14.2. First Law of Thermodynamics http://www.ck12.org
The internal energy of a system can increase or decrease by heat flowing into or out of the system. The internal
energy of a system can also increase or decrease when work is done on the system or the system does work on the
environment (we will describe how work is done on the system and how the system does work on the environment
later).
The relationship that was established in the 19th century among internal energy, heat, and work is now known as the
Law of Conservation of Energy. Though we have made use of conservation of energy in our work on mechanics, it
was not until the concept of heat was understood that a formal statement on the conservation of energy was proposed.
This formulation is now known as the First Law of Thermodynamics, stated below:
The change in the internal energy of a closed system is equal to the heat delivered into (or out of) a system, plus the
work done on the system (or by the system).
http://www.youtube.com/watch?v=3lrakjtd1hU
It is represented symbolically as∆U=Q+W, where∆Uis the change in internal energy(U),Qis heat, andWis
work.
It is important to follow the proper sign conventions in using the First Law:
- When heat flows into the system,Qis positive.
- When heat flows out of the system,Qis negative.
- When work is done on the system,Wis positive.
- When the system does work on the environment,Wis negative.
One further definition will be useful. It is often the case that we wish to transfer energy into or out of a system or
maintain a system at a constant temperature. To do so, we put the system in contact with aheat reservoir. The
reservoir is considered large enough so that any heat that flows into it or out of it does not affect its temperature. It is
incorrect to think of the heat reservoir as something that is necessarily hot. It is a reservoir that can accept or deliver
heat.
Illustrative Example 1
- An air-filled cylinder with a movable piston has a heat reservoir in contact with it,Figure14.10. The heat
reservoir transfers 500 J of energy into the cylinder. Due to the energy transfer, the piston does 300 J of work on the
environment.
a. What is the sign ofQ?
Answer:Since the heat flowed into the system,Qis positive.
b. What is the sign of theW?
Answer:Since the piston did work on the environment,Wis negative.
c. What is the change in internal energy of the system?
Answer:∆U=Q+W→∆U= + 500 J− 300 J= 200 J
Four Definitions
It is possible to carry out a thermodynamic process under several special conditions:
A thermodynamic process that takes place at constant temperature is called anisothermal process.
A thermodynamic process that takes place at constant volume is called anisochoric, or isovolumetric, process.
A thermodynamic process that takes place at constant pressure is called anisobaric process.