# Thermodynamics and daily life

## Main Question or Discussion Point

Hi everyone,
I'm reading thermodynamics and I can remember what's internal energy, enthalpy, entropy etc but I'm not sure whether I understand it completely or not as I couldn't find any real life reference to these concepts and what's the significance of learning all these things? Why we name the same thing differently as heat, enthalpy, internal energy etc? Please help me understand this.
Thanks:)

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Bystander
Homework Helper
Gold Member
"This" (thermo) is the foundation of the industrial revolution.
When you have specific difficulties with specific concepts, bring them to PF, and people are going to be more than happy to coach you through them. Check the PF posting guidelines for specifics on selecting sub-forums, and material to include in your requests for assistance.

I'm confused about the terms heat, enthalpy, internal energy and total energy of the system. internal energy is the heat energy contained in a system, work is the mechanical energy(P.E stored in system), heat is the internal energy transferred between system and the surrounding??? And enthalpy?? Is it the total energy of the system?? Or its the heat contained in a system at constant pressure? What happens to enthalpy if there's change in pressure? What's its relation with pressure?
Thanks.

Bystander
Homework Helper
Gold Member
total energy
heat energy contained
mechanical energy(P.E stored in system),
heat contained in a system at constant pressure
First source of confusion: there are no absolute ("total" or "stored") values for energies when working with the first law; you are doing bookkeeping only upon changes in these energies between (or among) two (or more) states of a system. Generally there is some initial or reference state (or you define one) to which you add or subtract work or heat.

If you really want to know more about real life applications, you should read more engineering books. In fact, many engineering courses have compulsory thermodynamics. You would learn that thermo is crucial to many parts of engineering.

Heat, enthalpy, and internal energy are so different. Firstly, internal energy U and enthalpy H are two different thermodynamic potentials that are minimized under different conditions, and U = H - pV. Secondly, heat Q refers to the internal energy change involved in a process which depends on the process, whereas internal energy itself is a state function which only depends on the state of the system. 1st law is usually written as dU = dW + dQ, so they are not exactly the same.

Internal energy: The sum of the kinetic and potential energies of the particles of a system.
Entropy: Quite simply put, it's a measure of the "disorderliness" of the particles/components of a system.(The degree of random placement which is more probable as a result of non-directional energy presence, such as that caused by diffusion)
Enthalpy: The total energy of a system.
It is worthwhile mentioning that only changes in energies of a system are observable. For convenience, we often end up defining a standardized "0" from which changes of a system are measured. You commonly use thermodynamic understanding in daily life without actually realizing it: by knowing that a teaspoon of water heats up faster than a pan full of it at the same initial temperature, for example.

OmCheeto
Gold Member
...What's its relation with pressure?
Thanks.
Enthalpy = Internal Energy + (Pressure)*(Volume)
If pressure goes up, and everything else stays the same, then enthalpy must go up.

Here is a great place to start if you haven't already found this site:
http://hyperphysics.phy-astr.gsu.edu/hbase/heacon.html#heacon
Thermo is a hard subject to study no matter how much you read, experimentation always helps. I like to think about the combustion engine. But to really get a feel for why it all works statistical mechanics helps, they math is a little funny to decipher but the concepts are rock solid and make sense.
In these lectures below are some really interesting demonstrations of thermodynamics and great explanations
Prof. C. J. Cramer (University of Minnesota)