First Law of Thermodynamics

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Claimed by Mlee406 for fall 2016

The science of thermodynamics originated in the 19th century in order to explain the mechanism of steam engines. The term 'thermodynamics' means power developed from heat, and thermodynamics deals with the relationship between heat and other forms of energy. One can apply thermodynamics to a problem, beginning with identifying a body of matter called system. A system's thermodynamics state is defined by several macroscopic properties that depend on the fundamental dimensions such as time, temperature, mass, length, and etc.

Background

Joule's Experiment: Mechanical Equivalent of Heat

James P. Joule's experiments made major contributions to the formation of the modern concept of heat. Joule put certain amounts of mercury, water, and oil into an insulated container and mixed the liquid using a rotating stirrer. He carefully measured how much work was done by the stirrer to the liquid and the change in temperature that the liquid experienced. Joule discovered that for a particular fluid, a fixed amount of work is required per unit mass in order to raise its temperature by one degree. Joule established that heat is a form of energy and that there is a quantitative relationship between heat and work.

Internal Energy

In Joule's experiment, work added to the liquid was transformed into heat. The energy in between the transition is contained in the liquid in a different form, called internal energy. It is named 'internal' in order to distinguish it from kinetic and potential energy, which can be considered 'external', as they depend on the substance's position or motion. Internal energy cannot be measured directly and its absolute values are unknown. However, this is no problem as we are only concerned with changes in internal energy in thermodynamics.

The Main Idea

It is crucial to recognize heat and internal energy as forms of energy through the previous section to understand the first law of thermodynamics. We can now extend scope of the law of conservation of mechanical energy from work and external potential and kinetic energy to heat and internal energy. First Law of Thermodynamics states, although energy comes in various forms, the total quantity of energy stays the same and the energy disappearing in one form must appear simultaneously in other forms.

A Mathematical Model

[math]\displaystyle{ ∆U = Q-W }[/math] where [math]\displaystyle{ ∆U }[/math] is change in the internal energy , [math]\displaystyle{ Q }[/math] is heat added to the system, and [math]\displaystyle{ W }[/math] is the work done by the system. In other words, change in internal energy is equal to flow of heat into a system minus work done on the system. While both [math]\displaystyle{ Q }[/math] and [math]\displaystyle{ W }[/math] are path functions, [math]\displaystyle{ ∆U }[/math] is a path-independent state function, which means that it only depends on the current state of the system.

A Computational Model

http://jersey.uoregon.edu/vlab/Thermodynamics/

This virtual experiment gives you visual representation of thermal equilibrium and how it is reached when beginning with different initial conditions.

Examples

Solving for Change in Internal Energy

If 500 J of work is done on a system, and it gains 100 J of heat from its surroundings, then what is the change in internal energy of the system?

Solution:

[math]\displaystyle{ ∆U = Q-W }[/math], [math]\displaystyle{ Q= 100 J }[/math], [math]\displaystyle{ W=-500 J }[/math]

[math]\displaystyle{ ∆U = 100 J -(-500 J) }[/math]

[math]\displaystyle{ ∆U = 600 J }[/math]

Connectedness

1. How is this topic connected to something that you are interested in?

  • Thermodynamics is seen in all aspects of life, in everything from melting ice cubes to flipping switches to cooking.

2. How is it connected to your major?

  • Thermodynamics is useful to know, and is covered in the Fundamentals of Engineering exam. Engineers interested in passing the FE and other similar tests should have a strong foundation in thermodynamics.

3. Is there an interesting industrial application?

  • Thermodynamics is relevant in the energy,transportation, and HVAC industries.

See also

Thermodynamics

Further reading

  • Thermodynamics (Dover Book on Physics) by Enrico Fermi
  • Engineering Thermodynamics by P. K. Nag
  • M. J. Moran and H. N. Shapiro, ‘Fundamentals of Engineering Thermodynamics’, Fourth Edition, Wiley, New York, 2000
  • Thermodynamics: An Engineering Approach by Cengel, Ya and Boles, M.A.

External links

References

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