Chapter4 Lecture

# Chapter4 Lecture - Chapter 4 The First Law of...

This preview shows pages 1–11. Sign up to view the full content.

Chapter 4 The First Law of Thermodynamics for a Closed System 1. Define boundary work 2. Apply the first law to a variety of different closed systems 3. Introduce the concept of specific heat

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
The First Law of Thermodynamics Energy can not be created or destroyed. A system will gain or lose energy through heat transfer, work, or mass transfer. Energy of the system = E = U +KE + PE Heat transfer = Q Work = W b + W sh + W e Mass transfer brings in mass that has energy E
Closed Systems No mass transfer! The change in energy of the system must be equal to the heat and work transfer during the process. in net in net W Q E E , , 1 2 PE KE U E E E 1 2 ) ( 2 1 2 1 2 2 V V m ) ( 1 2 z z mg system on net work system into heat net , , in out out in in net in out out in in net W W W Q Q Q

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Forms of 1 st Law for Closed Systems in net in net W Q E , , During a very small time dt, this becomes in net in net W Q dE , , Which can be converted to a rate form: in net in net W Q dt dE , , Can normalize the 1 st law by the system mass to get in net in net w q e , ,
Forms of 1 st Law for Closed Systems Often the changes in KE and PE are negligible (but not always) in net in net W Q U , , in net in net w q u , , Now that we know how to find internal energy for a substance, we can apply the first law to more complicated systems to obtain useful quantitative information. in net in net w q dt du , , Only applies if KE=PE=0

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Common Modes of Closed System Work Shaft work (power) rotation Torque on system due to motor (environment) t T W T W T W in sh in sh in sh , , , Electrical work (power) t VI W VI W VI W in e in e in e , , , ―paddle wheel‖ motor + - Remember, electrical work can be out of the system!
Boundary Work P Expansion and compression of gases: Force of piston on gas Force of gas on piston These forces are equal and opposite Gas expansion F ds 0 , Fds W out b Work is done by gas on the surroundings (W out is positive) 0 , Fds W out b F ds Gas compression Work done on gas by the surroundings (W out is negative) Lets use this force

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Boundary Work P ds 2 1 , , V V out b out b PdV W PdV PAds Fds W P is always positive (absolute pressure) dV > 0 expansion W b,out > 0 dV < 0 compression W b,out < 0 If W b,out < 0, means positive work was done on the system Only valid for quasi- equilibrium processes! One of the most important equations for MAE 204!
Consider a Piston-Cylinder System Neglect PE, KE Constant pressure process Assume quasi-equilibrium process (i.e. work is PdV) Process starts at state 1, ends up at state 2 in Q out b W , 2 1 ) ( 1 2 , V V out b V V P PdV W We know how to find the work: First law of thermodynamics: out b in W Q E E , 1 2

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Consider a Piston-Cylinder System out b in W Q E E , 1 2 System energy E only contains internal energy ) ( 1 2 1 2 V V P Q U U in in Q PV U PV U 1 1 2 2 in Q V P U V P U 1 1 1 2 2 2 1 2 H H Q in Heat transfer for a constant pressure process (closed system) is equal to the change in enthalpy.
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### Page1 / 39

Chapter4 Lecture - Chapter 4 The First Law of...

This preview shows document pages 1 - 11. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online