×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Temperature and Heat

Temperature and Heat

Temperature and Heat

(OP)
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!

RE: Temperature and Heat

not unless your 150F heat source gets warmer than 150F. ...which could happen under several circumstances.

RE: Temperature and Heat

This is a tricky question -- it needs to be defined in a better way ...

For example, take a thermometer.  If it is exposed  to the sun, its temperature will keep going up ... If it is kept in a shade, it will show a fixed temperature value after a certain period of time ...

I agree with DVD's answer, however, I would ask for some additional clarification ...

http://www.engineering-4e.com

RE: Temperature and Heat

Zeroth law, the one they forgot when naming the first law.

- Steve

RE: Temperature and Heat

Actually, Newton's law of cooling addresses that reasonably well: Q = k(T1-T2)

A positive temperature difference is required for a positive temperature change.  If you want something to get hotter, it needs to be exposed to something that is hotter.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: Temperature and Heat

(OP)
Hi Folks,

Thanks for your help and quick response.

No tricks, you answered my question. I thought the answer would be no, you can never exceed the temperature of the source but, I could not document any physical or scientific equation with the theory.

Feel free to add any further comments.

RE: Temperature and Heat

Wow, if we can find a way to heat something up and it actually got hotter than the heat source, we can solve our energy crises.  

There is a something called thermal momentum, where you heat up mass and then when you suddenly turn off the heat source, the temp of the mass will still keep going a few degrees before peaking and then come down.  
 

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

RE: Temperature and Heat

Thermal momentum will still need a driving force. You cannot observe this phenomenon starting from steady state. In other words, the temperature of the applied source would have to be higher to cause this temperature peak.

RE: Temperature and Heat

"There is a something called thermal momentum"

Inadequate measurement perhaps.  Temperature has no directionality.

- Steve

RE: Temperature and Heat

No it is different heat transfer coefficients of different materials.  Take the example of a pot on the stove:  The pot absorbs heat from the burner and passes it to the contents,  The outside of the pot will be measurably hotter than the inside.  After you turn the burner off, the temperature gradient across the pot bottom will equilibrate (raising the temperature of the contents).  This happens even if the process was in apparent thermal equilibrium (e.g., a pot of water at its boiling point does not change temperature until the liquid water is gone or the heat input is stopped).  That is "thermal momentum".

While temperature has no directionality, heat transfer always goes from hotter to cooler.

David

RE: Temperature and Heat

I've got to call this one! There is no such thing as thermal momentum. That is a flawed analogy to describe an observed phenomenon that is caused by heat capacity and thermal conductivity. It might be used as a shorthand term for the effect but shouldn't be used to to explain anything because it isn't real.

RE: Temperature and Heat


On an entirely different situation, one may encounter thermal runaway exothermic chemical reactions...

Thermal runaway can occur because, as the temperature increases, the rate at which heat is absorbed and removed increases, say, linearly but the rate at which heat is produced increases exponentially.

When control of the reaction is lost, temperature can rise rapidly leaving little time for correction. The elevated temperatures may initiate secondary, more hazardous runaways or decompositions.
 

RE: Temperature and Heat

Now THAT takes me back to my childhood days of weed-killer and sugar fires.

- Steve

RE: Temperature and Heat

David,

I think you are describing time constants, not momentum.  A failure of English language to describe engineering processes.  Like those TV journos who can "feel the power" of a car.
 

- Steve

RE: Temperature and Heat

You should see some of the arguments between me and the mrs. The hot object gets hotter without any heat source.  

RE: Temperature and Heat

Thermal momentum...?

I liken this effect to the analogous situation of energy stored in a capacitor - or a flywheel, for that matter.  Maybe the flywheel is a better example; in my mind, while you continue to add energy, the flywheel continues to accelerate, and the instantaneous cessation of energy input will result in the flywheel rotational speed to nonetheless rise to some maximum because of this inertia-driven acceleration (followed by deceleration).  That does not mean that energy continues to be added at this stage; all of the energy has already been provided by the source prior to cut-out.  What is observed afterwards is not a "momentum" effect so much as a "time lag" effect.

Regards,

SNORGY.

RE: Temperature and Heat

A flywheel will accelerate as long as energy is added to it by applying an external force. The instant that force is removed no more energy is added to the flywheel and acceleration instantly stops. Momentum is what resists acceleration.

Perhaps you are thinking of the wind-up that can occur in a drive shaft, which is energy storage in elastic deformation.

RE: Temperature and Heat

(OP)
I really enjoy the answers and related views. So let me ask this, why can rubbing two sticks together at a lower temperature create a flame at a higher temperature. Please, note, I understand combustion but it does flaw some earlier answers. Thanks!

RE: Temperature and Heat

The sticks are not at the lower temperature at the point where they rub.  Friction and the corresponding input of energy makes it locally hotter adjacent to the area where the sticks rub.  Wood is a decent insulator so the sticks don't become uniformly warmer.

Read up on the 2nd law of thermodynamics.  

RE: Temperature and Heat

If you suddenly reverse a pressure gradient, gas will flow against it for a period.  The same isn't true with temperature and heat flow.

- Steve

RE: Temperature and Heat

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: Eng-Tips.com Forum Policies

RE: Temperature and Heat

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.

RE: Temperature and Heat

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

RE: Temperature and Heat

(OP)
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!

RE: Temperature and Heat

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: Eng-Tips.com Forum Policies

RE: Temperature and Heat

(OP)
Hi IRstuff,

Information noted and confirmed.

Thanks.

RE: Temperature and Heat

SomptingGuy:

My apologies for being wrong.  (It happens.)

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

Regards,

SNORGY.

RE: Temperature and Heat

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.

RE: Temperature and Heat

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.  

RE: Temperature and Heat

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

RE: Temperature and Heat

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."  

RE: Temperature and Heat

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.

RE: Temperature and Heat

(OP)
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!

RE: Temperature and Heat

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.

RE: Temperature and Heat

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."  

RE: Temperature and Heat

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: Eng-Tips.com Forum Policies

RE: Temperature and Heat

(OP)
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.  

RE: Temperature and Heat

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: Eng-Tips.com Forum Policies

RE: Temperature and Heat

(OP)
ok, got it!

Thanks!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources