heat transfer between 2 mediums
Hi guys, I have a question regarding heat transfer. When I have a piece of heated metal which is left to cool down in air, it goes something like this right? Heat is transferred to air by conduction (metal to air) then from air heat is given off by convection currents. Also, radiation occurs. But I was wondering if the heat transfer from metal to air (conduction process) will change if another gas is used. I'm guessing it will have another effect because if water is used instead more heat will be transferred. So does it mean that the denser the covering medium, the faster the heat can escape? So for heat exchanges where hot water is pumped through a metal pipe, if 2 different metals are used will the better conductor metal take away more heat at the transferal part (water to metal). I don't really understand this though.
Also, if I have two pieces of metal rods which is heated in a fire and one becomes red hot and the other remains silver it means that the silver one is hotter at the other end (not in the fire) as the heat is able to transfer to the rest of the parts while the red hot one accumulates the heat at one place so the end will be not as hot as compared to the silver one. But if I were to take both of them out to cool in the same environment, then which one will cool faster? I'm guessing it should be the same as radiation will be the same as the silver one is hot throughout whole the red hot one is extremely hot at one part and cool at the other, so net radiation is equal. So it boils down to conduction through the 2 mediums, so I'm guessing it should be the same too?
Thanks for the help!