|Thermodynamics: First Law of Thermodynamics|
Energy can neither be created nor destroyed.
The term internal energy is often used synonymously with the energy of a system. It is the sum of the kinetic and potential energies of the particles that form the system. The last postulate in the kinetic molecular theory states that the average kinetic energy of a collection of gas particles is dependent only upon the temperature of the gas. Since ideal gases have no potential energy, the internal energy is directly proportional to the temperature:
If a system is more complex than an ideal gas, then the internal energy must be measured indirectly by observing any changes in the temperature of the system. The change in the internal energy of a system is equal to the difference between the final and initial energies of the system:
The equation for the first law of thermodynamics can be rearranged to show the energy of a system in terms of the energy of its surroundings. This equation indicates that the energy lost by one must equal the energy gained by the other:
The energy of a system can change by the transfer of work and or heat between the system and its surroundings. Any heat that is taken, given off, or lost must be complemented by an input of work to make up for the loss of heat. Conversely, a system can be used to do any amount of work as long as there is an input of heat to make up for the work done.
This equation can be used to explain the two types of heat that can be added to a system:
- Heat can increase the temperature of a system. This is sensible heat.
- Heat that does ONLY WORK on a system is latent heat.
The diagram above illustrates the sign (positive or negative) of the change in the energy of a system when heat and work are transferred between a system and its surroundings.
- When heat enters a system, resulting in an increase in the temperature, E is positive.
- When heat leaves a system resulting in a decrease in the temperature, E is negative.
- When a system does work on its surroundings, energy is lost, therefore E is negative.
- When the surroundings do work on a system, the internal energy increases, therefore E is positive.
- A property of a system is a state function if it depends on the state of the system and not the path used to get to that state.
- Equations of state - Equations that connect two or more properties that describe the state of a system.
- e.g. The ideal gas law, PV = nRT is an equation of state.
Are the following properties state functions?
- TEMPERATURE: YES. The net change in the temperature of a system (T) depends only on the initial and final temperatures of the system. It does not matter what changes in temperature happened in between these stages.
- INTERNAL ENERGY: YES. Since the internal energy of a system is directly proportional to the temperature of a system, internal energy is also a state function.
- WORK: NO. By definition, work is the product of the force and the distance that an object moves. This means that it does depend on the path use to get to the final state, and therefore work cannot be a state function.
- HEAT: NO. It was determined earlier that the change in the internal energy of a system is equal to the work and the heat transferred between the system and its surroundings:
We already determined that the internal energy of a system is a state function, but work is not. Since the change heat must complement the path that the work followed, heat cannot be a state function.
Properties that ARE state functions generally have CAPITALIZED variables.
- e.g. E, P, T, V, etc.
Properties that ARE NOT state functions generally have LOWER-CASE variables.
- e.g. q and w