thermodynamics chapter 10 section 1 relationships between heat and work
TRANSCRIPT
• Thermodynamics
Chapter 10Section 1 Relationships
Between Heat and Work
Section 1 Relationships
Between Heat and WorkChapter 10
Objectives
• Recognize that a system can absorb or release energy as heat in order for work to be done on or by the system and that work done on or by a system can result in the transfer of energy as heat.
• Compute the amount of work done during a thermodynamic process.
• Distinguish between isovolumetric, isothermal, and adiabatic thermodynamic processes.
Chapter 10
Heat, Work, and Internal Energy
• Heat and work are energy transferred to or from a system. An object never has “heat” or “work” in it; it has only internal energy.
• A system is a set of particles or interacting components considered to be a distinct physical entity for the purpose of study.
• The environment the combination of conditions and influences outside a system that affect the behavior of the system.
Section 1 Relationships
Between Heat and Work
Chapter 10
Heat, Work, and Internal Energy, continued
• In thermodynamic systems, work is defined in terms of pressure and volume change.
Section 1 Relationships
Between Heat and Work
( )
work = pressure volume change
A FW Fd Fd Ad P V
A A
W P V
• This definition assumes that P is constant.
Chapter 10
Heat, Work, and Internal Energy, continued
• If the gas expands, as shown in the figure, V is positive, and the work done by the gas on the piston is positive.
• If the gas is compressed, V is negative, and the work done by the gas on the piston is negative. (In other words, the piston does work on the gas.)
Section 1 Relationships
Between Heat and Work
Chapter 10
Heat, Work, and Internal Energy, continued
• When the gas volume remains constant, there is no displacement and no work is done on or by the system.
• Although the pressure can change during a process, work is done only if the volume changes.
• A situation in which pressure increases and volume remains constant is comparable to one in which a force does not displace a mass even as the force is increased. Work is not done in either situation.
Section 1 Relationships
Between Heat and Work
Chapter 10
Thermodynamic Processes
• An isovolumetric process is a thermodynamic process that takes place at constant volume so that no work is done on or by the system.
• An isothermal process is a thermodynamic process that takes place at constant temperature.
• An adiabatic process is a thermodynamic process during which no energy is transferred to or from the system as heat.
Section 1 Relationships
Between Heat and Work
Chapter 10
Energy Conservation
• If friction is taken into account, mechanical energy is not conserved.
• Consider the example of a roller coaster:– A steady decrease in the car’s total mechanical energy
occurs because of work being done against the friction between the car’s axles and its bearings and between the car’s wheels and the coaster track.
– If the internal energy for the roller coaster (the system) and the energy dissipated to the surrounding air (the environment) are taken into account, then the total energy will be constant.
Section 2 The First Law of Thermodynamics
Chapter 10
Energy Conservation
Section 2 The First Law of Thermodynamics
Chapter 10
Energy Conservation, continued
• The principle of energy conservation that takes into account a system’s internal energy as well as work and heat is called the first law of thermodynamics.
• The first law of thermodynamics can be expressed mathematically as follows:
U = Q – W
Change in system’s internal energy = energy transferred to or from system as heat – energy
transferred to or from system as work
Section 2 The First Law of Thermodynamics
Chapter 10
Signs of Q and W for a system
Section 2 The First Law of Thermodynamics
Chapter 10
Sample Problem
The First Law of Thermodynamics
A total of 135 J of work is done on a gaseous refrigerant as it undergoes compression. If the internal energy of the gas increases by 114 J during the process, what is the total amount of energy transferred as heat? Has energy been added to or removed from the refrigerant as heat?
Section 2 The First Law of Thermodynamics
Chapter 10
Sample Problem, continued
1. Define
Given:
W = –135 J
U = 114 J
Section 2 The First Law of Thermodynamics
Tip: Work is done on the gas, so work (W) has a negative value. The internal energy increases during the process, so the change in internal energy (U) has a positive value.
Diagram:
Unknown:
Q = ?
Chapter 10
Sample Problem, continued
2. Plan
Choose an equation or situation:
Apply the first law of thermodynamics using the values for U and W in order to find the value for Q.
U = Q – W
Section 2 The First Law of Thermodynamics
Rearrange the equation to isolate the unknown:
Q = U + W
Chapter 10
Sample Problem, continued
3. Calculate
Substitute the values into the equation and solve:
Q = 114 J + (–135 J)
Q = –21 J
Section 2 The First Law of Thermodynamics
Tip: The sign for the value of Q is negative. This indicates that energy is transferred as heat from the refrigerant.