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. (See Figure 2.) We will now examine Q, W, and Δ U further. Heat engines are a good example of this-heat transfer into them takes place so that they can do work. Note also that if more heat transfer into the system occurs than work done, the difference is stored as internal energy. So positive Q adds energy to the system and positive W takes energy from the system. We use the following sign conventions: if Q is positive, then there is a net heat transfer into the system if W is positive, then there is net work done by the system. W is the net work done by the system-that is, W is the sum of all work done on or by the system. Q is the net heat transferred into the system-that is, Q is the sum of all heat transfer into and out of the system. Here Δ U is the change in internal energy U of the system. In equation form, the first law of thermodynamics is Δ U = Q − W. 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. 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. If we are interested in how heat transfer is converted into doing work, then the conservation of energy principle is important. As the entire system gets hotter, work is done-from the evaporation of the water to the whistling of the kettle. The water in the kettle is turning to water vapor because heat is being transferred from the stove to the kettle. This boiling tea kettle represents energy in motion. Calculate changes in the internal energy of a system, after accounting for heat transfer and work done.įigure 1.Identify instances of the first law of thermodynamics working in everyday situations, including biological metabolism.Describe how conservation of energy relates to the first law of thermodynamics.Define the first law of thermodynamics.By the end of this section, you will be able to:
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