Work and Heat

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This is a topic from Thermodynamics:

Contents

Introduction

Heat is a process, not a physical substance or a property. Heat refers to the process whereby energy is transferred down a temperature gradient.
Sign convention:

  • Heat in is positive
  • Heat out is negative

Work is the energy transferred when motion is applied against an opposing or resistive force.
Sign convention:

  • The opposite of heat
  • Work in is negative
  • Work out is positive

Mathematical Definition for Work

Recall from the first year physics courses:

W = Fx

For an infinitesimal amount of work:

dW = Fdx
= PA dx
= P dV

Therefore, for a process moving from state 1 to state 2:

Screen Shot 2013-03-12 at 6.37.29 PM.png

Consider the following derivations using this defintion

Work for an Isobaric Process

  • Consider an isobaric process such that pressure is constant: P = c
  • Performing the integral:
Screen Shot 2013-03-12 at 6.32.26 PM.png
  • The work done is the area under the graph of the PV diagram:
30k.GIF

Work for an Polytropic Process

Screen Shot 2013-03-12 at 6.14.03 PM.png

Work for an Isothermal Process

Screen Shot 2013-03-12 at 6.21.46 PM.png
  • The work done is the area under the graph of the PV diagram:
30m.GIF

Types of Work

When a system is open it can be susceptible to more than one form of work.
Consider the following diagram:

Screen Shot 2013-03-28 at 9.58.04 AM.png
  1. The work done on/by the piston at the top of the diagram is the type of work with which we are mostly concerned with closed systems. It is known as Boundary Work and is denoted Wb. Boundary work, in which the system boundary undergoes a net motion, is compatible with both closed systems and open systems.
  2. The work done by the battery on the left of the diagram is called Electrical Work and is denoted We. It refers to the work done in the form of electrical power on the heating element embedded in the system.
  3. The work done by the rotor on the right of the diagram is called Shaft Work and is denoted Ws.
  4. The work done by the flow of mass in and out of the system seen in the bottom left and top right of the diagram is known as Flow Work and is denoted Wflow. Flow work is the work done when a volume of fluid flows against an opposing pressure. The points of exit and entry into the system is known as ports. This type of work is not possible in a closed system because it involved a transfer of mass across the system boundary.

The boundary work, electrical work and shaft work are generally grouped as Wsys such that:

W = Wb + We + Ws + Wflow becomes
W = Wsys + Wflow

Pump Work

Note: This type of work requires an understanding of concepts covered later in the notes and is not needed until the second law of thermodynamics has been covered.

w = (Pinvin + Vin2/2 + gzin) - (Poutvout + Vout2/2 + gzout) - (uout - uin - q)
  • However it can also be shown that qrev = uout - uin for a reversible process, or uout - uin > q for an irreversible process.
  • Therefore pump work can be rewritten as:
wrev = (Pinvin + Vin2/2 + gzin) - (Poutvout + Vout2/2 + gzout)
  • Ignoring kinetic and potential energies:
wrev = Pinvin - Poutvout
  • The flow is incompressible, and so vin = vout = v
  • Therefore we have:
wrev = v(Pin - Pout)


If work is equal to zero, we get Bernoulli's equation:

Pin/ρ + Vin2/2 + gzin = Pout/ρ + Vout2/2 + gzout
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