Toasting-what form of heating?

[pivoxa15]

Toasting in a toaster is usually considered cooking by (infra red) radiation. But the hot coils touch the toast so an element of heating by conduction occurs as well. Are they both equally as valid or is one more valid than the other. If the latter than which one and by how much?

On the other hand, if you cook a bread on top of a flame, would it be considered convection heating, radiation heating or conduction heating. I know that if you heat a saucepan over a flame it is considered heating by conduction. So if you replace the saucepan by a piece of bread does it change?

 

[Integral]

For the heating elements of a toaster to reach optimal temperature they CANNOT touch the toast. If they touch the toast conduction will carry way the heat to fast for the element to get hot. If the toast is touching an element you should be able to observe that that portion of the element is not red which indicates that the temperature is low.

 

[russ_watters]

Plus, the heating element is hot enough to quickly burn the toast - if the toast were touching the elements, it would have lines on it as if it were sitting on a grill.

 

[pivoxa15]

For the heating elements of a toaster to reach optimal temperature they CANNOT touch the toast. If they touch the toast conduction will carry way the heat to fast for the element to get hot. If the toast is touching an element you should be able to observe that that portion of the element is not red which indicates that the temperature is low.

Plus, the heating element is hot enough to quickly burn the toast - if the toast were touching the elements, it would have lines on it as if it were sitting on a grill.

fair points. So toasting in a toaster is only heat by radiation. What about my second senario? Which form(s) of heating is that?

 

[pivoxa15]

No one have answered my second question yet. I think it is all 3. Conduction is correct because like a saucepan, the flame is touching the toast and heating it one layer at a time. Radiation because the flame is emitting electromagnetic waves in the form of infrared as the temperture is quite high. Although this form may not provide a lot of heat to the toast. Convection occurs because warm particles rises and heats the toast although this form is not great either.

 

[Hootenanny]

Conduction only applies to solids (http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatra.html#c2), since the flame is essentially a liquid, heat would not be transferred by conduction.

 

[wxrocks]

In a sense -- all 3 forms of heating are involved in cooking no matter what. It just so happens 1 form is dominant (i.e. in a toaster). Sure the coils heat the air near them too, so the air gets some movement to it and thus some converction is taking place. The toast is also touching some type of metal grate that gets hot, and thus that contact is conduction. Overall though, it is mostly radiation.

Whenever you heat/cook something by a non-contact method in an atmosphere, there is always a certain amount of radiation and convection taking place.

 

[pivoxa15]

Conduction only applies to solids (http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatra.html#c2), since the flame is essentially a liquid, heat would not be transferred by conduction.

So you are only counting the flame as the heating object. The toast is heated by the flame so only the flame counts. And any heat transfer goes on in the flame. However, if I was to use the toast to heat something else (separated from the flame) than that something else is heated via conduction only.

This would be different if the pot was heated on a flame which is heating the water in the pot. The form of heat heating the water in that case would be conduction since it is only the pot that is heating it. The flame is not heating the water but heating only the pot via radiation and convection. If I was to put some vegetables in the pot (not touching it) than I could say the vegetables are heated through convection by the water only (the flame and hot pot would have nothing directly to do with the cooking of the vegetables).

In that way things can be studied separately.

 

[pivoxa15]

I was talking about energy used to heat things. It's good to classify the different forms of heating that can occur. Heat is an energy hence different forms of energy. It is kind of artificial as in the molecular level the difference is small but at the macroscopic level, the differences emerge.

 

[LeonhardEuler]

Don't forget about convection in the case of the toaster. This occurs because the heating elements heat the air, which then heat the bread.

In you second scenario, it is also primarily convection heating, but there is radiant heat transfer as well.

To Hootenanny: Conduction is generally most important in solids, but it also occurs in liquids and gases. If you had a stagnant liquid with a negligible thermal coefficient of expansion (e.g. water at 4°C) so that natural convection was very slow or essentially absent, then conduction would be the dominant mechanism of heat transfer in this case.

edit: Also, what do you mean by "the flame is essentialy a liquid"?

 

[pivoxa15]

Don't forget about convection in the case of the toaster. This occurs because the heating elements heat the air, which then heat the bread.

They should do an experiment in a vacuum and see whether toasting the bread in a vacuum environment is slower and by how much? When you put your and near a toaster, there is a deep warmth feeling. I wonder if that feeling is mainly due to the convection of the air or the radiant energy from the coils?

 

[Hootenanny]

To Hootenanny: Conduction is generally most important in solids, but it also occurs in liquids and gases. If you had a stagnant liquid with a negligible thermal coefficient of expansion (e.g. water at 4°C) so that natural convection was very slow or essentially absent, then conduction would be the dominant mechanism of heat transfer in this case.

True, although I was commenting on the specific case.

edit: Also, what do you mean by "the flame is essentialy a liquid"?

Sorry, the flame is 'fluid', careless error on my part.

 

[pivoxa15]

Sorry, the flame is 'fluid', careless error on my part.

Flame is condensed gas isn't it?

When one thinks about fluid, one thinks about liquid but

A fluid is defined as a substance that continually deforms (flows) under an applied shear stress regardless of the magnitude of the applied stress. It is a subset of the phases of matter and includes liquids, gases, plasmas and, to some extent, plastic solids.

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

A flame does deform to some extent uder an applied shear stress but not a lot.

 

[russ_watters]

Flame is condensed gas isn't it?

A flame is a plasma - ionized particles (sorta like a gas, but hotter).

A flame does deform to some extent uder an applied shear stress but not a lot.

Do you know what a shear stress is? Try blowing on a flame and see how much it deforms...

 

[pivoxa15]

Do you know what a shear stress is? Try blowing on a flame and see how much it deforms...

A shear stree is a sideways stress. I was thinking of using a solid to hit the flame on the side. I think it wouldn't deform much because the solid passes right through the flame but blowing it would cause a larger deformation.