Need help with thermal convection?

Question

Ok I have a couple of problems I am having a hard time understanding and my text book isn't a lot of help.

Why do we put rugs down on stone floors during the winter?
Is it because stone has higher thermal conductivity than the rug?

What about if I wanted to cool a cup of coffee?
should I stir it, put it in a short, wide cup or put it in a tall narrow cup?

Same cup of coffee - would it be cooler in a white cup, black cup, black and white striped cup or pouring it between a white cup and a black cup?

Answer 1

You are correct that stone conducts better than the rug. Heat travels from hot to cold. In the case of the stone floor, your feet would conduct heat out to warm the floor. The rug does not conduct as well, and is acting as an insulator of heat.

For the coffeee, putting a cold spoon in it would cool it. Stirring it would cool it because tit would bring new areas to the top. A wide cup would cool faster because there is more area to have coffe evaporate, a cooling process, and to convect from.

Good radiators of heat are good conductors of heat. The white would reflect heat back. The black would radiate heat out better. Pouring it back and forth would cool it because of exposing the coffee to the cooling air, and more area for evaporation.

In several cases here your heat transfer was radiation or conduction, only convection from the top of the cup.

Answer 2

We put rugs on the stone floor in winter because the stone floor will be chilling to our feet. Rugs are good insulators and so prevent the chill to come through. Between rug and stone, yes, stone has a higher thermal conductivity and rug less, which is what is needed.

Stirring is a good option since the cooler layers go down and the hotter layers come up to lose heat. A short wide cup is also a good choice because it radiates heat better compared to the tall narrow cup.

A black cup because it radiates heat better and cools coffee faster. Pouring it from a white cup to a black cup is also a good choice because the coffee's surface area is increased during the pouring and results in better radiation of heat.

The problems are more related to radiation than convection.

Answer 3

A thermal oven relies upon on the organic convection from direct warmth. forced convection makes use of a fan to pass the nice and comfortable air around the oven..giving a extra even distribution of warmth..