# Thermodynamics : Heat transfer : Conduction or convection?

I have an hot object, that is place in a room of 20°C. My goal is to be able to estimate how warm the object will be in T seconds. To be able to do that, I must first understand what is really happening; how the object is cooling down.

I am new to thermodynamics and I don't really understand with which Method the hot object will cool down. Is it conduction or convection?

Wiki: Conduction is the transfer of heat between substances that are in direct contact with each other. So in my case it should be the object and the ambiant air.

Wiki: Convection is the transfer of heat from one place to another by the movement of fluids. So in my case it should be the movement of the air

Both Method could happen in my case, that's why I am not sure which method is occuring here.

Is it possible for an object to cool down with 2 Methods simultaneously (radiation and convection for example)?

• All methods can occur simultaneously. Temperature change depends on so many things (material type, air motion, humidity, etc.) it is very difficult to calculate it from first principles. More common are empirical tables or limited-use models. Apr 29, 2016 at 9:50

## 2 Answers

Three processes are involved: Conduction: Heat flows from the object to its environment. Removal rate of heat from the interface further away from the object is proportional to the coefficient of conductivity (0.024 for air, 205 for aluminum -see http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html). Convection: The interface between the object and air is the same, but removal of heat from the interface is by replacing the interface volume, due to the flow of air (e.g: due to wind). Convection is more effective than conduction within the air - hence the familiar habit of blowing on hot food to cool it. Radiation does not depend on the immediate environment. There is, however, a balance between incoming radiation (from the sun, earth, space etc.) and outgoing radiation (from the object). With very hot objects - radiation heat transfer would be dominant.

This is in general a very complicated problem. There is three processes by which your object will cool down : conduction and convection which you already mentionned, as well as emission of blackbody radiation.

You can treat blackbody radiation with the help of Stefan's law that gives you the power emission with respect to temperature.

Conduction and convection are indeed happening simultaneously, you might want to check Newton's law of convection that gives you general principles of their interplay.

You will quickly probably realize that one of these mechanisms dominates the other, which will simplify your calculation. Good luck !