Is it Hot or Cold in Here? How Energy Transfer Affects How We Feel

When you are outside in the summertime, and you go into your house, you probably feel cooler once you are inside.  However, when you are outside in winter you probably feel warmer once you are inside the house (provided you live in a place where it is cold in winter!).  Yet the house is probably always about the same temperature.  Why does it feel warm in winter and cool in summer?

This difference in the way you feel in the house, despite that it is always the same temperature, comes from differences in thermal energy between you and the house.  Thermal energy is that portion of the energy in any body or object that is responsible for its temperature, according to Robert F. Speyer in his Thermal Analysis of Materials.  (In general, energy is just a property of any object or system that can be transferred to another object or system through some interaction.) Therefore, if one object has a higher temperature than another object, this object has more thermal energy. 

Like all forms of energy, thermal energy likes to transfer from an area of higher energy to an area of lower energy, until both areas are equal.  This means that if our boxes above touch, the one with more thermal energy will transfer some of that energy to the other box, until they both have the same amount of energy—that is, until they are at the same temperature.

The same sort of transfer happens between you and the air inside your house.  In winter, because your house has a heating system that is set by the thermostat, the air inside your house is kept warm, say 70°F.  Outside the air is much colder; say around 20°F depending on where you are.  If you are outside, and you enter your house, thermal energy is transferred to you from the air in the house, since you have less thermal energy than the air does. 

In the summer the opposite happens:  you come indoors, and you have more thermal energy on your skin than the air in the house does.  This means some of the thermal energy on your skin will transfer to the air in the house, until your skin and the air in the house are the same temperature. 

This transfer of thermal energy from some other object to you or from you to the other object is responsible for the feeling of warm or cold.  When you touch something that is warmer than you—that is, it has more thermal energy and is therefore at a higher temperature—the flow of energy from the object to you gives you the feeling of becoming warmer.  If you touch an object that is cooler than you, some of the thermal energy in your skin is flowing out and into the other object.  This is why your house at the same temperature can feel warm or cool depending on the time of the year—it all depends on who or what is transferring  thermal energy!

TRY IT!

Here’s what you’ll need:

1.       A large bowl of very warm water, but not hot enough to burn you, about 90°F (like a hot bath)

2.       A large bowl of ice water

3.       A large bowl of water at room temperature

4.       A watch with a sweep second hand, or a stopwatch

Here’s what you need to do:

1.       Place all the bowls of water on a table or counter, ice water and hot water on either side of you, and the room temperature water in front of you.

2.       Place one hand in the hot water, and your other hand in the ice water.

3.       Leave your hands this way for at least 30 seconds, or 60 if you can.

4.       After 30 seconds, pull both hands out of their respective bowls, and place them both in the room temperature water.

What are your hands feeling?  Is there any difference?  Why is that?  Be sure to write down what you feel and observe!

Speyer, R. F. (2012). Thermal Analysis of Materials. Materials Engineering. Marcel Dekker, Inc., New York, New York.