Shoudln't a fan heat instead of cool?

Question

it is taught that temperature is the kinetic energy of molecules. so something that is hot is simply vibrating/moving/has more kinetic energy than something that is cold.

this makes sense, until i thought about my boxfan in my room. as it is speeding up the molecules of air and giving them kinetic energy, shouldn't the air become hotter rather than cooler?

Answer 1

In a way, you're probably right - the air is being accelerated and is becoming hotter, but only infinitesimally so.  

The fan isn't cooling the air but the air is (hopefully) cooler than your body temperature and cools you down through 'forced convection'.  The moving cool air steals some of your body heat and takes it away.

Answer 2

You are right.  A fan will add to the thermal energy in the room.  

Fans are used to cool a room in 2 ways.  First, the 'wind chill' effect of air blowing over human skin, as noted above.  Second, which I didn't see above, is mixing of the air.  Cooler air settles, and a fan can mix it up to the level at which we are (generally sitting on a chair or standing...not so often do we lay on the floor).

Answer 3

Great question.

The fan on your desk has heat going into it from the plug. And it is dissipating all of that heat back into the room. Therefore the room is getting hotter. You feel cooler because your skin temperature is 93 and the room is 85. With no fan the the rate of (free convectection) heat transfer is low and you feel hot. When the fan is on you, the rate of (forced convection)heat  transfer is higher. Heat is being driven off your hot skin faster than if it were in still air. hence you feel cooler.

If you had the fan on for days and days or more the room temperature would go up and up possibly getting hotter than skin temperature. In that Case having the fan on would heat your skin up faster and turning the fan off would cool you off.

Answer 4

more heat is going to come from them motor making air move faster wont really heat it heat is more like a vibration on a microscopic scale not wind blowing around your room.

Answer 5

You need to consider heat more like a vibration.

In the case of your fan, it does not make the air vibrate, but create a continuous movement of air. Any vibration it would do on the air is dissipated such a way you can't feel it. The cooling feeling you get from it is because as air goes over your skin it gets a bit of your own heat through termal contact transfer (ambient air is ususally colder than your own temperature). It even gets more cool if you're sweating because it accelerates the swet drying, and water evaporation consumes calories, which helps cool you off.

There are two ways to heat something with a Fan :

- the obvious one : the air is hotter than the object you want to heat. For sure it will heat it, for the same reason explained above.

- the stream of air is so fast that the friction of the molecules on the surface of the object creates heat. That happens on fast supersonic planes, and also on space crafts entering the atmosphere, even if the air is actually very cold at these altitudes.

Answer 6

The fan is indeed heating up the air in the room by increasing the kinetic energy. This is why, in a closed system, any electrical device actually INCREASES the temperature of the room overall.

But the reason you FEEL cooler (or whatever the fan is aimed at) is because the moving air particles across the surface of the object increase heat transfer by forced convection. This is because your body temperature (or the skin temperature of whatever the fan is aimed at) is higher than the air temperature. It's only at very high air speeds that friction takes over and heats an object.