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CLOSE THE DOOR
You're letting the cold in!

Science Standards by State

 

 
 
 

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Have you ever heard someone say, “Close the door; you’re letting the cold in!”?  Does someone who “lets the cold in” really let the cold in, or do they let the warmth out?  When you put on a warm jacket or sweater in winter, is it to keep the cold air from your body, or is it to keep your body heat from escaping?  People often confuse the insulating of something because they don’t really understand what is being prevented.  What they feel may be quite different from what they think.

When any object is warmed, energy in the form of heat is added to it.   The energy that is added may come from something that is burning, such as gas, coal, or wood.  It may come from electricity that “fights its way” through a conductor such as a wire in a light bulb.  It may come from the friction of two things rubbing together, such as your hands, or two sticks.  It may even come from certain chemicals that get hot when they are brought together.  Of course, it may also come from the sun, where radioactivity releases tremendous quantities of energy in the form of heat.  Regardless of the source of heat energy, people have some strange notions of how it works, what it does, and what happens to it eventually.

You can feel the heat energy from a fire.  You can feel the energy from the white-hot wire inside an ordinary light bulb.  You can feel the heat (energy) from a warm body such as another person, or a puppy.  When something has more heat energy than you have, so that it leaves that place or object and travels to your body, you feel warmed.  And when you lose more heat energy than you get, you feel cold.  Whether heat energy comes to you, or leaves you, you feel it.  But what about things that have little or no heat energy?  What do they feel like?  They feel cold, and that may be confusing.  

Can cold be transferred from one thing to another, as heat can?  That’s a good question.  Think of having an ice-cube in your hand.  It feels cold because the heat in your hand melts the ice.  As the ice melts, does it do so because it’s losing something called “cold”, or does it melt because it’s getting something called “heat?”  If it’s gaining heat, maybe we should feel it getting warmer.  Unless the heat it gains does something other than make it warmer.  Maybe the heat it gains does something else, such as change it from ice to water.

People often confuse heat and cold.  When they go outside on a cold winter day, they put on “warm clothing.”  That doesn’t mean the clothing has a higher temperature than the air around it.  Rather, it slows the loss of body heat, and you feel warmer because you haven’t lost so much heat energy as you would without the jacket.  When you go to bed in winter, you may cover yourself with a “warm blanket.”  But if you check the temperature of the blanket before you put it over you, it may not be warm at all; it is merely the temperature of the room in which it lay.  But being an insulator, it slows the rate at which your body loses heat, and so it feels warm.  By itself, it isn’t warm.  But it can make you feel warm because it slows the escape of your own body heat.

In a similar way, something cold may feel cold because heat energy from our body escapes into it.  When we walk outside in winter, it isn’t something called “cold” that invades us.  Rather, our bodies lose heat energy to the cold air, and we feel that loss of energy.  It isn’t the cold getting into us; it’s the heat energy leaving our bodies.  There isn’t such a thing as “cold energy.”  But the lack or loss of energy may be felt as “cold.”  Heat energy can be produced in many ways.  It can be produced by burning a fuel.  It can be produced by friction.  It can be produced by electrical energy being forced through a conductor.  It can be produced when something radioactive sends out radiant energy.  But it cannot be produced by “removing the cold” from something.  Something cold lacks energy, and you cannot make a cold thing colder by giving it more “coolth.”  

How, then, do you make something colder?  You can do it by removing some of the heat energy in the thing you’re trying to cool.  In a refrigerator, things are cooled by removing heat energy from them.  This is done by using some of the heat energy in food to chill (and condense) a gas in the coils of the refrigeration unit. This is not easy, and it takes a lot of electrical energy to compress a special gas and make it liquid.  Then, as that liquid is circulated around the cooling places in a refrigerator, it takes energy from warmer foods, and that energy evaporated the liquid in the tubes of the cooling unit.   In the back of the refrigerator, heat is released from the gas, and it becomes liquid again, and gets circulated near food that loses its heat energy to the liquid.  This cycle goes on and on, and needs a lot of electrical energy to run it.  

So, when someone says, “Close the door; you’re letting the cold in”, you will know what’s really happening, and that knowledge may keep you warm inside!
 

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