Understanding Heat: Radiative, Convection & Conduction

[Wannabeagenius]

Hi All,

I'm trying to understand heat so I'm going to say what I understand and ask you to please tell me if I am correct. Thank you.

Radiative Heat - This is heat transferred in the form of electromagnetic radiation and does not need a medium. If in a vacuum, the radiation will travel through the vacuum unimpeded. If a medium, such as air, is present, the radiation may or may not interact with the medium. If it interacts, it will transfer some or all of this heat to the medium as it continues on its way.

Convection - Heat transferred by the movement of a medium. For example, if a fireplace is being used, the air gets heated up and a gust of wind could transfer this hot air from one room to another.

Conduction - There is a medium but it does not move. The heat from molecules is transferred from molecule to molecule across the medium. I think of a bucket brigade here.

With regard to climate we talk about Radiative heat, latent heat and sensible heat.

Radiative heat - Same as above

Latent heat - Heat that is absorbed or released due to a change of state. For example, ice to water and water to vapor.

Sensible heat - This is where I have a problem. Heat that is absorbed or released without a change of state. It involves a temperature increase or decrease. Now it seems that this is any type of heat transfer as long as there is not a change of state which means Radiative, conduction or convection but I'm sure this is incorrect. Climate textbooks talk about Radiative heat, latent heat and sensible heat and radiative heat is definitely different from sensible heat.

Thank you,
Bob

 

[Andy Resnick]

Hi All,

I'm trying to understand heat so I'm going to say what I understand and ask you to please tell me if I am correct. Thank you.

Nearly everything you wrote was correct. "sensible heat" is a very archaic term; we currently use 'specific heat' or 'heat capacity'.

Additionally, radiation/conduction/convection are three methods of heat *transfer*. That is, they refer to the physical mechanism by which heat flows from one object to another.

When heat Q flows into an object (however it is transferred), the following relationship holds:

Q=$$\int [\Lambda_{V} (V, T) dV + C_{V} (V,T) dT]$$

Where the integral is over the path of the process. $\Lambda_{V} (V, T)$ is the latent heat at constant volume, $C_{V} (V,T)$ the specific heat at constant volume.

Usually this is presented in a simplified form, for example a constant-volume process, and the specific heat is constant: Q = mc_V$\Delta T$.

http://en.wikipedia.org/wiki/Heat#Specific_heat

 

[Wannabeagenius]

Nearly everything you wrote was correct. "sensible heat" is a very archaic term; we currently use 'specific heat' or 'heat capacity'.

Additionally, radiation/conduction/convection are three methods of heat *transfer*. That is, they refer to the physical mechanism by which heat flows from one object to another.

When heat Q flows into an object (however it is transferred), the following relationship holds:

Q=$$\int [\Lambda_{V} (V, T) dV + C_{V} (V,T) dT]$$

Where the integral is over the path of the process. $\Lambda_{V} (V, T)$ is the latent heat at constant volume, $C_{V} (V,T)$ the specific heat at constant volume.

Usually this is presented in a simplified form, where for example the latent heat is zero, and the specific heat is constant: Q = mc_V$\Delta T$.

http://en.wikipedia.org/wiki/Heat#Specific_heat

Thanks for the response.

I have been studying climate change and the term "sensible" heat is still used by climatologists. I know what specific heat means and it's not that. Let me give you the full context.

Visible sunlight enters the atmosphere and arrives on the ground and heats it up. The heat leaves the ground by three methods; Radiation, Latent Heat, and Sensible Heat.

Radiation is Infrared. No problem for me.

Latent heat. A parcel of air gets heated up, expands and rises up into the atmosphere. The temperature drops as the parcel gets higher and the water vapor in the parcel of air condenses releasing latent heat into the atmosphere.

And then there is sensible heat! It seems to me that the only methods left are conduction and convection so that's what I'm seeing as sensible heat. And conduction and convection as I have defined these terms.

I fully understand Radiative heat and latent heat so for sensible heat I used the process of elimination.

Do you see sensible heat as I do from a climatological viewpoint?

Thanks,
Bob

 

[Andy Resnick]

Do you see sensible heat as I do from a climatological viewpoint?

No, I don't understand.

What you refer to as 'latent heat', I refer to as 'convective flow'.

 

[russ_watters]

Nearly everything you wrote was correct. "sensible heat" is a very archaic term; we currently use 'specific heat' or 'heat capacity'.

I'm not so sure that's correct. Both terms are used in engineering and they are not interchangeable. Specific heat, as the wiki says, is the amount of heat required to change the temperature by one degree. Sensible heat is the total amount of heat added to change the temperature: ie, sensible heat is specific heat times delta-t times mass.

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

What you refer to as 'latent heat', I refer to as 'convective flow'.

Huh? I've never heard the term "convective flow", but it doesn't imply to me anything close to the definition of latent heat. Latent heat, as the OP said, is heat added to change state. In the atmosphere (or HVAC), that's the energy associated with boiling water or condensing water vapor.

That paragraph from the OP talked about thermals rising, but the heat s/he was talking about was specifically the heat from condensing water, not the heat transfer from convection:

Latent heat. A parcel of air gets heated up, expands and rises up into the atmosphere. The temperature drops as the parcel gets higher and the water vapor in the parcel of air condenses releasing latent heat into the atmosphere.

A parcel of air gets heated by the ground is conduction and convection.

The air rises - that's convection.

The air cools as it rises - that's not a transfer of heat, it's adiabatic expansion.

The water vapor condenses - latent heat is converted to (released into) sensible heat.

 

[Andy Resnick]

I'm not so sure that's correct. Both terms are used in engineering and they are not interchangeable. Specific heat, as the wiki says, is the amount of heat required to change the temperature by one degree. Sensible heat is the total amount of heat added to change the temperature: ie, sensible heat is specific heat times delta-t times mass.

http://en.wikipedia.org/wiki/Sensible_heat Huh? I've never heard the term "convective flow", but it doesn't imply to me anything close to the definition of latent heat. Latent heat, as the OP said, is heat added to change state. In the atmosphere (or HVAC), that's the energy associated with boiling water or condensing water vapor.

That paragraph from the OP talked about thermals rising, but the heat s/he was talking about was specifically the heat from condensing water, not the heat transfer from convection: A parcel of air gets heated by the ground is conduction and convection.

The air rises - that's convection.

The air cools as it rises - that's not a transfer of heat, it's adiabatic expansion.

The water vapor condenses - latent heat is converted to (released into) sensible heat.

"archaic" may have been a poor choice of words, but I hope you can see that starting from the integral relation I posted, there are two simple limiting cases- an isochoric and an isothermal. The isochoric process, you refer to as 'sensible heating'. It only involves the specific heat of the material.

I used convective flow as shorthand for buoyancy-driven flow with heat convection. Hot air rising is buoyancy-driven flow; heat is thus convected along by the flow. A hair dryer uses forced flow to convect heat (forced convection).

http://www.city.ac.uk/sems/mathematics/research/fluids/thermalconv.html

While the condensation of water vapor definitely involves latent heat, none of this addresses the OP's confusion between heat transfer mechanisms (radiation, convection, conduction) and the effects of heat on matter (latent, sensible).

 

[russ_watters]

"archaic" may have been a poor choice of words...

Well, archaic means "not used", which is what I think you meant...

...but I hope you can see that starting from the integral relation I posted, there are two simple limiting cases- an isochoric and an isothermal. The isochoric process, you refer to as 'sensible heating'. It only involves the specific heat of the material.

I understand, but your usage of the term "specific heat" in that sentence is not the same as your usage in the previous post. I just want to make sure we're clear that "sensible heat" and "specific heat" are not the same thing so "specific heat" can't be used as a replacement for "sensible heat". Heck, in the equation:

Q=m Cp dT

Q=sensible heat
Cp=specific heat

Specific heat...or, specific heat capacity, is a material property, not a type of heat.

While the condensation of water vapor definitely involves latent heat, none of this addresses the OP's confusion between heat transfer mechanisms (radiation, convection, conduction) and the effects of heat on matter (latent, sensible).

Well ok, but this stuff about specific heat doesn't have anything to do with the OP's question - it's a side issue that probably shouldn't have been brought-in.

The OP's confustion:

Sensible heat - This is where I have a problem. Heat that is absorbed or released without a change of state. It involves a temperature increase or decrease. Now it seems that this is any type of heat transfer as long as there is not a change of state which means Radiative, conduction or convection but I'm sure this is incorrect. Climate textbooks talk about Radiative heat, latent heat and sensible heat and radiative heat is definitely different from sensible heat.

...is just in thinking that "sensible heat" is a heat transfer process like convection and radiation. But it isn't: Sensible heat is a type of stored/released heat. You can say "sensible heating", but sensible heating can happen via any of the 3 heat transfer processes.

 

[Andy Resnick]

Well ok, but this stuff about specific heat doesn't have anything to do with the OP's question - it's a side issue that probably shouldn't have been brought-in.

If you say so.

Sensible heat - This is where I have a problem. [...] It involves a temperature increase or decrease.

 

[russ_watters]

If you say so.

What was the point of that quote? You didn't explain anything. Are you intending to imply that that you are still saying that sensible heat = specific heat?

The OP didn't use the term "specific heat" - you brought it up and it seems to me that you brought it up because you misunderstand the definition of "sensible heat".

I'm thinking maybe it is because you think "latent heat" is short for "latent heat capacity" and "sensible heat" is short for "sensible heat capacity" the way "specific heat" is short for "specific heat capacity". But the first two are not. Latent heat and latent heat capacity are not the same thing. Sensisble heat and sensible heat capacity are not the same thing. Specific heat and specific heat capacity are the same thing - and they are also the same as sensible heat capacity:

Q = latent heat, sensible heat
Cp = sensible heat capacity, specific heat, specific heat capacity

so

specific heat =/ sensible heat

Do you have any references for this? The references you have provided do not agree with what you have said.

 

[Andy Resnick]

Are you intending to imply that that you are still saying that sensible heat = specific heat?

Please stop imputing my motives.

The OP clearly attributed 'sensible heat' (which I clearly stated was a term I was unfamiliar with in post #4) with an increase in material temperature. And I clearly stated that the material effect of applying heat to a change in temperature is due to specific heat, as opposed to latent heat.

I don't understand why you are challenging me, instead of helping the OP.