Thermal transfee in solids

1. Apr 9, 2012

sgstudent

I learnt that solids conduct heat by vibrating and thus transferring the energy from one particle to another particle. However, my teacher gave us a question: two metal rods of different material are placed in a fire. one of them turns red hot at the end while the other remains silver coloured throughout. which one feels hotter she explained that the one they does not glow is able to conduct the best as for normal by lattice vibrations and the movement of electrons. while the other one accumulates the heat so the thermal conductivity of that rod is lesser as compared to the one that remained silver. so the heat will accumulate at the end and it is observed as the red hot part. is this correct?

So what happens when I remove both of them from the fire? Since no more best is provided will the silver one radiate heat to the surrounding area for the totally silver one so it will drop in temperature. then what about the metal rod with the red hot end? Will the heat energy from the red hot end be conducted to the entire metal rod before radiating the heat? how will the heat travel? Will it reach an equilibrium before radiating or will the process occur simultaneously where it is being radiated out and the heat also flows by conduction.

also, when an object is a bad conductor of heat, once it reaches its limit of thermal conduction will the heat accumulate at one place and get heated up. so if a wooden rod can only transfer 10J of energy per unit time once it exceeds that amount will the exceeded amount be retained as heat being accumulated at that point. eg 15J is provided per unit time so every unit time 5J of heat is accumulated until it burns or melt?

Lastly, for thermal radiation, how do I tell if an object is emitting radiant heat or absorbing it? At first I thought it was to determine the object in question to the surrounding. If it is hotter as compared to the surroundings it is emitting and if opposite, then it is absorbing heat. But in a textbook example they seem to work both ways. For the absorbing radiation's explanation this is what they face: two aluminium foil are connected to data loggers. One of them is blackened. The blackened one absorbed radiation at a faster rate as compared to the shiny one. Hence, black surfaces absorb infrared radiation faster than shiny ones.
For the emitting one this is what they gave: a black tin and white tin is filled with boiling water, also data loggers are placed on the top of the two tins. The temperature drop for the black tin is faster than the shiny tin.

Aren't they repeating the same experiment just in different manner? And even worse I can't tell whether they are trying to refer it to absorption or emission of the radiant heat. I'm hoping that you awesome geniuses can help me find a way to determine which one occurs when a question is thrown st me. Thanks for the help guys!

Last edited: Apr 9, 2012
2. Apr 10, 2012

sgstudent

Hi I'm quite curious about how to tell I'd something is emitting or absorbing radiant heat. Could I get some help? Thanks!

3. Apr 10, 2012

Whovian

Well, I'm guessing since radiant heat is just EM radiation, we could use something that can pick up various levels of radiation. Anyways, all objects are both absorbing and emitting radiant heat. What matters is which is occurring more, i.e., if it has any net gain or loss of heat. Please note that thermodynamics isn't my strong suit.

4. Apr 11, 2012

sgstudent

But in the case of a teapot with boiling water most people would say having a black surface makes it a good emitter of radiant heat. But isn't the inner surface of the teapot absorbing heat too? So I'm not sure how to differentiate between which is the net effect... thanks for the help!

5. Apr 11, 2012

Whovian

Well, either check if it's gaining or losing heat, and maybe there are other ways that aren't coming to mind at the moment. :)

6. Apr 11, 2012

Integral

Staff Emeritus
When one end of a thermally conductive rod is placed in a fire, the end in the fire will start to heat up at a rate determined by the difference in temperature of the fire and the cold rod, it will continue to heat up until the rod reaches the temperature of the fire. The other end will begin to heat up as thermal energy is conducted along the rod. If the non heated end is thermally isolated from the surroundings it will heat up until it reaches the same temp as the fire. If it is exposed to the environment it will heat up until the rate of heat loss to the environment equals the heat conducted from the fire through the rod. The main rate of heat loss to the environment is usually convection, but let us suppose that this fire is burning in vacuumn so there is no convection possible. This leaves only radiation as a mechanism of heat loss. A body radiates heat according to the Stefan-Boltzman law. E= σT4 This law applies to every body all the time. So when the exposed end of the rod reaches a temperature at which it is radiating as much energy as it receives, both from the hot end and the environment, its temperature will stabilize.

A very good conductor of heat (say copper) will transfer more energy to the cold end, therefore it will be at a higher temperature then a poor conductor (say stainless steel).

When removed from the fire the hot end will be radiating more heat then the cooler end so will cool down faster, but soon the rod will equalize in temperature, then will radiate away heat until the rod reaches the temperature of the surroundings.

7. Apr 11, 2012

sgstudent

But is it right to say that the glowing piece of metal feels less hot at the end as it accumulates the heat as compared to the metal rod that remains silver since it is able to conduct away all the heat easily with accumulating them? Then also when both rods are taken away which one would remain hotter for a longer period of time? My guess was the one that is red hot since the heat is accumulated at one point and conduction would occur. but then again since its red hot besides the conduction occurring emission of radiant heat would be more since it is red hot. so for the silver one radiation is minimized hence more heat would retain? I'm quite confused about this part over here.. thanks for the help integral!

8. Apr 13, 2012

sgstudent

Um so when I remove both rods which one would cool down first? I'm guessing since the copper will have an even temperature while the steel has a less even temperature (hot end very hot other end relatively cooler) so the net radiation lost is the same so what determines the rate of cooling? Thanks for the help!