Download presentation

1
**Energy Transfer by Heat, Work, and Mass**

CHAPTER 3 Energy Transfer by Heat, Work, and Mass

2
**Heat Transfer Heat, means heat transfer. Adiabatic – no heat transfer**

Energy transfer driven by temperature difference always hotter to cooler Adiabatic – no heat transfer same as isothermal? Symbols used: Q and q Q Caloric?

3
**Work Energy transfer not driven by a temperature difference. Examples**

Rising piston rotating shaft electric wire crossing the system boundaries Symbols used: W and w W

4
**Formally: Qin and Wout are positive, Qout and Win are negative**

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. FIGURE 3-9 Specifying the directions of heat and work. Formally: Qin and Wout are positive, Qout and Win are negative 3-1

5
**Heat and Work Both heat and work are boundary phenomena.**

Systems possess energy, but not heat or work. Both are associated with a process, not a state. Both are path functions Magnitudes depend on paths as well as end states

6
Processes Process line, or path State 1 State 2 P1 P3 P2

7
Electrical Work We = VI so We = VIΔt if V and I are constant.

8
Mechanical Work m

9
**Work at a system boundary...**

Quasi – equilibrium processes, best case. Work at a system boundary... There must be a force acting on the boundary. The boundary must move.

10
**Copyright © The McGraw-Hill Companies, Inc**

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. FIGURE 3-19 A gas does a differential amount of work dWb as it forces the piston to move by a differential amount ds. 3-2

11
**Work transfer at a boundary**

System Surroundings W > 0 W< 0 System Boundary

12
Work of Expansion

13
**Work of Expansion: p-dV work**

14
**Evaluating a equilibrium expansion process**

V = Ax V1 V2 p1 p2

15
**Copyright © The McGraw-Hill Companies, Inc**

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. FIGURE 3-20 The area under the process curve on a P-V diagram represents the boundary work. 3-3

16
**Copyright © The McGraw-Hill Companies, Inc**

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. FIGURE 3-22 The net work done during a cycle is the difference between the work done by the system and the work done on the system. 3-4

17
**PROCESSES INVOLVING IDEAL GASES**

18
Polytropic processes...

19
**The polytropic process: PVn=Const.**

State 1 State 2

20
**Changes in KE and PE are zero Quasistatic process Polytropic process **

Assumptions Changes in KE and PE are zero Quasistatic process Polytropic process Ideal gas

21
Expression for work: Process equation:

22
**Evaluating the integral:**

Note that n cannot equal one, which is the general case.

23
**For the special case when n = 1:**

24
**Polytropic processes p n > 1 V1 V2 V T1 T2**

Isothermal Process (n = 1) n > 1 p1 p2

25
**Alternative expressions for W1-2**

26
**Constant pressure processes...**

27
**Constant pressure process**

Consider as a limiting case of the general polytropic process. P = Constant Evaluation of the work integral

28
**P V Constant pressure, constant temperature and polytropic processes:**

1 2 P V P = Constant (n = 0) Isobaric process Constant pressure, constant temperature and polytropic processes:

29
**Shaft Work Work = F∙d Wsh = T(2πn) or**

Replace force with torque, T Replace distance with angle rotated = 2πn where n is number of rotations Wsh = T(2πn) or Wsh = T(2πn) where n is frequency in Hz

Similar presentations

© 2019 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google