20.7:
First Law Of Thermodynamics: Problem-Solving
The first law of thermodynamics states that the heat added to a system is utilized in performing the work and increasing the system's internal energy.
Consider an example where 100 grams of water at one atmospheric pressure converts into steam at 100 degrees Celsius. What is the change in the internal energy for this thermodynamic process?
This thermodynamic system changes from the initial liquid to the final gaseous state at constant temperature and pressure when water converts to steam.
The heat transfer into the system is the product of mass and the latent heat of the vaporization of water.
The work done is equal to the product of pressure and volume change. Using the value of mass and density of the steam and water, the volume change and so the work done is estimated.
Here the unknown quantity is the change in the internal energy, while the known quantities are heat supplied and work done by the system. So, applying the first law of thermodynamics, a change in internal energy can be estimated.
20.7:
First Law Of Thermodynamics: Problem-Solving
The first law of thermodynamics states that 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 and can be applied to any thermodynamic process.
The following strategies can be used to solve any problem involving the first law of thermodynamics.
- The thermodynamic system should be identified.
- The initial and final states of the thermodynamic process should be listed.
- The known and unknown quantities for the given problem are identified. It should be ensured that the units for all the quantities are consistent. For example, if pressure is in Pascals and volume is in cubic meters, the work should be expressed in Joules.
- The change in internal energy is path independent. As a result, it can be estimated for all possible paths if the internal energy for the initial and final states is known.
- Heat, work, and change in internal energy for any process are related via the equation for the first law of thermodynamics.
- Conventionally, Q is positive when heat is added to the system and negative when the heat is removed from it. Also, W is positive when work is done by the system and negative when work is done on the system. This means that the polarity of work and heat should be considered in the first law of thermodynamics.
- The known quantities can then be plugged into the equation for the first law of thermodynamics to evaluate the unknown quantity.
Suggested Reading
- Young, H. D, and Freedman, R.A. (2012). University Physics with Modern Physics. San Francisco, CA: Pearson. pp 632.
- 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