Temperature and Heat

[XELR8]

Will an object that is heated ever get to a higher temperature then the heat input temperature itself. For example, if I heat a piece of metal with a heat source at 150F, will the temperature of the metal ever exceed 150F if the heat source input remains constant for a long period of time? Thanks!

 

[IRstuff]

Rubbing of sticks is not a pure thermal phenomenon, so it's not a question of simply two lower temperature objects achieving a higher temperature on their own. The energy input comes from the rubbing together and overcoming the induced friction caused by the pushing together of the sticks.

As for the "momentum" question, that's a complete misunderstanding of what the initial conditions are, and where the heat is stored within the system. Consider the simple case of a solid object with a constant T1 on one side, and natural convection on the other, resulting in a surface temperature of T2. There is a thermal gradient, which causes heat to flow from the T1 surface to the T2 surface. Remove the heat source on the T1 side and insulate both sides. The T2 side will increase in temperature, while the T1 side will decrease. over time, the temperature will equilibrate to (T1+T2)/2, the average temperature. Pure conservation of energy, and pure Fick's law (gradient drives flow).

In many supposed thermal momentum allegations, it's simply because people cannot observe all the state variables, and see only T2 increasing, and assume there's a "momentum" involved.

TTFN

FAQ731-376

 

[SNORGY]

Compositepro:

No, what I was thinking - but verbalizing poorly - with the flywheel is that, at the instant the accelerating load is removed, I conceptualize that the rotational speed will nonetheless increase to a maximum (with "acceleration" declining) before decreasing, and this is the "lag" to which I was referring. In other words, I would expect that the transition in angular speed is continuous and cuspless. Put another way, I suspect the speed versus time curve is smooth after the cessation of the applied force, and the duration in this region depends on whatever inertia and damping are present.

But I could be wrong.

Regards,

SNORGY.

 

[SomptingGuy]

SNORGY, there is no polite way to say this - you are simply wrong. You seem to be confusing acceleration with velocity, probably confusing inertia with momentum too.

Back to the heat analogy. That's wrong too. Heat has no equivalent of moving mass to give it an equivalent of momentum.

- Steve

 

[XELR8]

Hi Expert Help,

So, a material when heated (properties willing) can reach the same temperature of the heat source and can never exceed the temperature of the higher temperature heat source?

This assumes the higher temperature source has an infinite supply of energy to remain constant.

( I understand that when two bodies at different temperatures come in contact, the heat will flow from the higher temperature body to the lower temperature body until they come to an equal temperature.)

Thanks!

 

[IRstuff]

I'm not sure what you don't understand of the numerous answers that you have received that all say basically the same thing.

"This assumes the higher temperature source has an infinite supply of energy to remain constant" is not required in real problems, and cannot be achieved in the real world. Constant temperature merely requires that net heat flow be ZERO. If net heat flow is zero, then be definition, the temperature is constant.

TTFN

FAQ731-376

 

[XELR8]

Hi IRstuff,

Information noted and confirmed.

Thanks.

 

[SNORGY]

SomptingGuy:

My apologies for being wrong. (It happens.)

No worries...you were quite polite about it.

Regards,

SNORGY.

 

[SNORGY]

In retrospect, Compositepro was (in my - perhaps also wrong - opinion) correct. The effect that I poorly (incorrectly) described requires some elasticity and elastic deformation in order for there to be no cusp in the velocity-time curve or the acceleration-time curve. I simply had difficulty with accepting that changes in acceleration from "some value" to "zero" occur instantaneously.

Perhaps I just don't understand jerks (like me) as well as I ought to.

(Poor attempt at humour there.)



Regards,

SNORGY.

 

[Compositepro]

Snorgy, kudos for taking the time to do a little research and bettering your understanding. Your response let's others know that the time they have invested in responding has done some good. It also clarifies the facts for all who read the thread but remain silent. This is what makes ENG-TIPS so educational.

 

[rmw]

And thanks for starting a really interesting thread to read. Sorry I wasn't around earlier in the day to get in on the middle of it.

rmw

 

[Twoballcane]

Wow, just came back from business travel. Sorry for causing this tangent. This is really the case of transient heating. I have even observed it during test. While the electronics are heating up and then you turn off (no more energy) the devices, the temp will still rise a degree or two. Nothing much, all less than a second and then cool down. However, there is no official term (well that I know of) of thermal momentum other than chefs. I think it is energy still dissipating thru the thermal mass which causes the temp to go slightly higher.

Tobalcane
"If you avoid failure, you also avoid success."

 

[Compositepro]

But there is an official name for it - its called time lag. Its the time delay between applying heat and detecting it with your sensor. Control systems have to deal with it all the time. Knowing that it is a time lag and not thermal momentum helps to come-up with appropriate solutions.

 

[XELR8]

Great information and insight but are you talking about temperatures below the heat supply temperature and probably not perfectly insulated from other cooling/heating source like radiation or convection, if possible? I still think the heated object temperature (object at lower temperature) can never exceed your source temperature (object at higher temperature). Thanks!

 

[Compositepro]

Dracula, heat cannot flow from lower to higher temperature. I think you are unclear on what a heat source is. Very few are constant temperature. Heat may come from a flame, electric resistance, chemical reaction. These are not constant temperature. The wire in an electric resistance heater may be 1000F hotter than the "constant temperature" object that is heating a lower temperature object. If you turn off the electricity, then electric power stops instantly, but there is heat stored in the high temperature wire that will still flow for a time. That is time lag effect.

If there is heat flow there must be a temperature difference. You cannot have heat flow at constant temperature.

 

[Twoballcane]

That is correct that you can not get higher than the heat source temp, but I think that is the "momentum" or really transient. The mass is coming to equilibrium. While the hot temp is cooling, the other end is still heating up till both sides meet temperature wise (somewhere between hot temp and room temp) and then come to room temp.

Tobalcane
"If you avoid failure, you also avoid success."

 

[IRstuff]

The only possible way for something to get hotter than its source of heat is if there is another, unaccounted for source of heat.

TTFN

FAQ731-376

 

[XELR8]

Compositepro, sorry, I meant to say higher temperature to lower temperature. I just wrote it wrong. Thanks for correcting me.

To all, even though the temperature is rising for the cooler material and is still less then T high. I am sure there is a conduction within the object at T low absorbing heat within itself (its not at uniform temperature from the surface to the inner core) and therefore may absorb more heat but temperature at the surface will not increase if T high is removed. Besides (thermal expansion and other factors) change that energy into something equal but not increase the heat.

 

[IRstuff]

That's the thermal lag phenomenon that's been discussed in several responses in this thread. Nonetheless, regardless of the degree of lag, the final temperature cannot exceed the temperature of the original source of heat.

TTFN

FAQ731-376

 

[XELR8]

ok, got it!

Thanks!