Really need clarification for this.

Question

in a thermal pot, there is a layer of vacuum in the outer wall, an inner pot lid and an outer pot lid. 

 

Please help me to clarify these questions:

 

 

1. Does hot food radiate heat? Or does radiation only come from a heat source?

 

2. does radiation occur horizontally? If it occurs vertically then it's convection?

 

3. I thought radiation means heat escaping from the food in a thermal pot so why isn't the function of the inner pot lid reducing convection AND RADIATION? 

 

Please explain every single question clearly, thanks so much! I really need help asap!

 

 

Answer 1

Hi Amy!

 

My thoughts on your questions:

 

1.  Following Wien's law, any body with a non-zero temperature (that is to say, any body) will radiate.  All you need is to have some internal linear motion in your atoms and molecules, and radiation will occur.  In fact, you are radiating right now -- though the electromagnetic radiation you emit is mainly in the infrared, so you cannot see it directly.  If our eyes were sensitive in the near infrared, we would all look like bright light bulbs to each other.

 

2.  Radiation occurs, in many cases, equally in all directions (in three dimensions) around an object (the word that describes this is isotropic).  Think of a bare light bulb.  If you look at it from any direction, you will see light, because it is radiating in all directions, and that light will likely be just about as bright no matter which direction you look from.

 

Convection is a different process altogether, in which thermal energy is transferred not by electromagnetic waves, but rather by the motion of a medium like air or water.  When one part of the medium heats up, it moves to another part of the medium and takes the absorbed thermal energy with it.  In a pot of water, for example, convection is seen in a "rolling boil," where water heated on the bottom rises to the top (and then transfers thermal energy to the air above).

 

So radiation is thermal energy transfer by electromagnetic waves, and convection is thermal energy transfer by the motion of a physical material.  Two completely different thermal energy transfer mechanisms.

 

3.  The thermal pot lid would stop the air inside the pot from moving farther upward, thus stopping convection.  It also can reflect electromagnetic waves, which will prevent some radiation of thermal energy from the pot, as well (this is why the insides of some coolers and cooking devices are coated with highly reflective materials).  However, the lid will absorb some of the radiated thermal energy from inside, and then re-radiate it to the outside.  So, if the lid's seal is tight, it can likely stop all convection to the outside, because it provides a pretty absolute physical barrier to the movement of air from inside to outside.  But some of the radiation will  effectively get through by beign absorbed by the lid and re-radiated, so it is rather less effective at stopping radiation.

 

I hope this helps some!  If you have further questions, or want to talk about any of this more, do not hesitate to ask.

 

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