Isothermal means constant temperture process

1. Mar 2, 2007

pivoxa15

Isothermal means constant temperture process. But thermal means heat. Presumably iso is short for isolated. So it should be isolated heat process or constant heat process rather than constant temperture process. Instead adiabatic means constant heat process. Even though it is a trivial matter, I feel it still is a bit strange and somewhat unsatisfied.

2. Mar 2, 2007

ZapperZ

Staff Emeritus
What exactly is your point here? You don't like how something is named?

Zz.

3. Mar 2, 2007

Staff: Mentor

Not really. The (Greek) prefix "iso" means "equal"; the word "isolated" has a different origin, from the Latin word for island.
The origin of this word, again from the Greek, means "impassable"--which is close to it's current meaning that heat cannot flow in or out.

While word origins can be interesting, you still have to learn how words are used in their current scientific context--it's dangerous to guess the meaning of a word. (Just deal with it.)

4. Mar 2, 2007

FredGarvin

One more point...an adiabatic process is not necessarily a constant heat process. It means there is no heat transfer out of the control volume. For example, you can have an adiabatic combustor that increases the internal temperature of the CV but still have no heat transfer.

You may also look at weather maps that use "isobars" (constant pressure lines) and "isotherms" (constant temperature lines).

5. Mar 2, 2007

pivoxa15

Yeah. If you break isothermal into components it's iso which is 'equal' and thermal which is heat. So 'logically' the definition of isothermal should be constant heat process (no heat in nor out) but instead it's a constant temperture process.

Looks like adiabatic could also represent constant heat or no heat in nor out but at least they should define another word for constant temperture processes. However, this type of thing is not unusual in science and physics is it. Myabe it's got to do with the history and scientists getting theories wrong but the definitions for words they made stuck with us even though the theory has improved.

6. Mar 3, 2007

ZapperZ

Staff Emeritus
You need to get over this very quickly. There are, frankly, more daunting issues to deal with when learning physics. The last thing you want to do with waste time and effort on something that does not make any difference.

Besides, both Doc Al and FredGarvin have described where you got this one wrong.

Zz.

7. Mar 3, 2007

Andrew Mason

There a many things in thermodynamics that reflect the fact that the early pioneers in this field did not fully understand what heat was.

The concept of heat in thermodynamics is really heat flow, or the movement of heat energy into or out of a substance rather than the amount of heat in a substance. The former is Q, the latter is measured by temperature.

The term "isothermal" says that the heat energy contained in a substance does not change. It does not say that there is no heat flow into or out of the substance (ie. does not say Q = 0).

On the other hand, "adiabatic", as Doc Al correctly notes, refers to heat flow into or out of the substance being nil - heat does not flow: Q = 0.

So I think the terms are accurate. They just refer to two different aspects of heat: heat content and heat flow.

Adiabatic combustion is a confusing term. If combustion occurs, there is heat flow into the substance.

The "adiabatic" must refer to the fact that there is no heat flow in or out (Q=0) after combustion so the expansion of the hot post-combustion gas is adiabatic. In other words, the heat generated in combustion does not flow to the surroundings.

AM

8. Mar 3, 2007

pivoxa15

Good point. Temperture is a measure of the amount of heat in the system. "The temperature of a system is related to the average energy of microscopic motions in the system."

http://en.wikipedia.org/wiki/Temperture

So constant heat content => constant temperture. Heat can still flow but as long as heat inflow=heat outflow. Fixed volume and pressure.

However with adiabatic, does it imply no heat inflow nor outflow, even if both cancel and you have 0 net heat flow?

9. Mar 4, 2007

Staff: Mentor

I think you missed part of Andrew's point.
"Heat" refers to energy transferred due to a temperature difference; systems contain internal energy, not "heat"; temperture is not a measure of the amount of internal energy in a system.
Right!

Again, you are confusing "heat content" (internal energy?) and temperature with heat flow, symbolized by Q. In an isothermal process, temperature is constant but heat may flow into/out of the system (Q could be nonzero) and internal energy may change.

10. Mar 4, 2007

Andrew Mason

I see that I may have added some confusion. It might be best to avoid use of "heat energy" and "heat content" (as I have used them, as distinct from "heat" or "heat flow") and use the terms "thermal energy" or "thermal energy content" to connote the measure of the average kinetic energy of the molecules.

As has been said, in thermodynamics we use "heat" to mean "heat flow" (Q). If we use "thermal energy content" (U) to refer to the internal energy, which is a function of temperature only, the confusing uses of the word "heat" can be avoided.

Aside: The original thermodynamic concept of heat is that of a substance that flows into or out of a system. That concept is still at the heart of thermodynamics. It is part of the reason that thermodynamics remains a difficult subject for many. In my view, the study of thermodynamics could greatly benefit from an approach that gets rid of the distinction between Q and U and talks about potential energy change and work.

AM

11. Mar 4, 2007

pivoxa15

For an ideal gas U=(f/2)NkT so temperture is a measure of the amount of internal energy of an ideal gas although also needing the degrees of freedom of that gas.

Does adiabatic mean no net heat flow or no heat flow whatsoever (even if heat in=heat out)? This is an important point which must be clarified?

If we take your point that ""Heat" refers to energy transferred due to a temperature difference" then temperture difference => heat transfer. So when there is no heat transfer, there cannot have been a temperture difference. Back to iso=same and thermal=heat. Sameheat <=> no heat transfer (whatsoever) hence when an isothermal process occurs there cannot be a temperture difference. Hence we have constant temperture for an isothermal process.

However, adiabatic is the process to describe constant heat or no heat flow but is it no net heat flow? If it is then my above point may be valid because the above describes no heat flow whatsoever (i.e. it can't even be heat inflow=heat outflow) which would not be the same to all adiabatic processes. However, it would mean all isothermal processes are adiabatic which is incorrect. Maybe there are other factors that lead to heat flow apart from temperture differences. Thermodynamics is more than just heat and temperture (and energy). However, back in the pioneering days things were hazy like it is to me and they made definitions which are not fully reasonable or logical compared to our (excluding me as I am still learning) understanding today.

Last edited: Mar 4, 2007
12. Mar 4, 2007

Andrew Mason

Quite right. Internal energy is a function of temperature only. An Isothermal change necessarily means no internal energy change (dU = nCvdT, so dU = 0 if dT = 0) but it does NOT mean no heat flow. In other words dU = 0 but $dQ \ne 0$ if dT = 0. This means that dQ = dW for isothermal processes.

Yes. No heat flow in or out even if heat in = heat out.

No. Doc Al is referring to a situation where the reservoirs at different temperatures are in thermal contact. In that case there is a flow from the hotter reservoir to the cooler one. An adiabatic wall would prevent thermal contact.

No. There is always heat flow in an isothermal change. If dU = 0 (isothermal: dT=0), dQ = dW. If dW = 0 and dU = 0 there can be no change at all. So in any change where dU = 0, there has to be heat flow (Q) and work done on or by the system.

AM

13. Mar 4, 2007

Staff: Mentor

Note that that (for an ideal gas) the temperature plus N tells you the internal energy, not temperature alone. (That's part of what I meant.)

For an ideal gas, but not in general. Consider heating an ice cube and the associated phase change--it's isothermal, but requires an increase in internal energy.

14. Jan 26, 2009

niikki

Re: isothermal?

im tring to understand, im currently learning to use a isothermic system and im puting heat on at all times and its referred to as a isothermic process how is that so? someone please explain

15. Jan 26, 2009

Re: isothermal?

Heat and temperature are different things and it is possible for heat to flow without a resulting change of temperature.If you look up latent heat that may help.

16. Jan 27, 2009

Sedunov

Re: isothermal?

Dear Pivoxa!
Isothermal never means isolated. You cannot have really isothermal conditions without good thermal contact with some thermostate. So, isolation in this case is a misleading concept. Isothermal processes are always accompanied by heat flows through the boundary of the system. When you compress some volume of gas, the extra thermal energy flows out of the volume, and the process remains isothermal (with constant temperature), if the thermal contact is good.
But adiabatic means really isolated. No heat flow can cross the boundary. When you compress the gas in adiabatic conditions, the temperature grows because the produced by this process heat energy cannot escape from the volume.

Last edited: Jan 27, 2009