20.6:
First Law of Thermodynamics
Consider a sky lantern with a candle. The candle's heat increases the internal energy of the air molecules inside the lantern, which causes the expansion of air and inflates the lantern. Due to the expansion, work is done by the air molecules on the lantern. The air density inside the inflated lantern is reduced, which creates lift.
In this example, the change in internal energy of the system is equal to the net heat transfer into the system minus the net work done by the system. This equation is a generalized form of energy conservation known as the first law of thermodynamics.
Conventionally, Q is positive or negative, depending on whether heat is added to the system or removed from the system. Also, W is positive when work is done by the system and negative when work is done on the system.
For any thermodynamic process, the change in internal energy is path independent and depends only on the system's initial and final equilibrium states. Thus, the internal energy is a state function.
20.6:
First Law of Thermodynamics
A change in the internal energy of a system depends on the the net heat transfer into the system and the net work done by the system. The first law of thermodynamics, which is a generalized form of energy conservation, relates these three quantities mathematically. It states that the change in the internal energy equals the difference between the heat transfer and work done by the system.
The applied heat increases the internal energy of a system. Hence, conventionally heat is considered positive when added to the system and it is negative when removed from the system. When a gas expands, it does work, and its internal energy decreases. Thus, work is considered positive when it is done by the system and negative when it is done on the system.
Although heat and work both depend on the thermodynamic path taken between two equilibrium states, the change in internal energy depends only on the system's initial and final equilibrium states. Similar to the change in potential energy, the change in internal energy is a path independent quantity. The internal energy is a function of thermodynamic variables like pressure, temperature and volume. Functions such as internal energy and potential energy are known as state functions because their values depend solely on the state of the system.
Suggested Reading
- Young, H. D, and Freedman, R.A. (2012). University Physics with Modern Physics. San Francisco, CA: Pearson. pp 630.
- OpenStax. (2019). University Physics Vol. 2. [Web version]. pp 116 Retrieved from https://openstax.org/books/college-physics/pages/15-1-the-first-law-of-thermodynamics