ½k(x 2 2 –x 1 2 ) If the spring is initially unstretched, x 1 = 0 Spring and gas work displacement of spring D V = A D X

bella
Télécharger la présentation

First Law of Thermodynamics

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. First Law of Thermodynamics Physics 313 Professor Lee Carkner Lecture 8

  2. Exercise #6 Spring • Work done on spring • W =  F dx =  kx dx = ½kx2 = ½k(x22 –x12) • If the spring is initially unstretched, x1 = 0 • Spring and gas work • displacement of spring DV = A DX • 0.025 = 0.25 Dx, Dx = 0.1 m, x2 = 0.1 m • W = ½kx22 = (½)(150)(0.1)2 = • P = W/DV = 0.75/0.025 = 30 kPa • PV = nRT • T = PV/nR = (30000)(0.025)/(1)(8.31) =

  3. Heat • What is heat? • Heat is not a state variable • No heat transfer if: • Heat is energy and can occur with different processes

  4. Isochoric Heat • If heat is added the temperature of the system will rise • Any heat exchange directly affects the internal energy

  5. Adiabatic Work • In an adiabatic system no heat can flow • For any adiabatic process by which the system move from state 1 to state 2 the total amount of work is a constant • This is not normally true • This work changes the internal energy

  6. Internal Energy • When heat flows into the system or work is done on a system the system gains energy • The internal energy is a property of a system and can be expressed in terms of thermodynamic coordinates • We will often discuss a change in internal energy (DU or dU)

  7. First Law of Thermodynamics • Energy is conserved • Can write in differential form as • dU is a change in internal energy • dQ and dW are small amounts of heat or work

  8. Notes on the First Law • Heat is defined thermodynamically by the first law: • Can also write for work: • Sign Convention • Heat into a system is positive • Work done on the system is positive • This convention can be changed but the first law then also must be changed

  9. Notes on Heat • Heat was once thought to be a fluid within a body • People began to suspect that heat was a form of energy in the early 1800’s, but couldn’t prove it • Joule demonstrated the equivalence of heat and work in the 1840’s

  10. Special Cases • Adiabatic: DU = W • Isochoric: DU = Q

  11. Heat and Internal Energy • If a Styrofoam block and a steel block are both heated the same amount which is hotter? • Why? Q = C DT

  12. Specific Heat • We will also use the heat capacity per mole: • Where n is number of moles or (m/M) total mass divided by molar mass

  13. Heat Capacities • We can express the heat capacity in terms of differential changes in temperature and heat C = dQ/dT • We can then define two specific quantities: CV = (dQ/dT)V CP = (dQ/dT)P • Note that C is a function of temperature

  14. Internal Energy • We can write the heat flow into a system as: • For an isochoric system • So: • The change in internal energy is a function of the change in temperature

  15. Heat Conservation • All objects within the boundary will exchange heat until they are in thermodynamic equilibrium (equal T) • Lost to surroundings

  16. Calorimetry • A calorimeter must : • produce a well defined amount of heat • Monitor temperature • Heat produced must all go into raising temperature of sample

More Related