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Learning objectives

By the end of this section, you will be able to:

  • Define the first law of thermodynamics.
  • Describe how conservation of energy relates to the first law of thermodynamics.
  • Identify instances of the first law of thermodynamics working in everyday situations, including biological metabolism.
  • Calculate changes in the internal energy of a system, after accounting for heat transfer and work done.

The information presented in this section supports the following AP® learning objectives and science practices:

  • 4.C.3.1 The student is able to make predictions about the direction of energy transfer due to temperature differences based on interactions at the microscopic level. (S.P. 6.1)
  • 5.B.4.1 The student is able to describe and make predictions about the internal energy of systems. (S.P. 6.4, 7.2)
  • 5.B.7.1 The student is able to predict qualitative changes in the internal energy of a thermodynamic system involving transfer of energy due to heat or work done and justify those predictions in terms of conservation of energy principles. (S.P. 6.4, 7.2)
The photograph shows water boiling in a tea kettle kept on a stove. The water vapor is shown to emerge out of the nozzle of the kettle.
This boiling tea kettle represents energy in motion. The water in the kettle is turning to water vapor because heat is being transferred from the stove to the kettle. As the entire system gets hotter, work is done—from the evaporation of the water to the whistling of the kettle. (credit: Gina Hamilton)

If we are interested in how heat transfer is converted into doing work, then the conservation of energy principle is important. The first law of thermodynamics applies the conservation of energy principle to systems where heat transfer and doing work are the methods of transferring energy into and out of the system. The first law of thermodynamics    states that the change in internal energy of a system equals the net heat transfer into the system minus the net work done by the system. In equation form, the first law of thermodynamics is

Δ U = Q W . size 12{ΔU=Q - W} {}

Here Δ U size 12{ΔU} {} is the change in internal energy U size 12{U} {} of the system. Q size 12{Q} {} is the net heat transferred into the system —that is, Q size 12{Q} {} is the sum of all heat transfer into and out of the system. W size 12{W} {} is the net work done by the system —that is, W size 12{W} {} is the sum of all work done on or by the system. We use the following sign conventions: if Q size 12{Q} {} is positive, then there is a net heat transfer into the system; if W size 12{W} {} is positive, then there is net work done by the system. So positive Q size 12{Q} {} adds energy to the system and positive W size 12{W} {} takes energy from the system. Thus Δ U = Q W size 12{ΔU=Q - W} {} . Note also that if more heat transfer into the system occurs than work done, the difference is stored as internal energy. Heat engines are a good example of this—heat transfer into them takes place so that they can do work. (See [link] .) We will now examine Q size 12{Q} {} , W size 12{W} {} , and Δ U size 12{ΔU} {} further.

The figure shows a schematic diagram of a system shown by an ellipse. Heat Q is shown to enter the system as shown by a bold arrow toward the ellipse. The work done is shown pointing away from the system. The internal energy of the system is marked as delta U equals Q minus W. The second part of the figure shows two arrow diagrams for the heat change Q and work W. Q is shown as Q in minus Q out. W is shown as W out minus W in.
The first law of thermodynamics is the conservation-of-energy principle stated for a system where heat and work are the methods of transferring energy for a system in thermal equilibrium. Q size 12{Q} {} represents the net heat transfer—it is the sum of all heat transfers into and out of the system. Q size 12{Q} {} is positive for net heat transfer into the system. W size 12{W} {} is the total work done on and by the system. W size 12{W} {} is positive when more work is done by the system than on it. The change in the internal energy of the system, Δ U size 12{ΔU} {} , is related to heat and work by the first law of thermodynamics, Δ U = Q W size 12{ΔU=Q - W} {} .

Questions & Answers

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classification of plants, gymnosperm features.
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In biology, a pathogen (Greek: πάθος pathos "suffering", "passion" and -γενής -genēs "producer of") in the oldest and broadest sense, is anything that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ. The term pathogen came into use in the 1880s.[1][2
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Definition of respiration
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respiration is the process in which we breath in oxygen and breath out carbon dioxide
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in the mouth
EZEKIEL
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stimulates the follicle to release the mature ovum into the oviduct
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endocrine secrete hormone and regulate body process
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Biology is the study of living organisms, divided into many specialized field that cover their morphology, physiology,anatomy, behaviour,origin and distribution.
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Biology is the study of how living organisms live and survive in a specific environment
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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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