'Keep It Cool Box' Problem.

[esmeralda4]

Hello again.

I come with another physics concept I am struggling with. At first I thought I understood this but the more I think about it the more confused I get. Help!

This post is with regard to the insulated 'Keep it Cool' boxes you can buy for taking on picnics or carrying cans of beer.

I had always assumed that once you had put everything you want into the box you should then pack the spaces with something insulating to stop heat lose and movement by convection. However, a friend recently said to me that he puts metal things into his to keep the food cool because metal is a good conductor of heat and will therefore conduct the heat away from the food. This, to me, sounds physically sound. - shove a lump of cold metal in with the picnic and if any heat does 'get into' the box it will go to the metal instead of your food.

Which theory is right and why??

Thanks.

 

[turbo]

Hello again.

I come with another physics concept I am struggling with. At first I thought I understood this but the more I think about it the more confused I get. Help!

This post is with regard to the insulated 'Keep it Cool' boxes you can buy for taking on picnics or carrying cans of beer.

I had always assumed that once you had put everything you want into the box you should then pack the spaces with something insulating to stop heat lose and movement by convection. However, a friend recently said to me that he puts metal things into his to keep the food cool because metal is a good conductor of heat and will therefore conduct the heat away from the food. This, to me, sounds physically sound. - shove a lump of cold metal in with the picnic and if any heat does 'get into' the box it will go to the metal instead of your food.

Which theory is right and why??

Thanks.

If the metal is at a temperature higher than that of the food, it will warm the food, not cool it. The idea that thermally conductive materials automatically will cool things in contact with them is wrong. Now, if he had put a heavy piece of copper or some other metal in a freezer and chilled it for hours and then put THAT in the cooler, it would help keep the food cool. Of course you could do the same thing with a zip-lock bag full of ice cubes, which would be much cheaper, lighter and more efficient than a cold piece of metal. Ice has a great advantage in that it requires a lot of heat to make the phase-change from solid to liquid.

 

[russ_watters]

To add - for a cooler, the primary function is to keep heat from flowing through the side of the box. Adding a piece of metal will not change the heat flow rate. It can only help if, like Turbo-1 said, it is cold and acts like a heat sink, absorbing the heat that is flowing in so it doesn't go into the food.

 

[uby]

I think there is a bit of a misconception here. Thermal conductivity relates the RATE at which heat is transferred to a material. What you really want here is a material which has a high heat capacity. That is, it would take a large amount of absorbed heat energy to raise the temperature of the material. This would permit it to act as a thermal sink, assuming that the thermal conductivity isn't SO low that the material cannot absorb heat as fast as it enters the container. (if you've packed the food inside a thermal insulator in the first place, this shouldn't be an issue)

It so happens that many materials with high heat capacities also have high thermal conductivities, especially metals. This is not always the case though. You can read up on thermal diffusivity (which is a ratio of a materials thermal conductivity to its heat capacity) to see what materials would get the job done.