What are some common mistakes students make with heat in thermodynamics?
1 Answer
Explanation:
What a great question!
You can think of heat as the net energy transferred from one object to another (thermal interaction) due to a temperature difference. This is a process usually accomplished by conduction or radiation (+ convection, evaporation, etc.).
One of the most common misconceptions/mistakes I see have to do with not distinguishing well between heat, work, thermal energy, and temperature.
Outside of physics/chemistry/etc., we often talk about "heat" in ways which are not scientifically accurate, which probably doesn't help; before we first learn about the concept of heat in physics or chemistry or what have you, the odds are we've been using the term for many years.
Heat vs. Work:
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Unlike a mechanical interaction in which work is done, heat requires no macroscopic motion of the system
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Energy is transferred when the faster molecules in the hotter object collide with the slower molecules in the cooler object
-Net result is energy transfer from hotter object to cooler object
Heat Vs. Temperature
- Temperature is a state variable that quantifies the "hotness" or "coldness" of a system
- Heat is not a state variable. It makes no sense to talk about how heat changes.
- Heat transfer is a consequence of a temperature difference between objects
Heat Vs. Thermal Energy
- Thermal energy is an energy of the system due to the motion of its atoms and molecules
- Thermal energy it is a state variable; it makes sense to talk about how
#E_"th"# changes during a process - Heat is not a state variable
- Heat may cause the system's thermal energy to change
On the same subject, students often want to assume that an observed increase in temperature implies an increase in heat, but heating a system is only one way to change its temperature! You can also change the system's temperature by doing work on the system (such as with friction).
For example, in an adiabatic process,
#W=DeltaE_"th"#
