![]() Internal energy is a form of energy completely different from either heat or work. However, both can change the internal energy of a system. Heat transfer and work are both energy in transit-neither is stored as such in a system. Once the temperature increase has occurred, it is impossible to tell whether it was caused by heat transfer or by doing work. Heat transfer into a system, such as when the Sun warms the air in a bicycle tire, can increase its temperature, and so can work done on the system, as when the bicyclist pumps air into the tire. For example, both can cause a temperature increase. Nevertheless, heat and work can produce identical results. Work, a quite organized process, involves a macroscopic force exerted through a distance. Heat transfer, a less organized process, is driven by temperature differences. Heat transfer ( ) and doing work ( ) are the two everyday means of bringing energy into or taking energy out of a system. The first law gives the relationship between heat transfer, work done, and the change in internal energy of a system. The first law of thermodynamics is actually the law of conservation of energy stated in a form most useful in thermodynamics. MAKING CONNECTIONS: LAW OF THERMODYNAMICS AND LAW OF CONSERVATION OF ENERGY The change in the internal energy of the system, ΔU, is related to heat and work by the first law of thermodynamics, ΔU=Q−W. W is positive when more work is done by the system than on it. W is the total work done on and by the system. Q is positive for net heat transfer into the system. Q represents the net heat transfer-it is the sum of all heat transfers into and out of the system. 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 and further. Heat engines are a good example of this-heat transfer into them takes place so that they can do work. Thus Note also that if more heat transfer into the system occurs than work done, the difference is stored as internal energy. So positive adds energy to the system and positive takes energy from the system. We use the following sign conventions: if is positive, then there is a net heat transfer into the system if is positive, then there is net work done by the system. is the net work done by the system-that is, is the sum of all work done on or by the system. is the net heat transferred into the system-that is, is the sum of all heat transfer into and out of the system. ![]() Here is the change in internal energy of the system. In equation form, the first law of thermodynamics is 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. ![]()
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