Kumpulan Makalah

If we think of a macroscopic system as consisting of a large number of interacting particles, we know that it has a well defined total energy which satisfies a conservation principle. This simple justification of the existence of a thermodynamic energy function is very different from the historical development because thermodynamics was developed before the atomic theory of matter was well accepted. Historically, the existence of a macroscopic conservation of energy principle was demonstrated by purely macroscopic observations as outlined in the following.

Consider a system enclosed by insulating walls – walls that prevent the system from being heated by the environment. Such a system is thermally isolated. A process in which the state of the system is changed only by work done on the system is called adiabatic. We know from overwhelming empirical evidence that the amount of work needed to change the state of a thermally isolated system depends only on the initial and final states and not on the intermediate states through which the system passes. This independence of the path under these conditions implies that we can define a function E such that for a change from state 1 to state 2, the work done on a thermally isolated system equals the change in E:

W = E2 − E1 = ΔE (adiabatic process)

The quantity E is called the (internal) energy of the system.7 The internal energy is measured with respect to the center of mass.8 The energy E is an example of a state function, that is, it characterizes the state of a macroscopic system and is independent of the path.

Problem

What the difference between the total energy and the internal energy?

If we choose a convenient reference state as the zero of energy, then E has an unique value for each state of the system because W is independent of the path for an adiabatic process. (Remember that in general W depends on the path.)

If we relax the condition that the change be adiabatic and allow the system to interact with its surroundings, we would find in general that ΔE = W. (The difference between ΔE and W is zero for an adiabatic process.) In general, we know that we can increase the energy of a system by doing work on it or by heating it as a consequence of a temperature difference between it and its surroundings. In general, the change in the internal energy of a closed system (fixed number of particles) is given by
ΔE = W + Q (first law of thermodynamics)

The quantity Q is the change in the system’s energy due to heating (Q > 0) or cooling (Q < style=”font-weight: bold;”>Problem

A cylindrical pump contains one mole of a gas. The piston fits tightly so that no air escapes and friction in negligible between the piston and the cylinder walls. The pump is thermally insulated from its surroundings. The piston is quickly pressed inward. What will happen to the temperature of the gas? Explain your reasoning.

So far we have considered two classes of thermodynamic quantities. One class consists of state functions because they have a specific value for each macroscopic state of the system. An example of such a function is the internal energy E. As we have discussed, there are other quantities, such as work and energy transfer due to heating, that do not depend on the state of the system. These latter quantities depend on the thermodynamic process by which the system changed from one state to another.

Originally, many scientists thought that there was a fluid called heat in all substances which could flow from one substance to another. This idea was abandoned many years ago, but is still used in everyday language. Thus, people talk about adding heat to a system. We will avoid this use and whenever possible we will avoid the use of the noun “heat” altogether. Instead, we will refer to a process as heating or cooling if it changes the internal energy of a system without changing any external parameters (such as the external pressure, electric field, magnetic field, etc). Heating occurs whenever two solids at different temperatures are brought into thermal contact. In everyday language we would say that heat flows from the hot to the cold body. However, we prefer to say that energy is transferred from the hotter to the colder body. There is no need to invoke the noun “heat,” and it is misleading to say that heat “flows” from one body to another.

To understand better that there is no such thing as the amount of heat in a body, consider the following simple analogy adapted from Callen A farmer owns a pond, fed by one stream and drained by another. The pond also receives water from rainfall and loses water by evaporation. The pond is the system of interest, the water within it is analogous to the internal energy, the process of transferring water by the streams is analogous to doing work, the process of adding water by rainfall is analogous to heating, and the process of evaporation is analogous to cooling. The only quantity of interest is the water, just as the only quantity of interest is energy in the thermal case. An examination of the change in the amount of water in the pond cannot tell us how the water got there. The terms rain and evaporation refer only to methods of water transfer, just as the terms heating and cooling refer only to methods of energy transfer.

Take a small plastic container and add just enough water to it so that its temperature can be conveniently measured. Then let the water and the bottle come into equilibrium with their surroundings. Measure the temperature of the water, cap the bottle, and shake the bottle until you are too tired to continue further. Then uncap the bottle and measure the water temperature again. If there were a “whole lot of shaking going on,” you would find the temperature had increased a little.

In this example, the temperature of the water increased without heating. We did work on the water, which resulted in an increase in its internal energy as manifested by a rise in the temperature. The same increase in temperature could have been obtained by bringing the water

into contact with a body at a higher temperature. But it would be impossible to determine by making measurements on the water whether shaking or heating had been responsible for taking the system from its initial to its final state. (To silence someone who objects that you heated the

water with “body heat,” wrap the bottle with an insulating material.)

OK, see you again.
source : http://gambutku.com/the-first-law-of-thermodynamics