op  Energy and heat flow; heat flow in nature; radiation and conduction.
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Heat flowing out of a man's body.
More heat is flowing out than his metabolism can generate. He is cold.
Heat flowing into a man's body.
More heat is flowing into his body than is flowing out. He is hot.

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Energy, in the process we call heat or heat flow, is constantly flowing into and out of all objects, including living objects.  Heat flow moves energy from a higher temperature to a lower temperature.    The bigger the difference in temperature between two objects, the faster heat flows between them.   When temperatures are the same there is no change in energy due to heat flow. Radiation and Conduction are the two methods of heat transfer. Convection is a special type of conduction. Heat has the units of energy; heat flow has the units of power.
  Kent contemplates the mysteries of heat flow.
     Kent contemplates the mysteries of heat flow.  
     Where does the "hotness" go?
     Will it be home for dinner?
     Is there such a word as "hotness"?
  You Can't Stop the Heat Flow
     Energy is flowing into and out of your body, and everything else, all the time. I'm not talking about energy from the food you eat as a card carrying member of the Food Chain, though energy does get into animals in that way too.

     Energy flows into your body from the surrounding air, from surrounding objects, and from the sun when you're outside, or even light bulbs when you're inside. Energy also flows out of your body into the surrounding air, into the surrounding objects, and even into outer space, most notably if you are outside on a clear night.

     This type of energy transfer from one place to another place is driven by differences in temperature and is called heat flow. It is a really big deal - one of the major driving forces of nature (if you're interested in major driving forces - and who isn't? - not to be confused with Tiger Woods or Barry Bonds).

     Heat flow from the sun to the earth is the force that makes weather. The mother of all earthly driving forces.
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    You can't use heat flowing into your body as energy like you can use the energy in food. But it does help to keep you warm - or too warm. Animals in cold climates don't have to burn as many calories in the summer as in the winter. How come?

David Watson with insulation to slow heat transfer from his body.
This man had to cover himself with longjohns and many other layers in order to stay warm while his active daring children ice skated and kept warm with hardly any layers because they were generating a lot more internal energy by exercising while he stood around like a sissy.

A steam radiator heats up a room.
These great old radiators used a combination of radiation and conduction to heat a room. Down in the basement was usually a steam boiler converting the chemical energy stored in coal (in the old days) or fuel oil or natural gas (more common nowadays), into "heat" which was used to convert liquid water into gas phase water (steam to you laymen). The steam flowed up into the radiators where its heat was transferred by conduction to the air and by radiation to the objects in the room.

         Heat flow from the sun to the plants and algae and some types of bacteria is the energizer of photosynthesis - the provider of food for almost all living things.

     For animals heat flow is a really big deal too. The way that each animal is made and behaves and where they can live, has a lot to do with heat flow.
     Because of heat flow, cold-blooded reptiles and insects that live in cold climates only come out in the summer. Heat flow is why there are a lot more reptiles and insects in equatorial climates than arctic climates.
Sources of heat flow in an animal's body      Warm-blooded mammals and birds with their high metabolisms and hotter bodies can live and move in some pretty cold places. But, because of heat flow, they have to have good dry fur or feathers, or a good layer of blubber, and plenty of high energy food to keep that body heat coming.

     When you put on a jacket in the winter, or wear shorts on a hot day, you are adjusting your heat flow, trying to help your body keep its temperature very close to 38 degrees C (98.6 deg. F). It's a never ending struggle. Energy as heat is always flowing out of, and into, your body.
      It's dangerous to be too hot and dangerous to be too cold. Just like Goldilock's porridge, our body temperature has to be "juuust right".

     Animals and human engineers use all kinds of clever ways to transfer heat. Elephant ears act like giant radiators to transfer heat out of their bodies. Car radiators are used to transfer heat out of the car engine. There are countless fascinating examples. In pages yet to come we will explore some fascinating examples of how animals and human made machines use the same basic principles of heat flow, along with some pretty clever techniques, to keep their operating temperatures "juuust right".

     But first we must learn three simple things about how energy heat flow works.

    Which Way Does It Go and How Fast Does it Go?
     What happens when you put a cup of hot liquid on the counter in the kitchen?
     (Confused pause while you wonder if this is a trick question.)    Everybody knows that if a cup of hot water sits in a room, the water in the cup will gradually get less hot.
Assuming, of course, that the air in the room is at a temperature that humans find comfortable, and that gravity holds the water in the cup and the cup on the counter.

Kent experiences heat flow into his finger.
     Kent demonstrates how not to determine if the water is hot.
       But what really happened to the water in the cup on the counter?  How did the "hotness" escape?  Where did the hotness go?  Why did it go? It never stays for dinner.  Didn't it like us?   What is "hotness" and "coldness"?  Are those real words?  And why do I keep losing socks?

      Here's some more questions for you.   What temperature will the hot chocolate cool down to?   Why doesn't it just keep getting colder and colder until it freezes?   It seems ridiculously obvious that the hot water won't freeze, but when we are asked to explain it, most of us go, "uh...".   Or we resort to the cowardly response,   "That's just the way it is".   Or the truly brave approach, "I haven't got a clue".

     In this page, and others, we'll talk about where the "hotness" went.   There are three fundamental things about heat flow that will answer the above questions (except for the ones about where my socks go).
    Heat flows from higher temperature to lower temperature.The Three Things To Know
      Here are three easy things to know about the way heat flows:

1)  There has to be a temperature difference. Energy only flows as heat if there is a temperature difference.

2)   Energy as heat flows from a higher temperature to a lower temperature.

3)   The greater or larger the difference in temperature, the faster the energy flows.

(See The Second Law of Thermo Pages)
Heat flow from a cup of hot chocolate.      Let's consider a cup of hot chocolate in the light of our new found wisdom.
     The cup will only cool down if the air surrounding the cup is cooler than the hot chocolate. The energy doesn't disappear. It flows by conduction into the air around the cup and the counter top it is sitting on. It also travels by radiation into the cooler surfaces around the cup, like walls and cupboards.
     The cup cooled down, the room heated up.
     That's right. The air in the room heated up.
     You didn't notice it because the large room full of air can easily absorb that amount of added heat energy without much change in temperature.
      But if you had a thermometer that could measure temperature to very small fractions of a degree and if you could make the kitchen airtight so that no air could flow in or out, then you would be able to measure a slight increase in the temperature of the kitchen air. And if you put hundreds of cups of hot chocolate in the kitchen it might get pretty darn hot and muggy in there.
      According to the First Law of Thermodynamics, the energy that flowed into the air and the other surfaces in the room exactly equals the energy that flowed out of the cup. It has to be that way because energy can't just disappear into nothingness. The total amount of energy never changes.

The laws of thermodynamics hold true.     The energy in the cup moves, as it always does, from the hotter "stuff" to the cooler "stuff".  The hot stuff has more energy in its atoms than the stuff that is cooler. The higher energy atoms transfer their energy to the lower energy atoms.

     The hot chocolate will cool down until it becomes the same temperature as the surrounding air.
(Check out 2nd Law of Thermo)

The stuff below assumes you've had a little physics, maybe highschool or older (or are smarter than I was in junior high):

Heat Flow as a Rate
Units of Heat, Energy, Heat Flow, & Power

     Heat flow is energy moving. It has the same units as power - energy per unit time - or energy/time. It means that during the given amount of time, during which heat is flowing, a certain amount of energy is transferred or moved from one place to another place. Confused? Let's make up an energy unit. Anyone can do it.

     Let's pretend units of energy are measured in wompy gloopers (WG). Let's also say that 60 wompy gloopers of energy flows from an average adult human male body to the surrounding air (temp. at about 24 degC) during every minute (not a very exact definition). Then the rate of heat flow is 60 wompy gloopers per minute, or 60 WG's/min. See? Units of energy per unit of time. How about in seconds?
Yup, 60 WG/minute equals 1 WG per second
or 1 WG/sec (60 divided by 60).
What is the heat rate in hours? There are 60 minutes in an hour, so 60 WG/min equals 3600 WG's/hour (60 times 60). I am an average adult human male. As I write this, heat energy in my cells, generated by the metabolism of sugar to energy during cellular respiration, is flowing out of my body into the room at the rate of 3600 WG/hr.

     But alas, nobody is measuring heat flow energy in wompy gloopers. You might be more familiar with units like joules or calories or kilowatt-hours (Kw-hr), or even BTU's (British Thermal Units), an old favorite of mine.

     One joule (J) is the amount of energy generated when a force of 1 newton (N) is exerted over a distance of 1 meter (m). So a joule has the units of newtons times meters, or N-m. A joule is the official unit for energy, heat, and work, in the International System (SI). Heat flow is measured in joules per second. Most of us are more familiar with watts as the unit of heat flow rate and power. Watts are a measure of power, not energy. Power is a measure of how fast energy is "happening". One watt means that 1 Joule of energy is being transferred every second, or 1 J/s. A killowatt (Kw) is 1000 joules per second or 1000 watts.

     A typical adult male human being, sitting around in a room of normal temperature, not doing much (hopefully reading rather than watching TV), generates about 100 watts of heat flow into the air surrounding him. From my WG definition above, we can say that 60 wompy gloopers equals about 100 watts. So 1 WG = 1.67 W (100 divided by 60). Just like that, we invented a new unit of energy.

     What happens if the furnace breaks and the air temperature in the house gets colder?
     In the first case the air temperature was 24 degC and my body temperature was at the normal 38 degC then the temperature difference was 14 degC (38 minus 24). So at a temperature difference of 14 degrees C, energy flows out of my body into the surroundings at 100 Watts or 60 WGs.

     After a few hours with a broken furnace the air temperature in my house falls to 0 degrees C (It's cold outside. This is Wisconsin). Now the temperature difference between my body and the air is 38 degrees C. If I don't have enough sense to put on more clothes, or wrap myself in a few blankets (insulation - which is a subject for another page), I am going to feel very uncomfortable. How come? Because as we know from our discussion above, the temperature difference has become larger, meaning heat will flow much faster from my body into the surrounding air
     If the heat flow rate from my body is faster than the internal energy produced by my metabolism, I will start to feel very uncomfortable. My body temperature will start to drop because I cannot produce internal energy as fast as it is flowing out of my body. In addition to being miserable, I will be in danger of hypothermia. Hypothermia is what they call it when your body temperature gets lower than approximately 38 deg C (98.6 degF). A degree or two lower is no big thing occasionally, but if it gets much more than that for very long, I could be in trouble. Ah, the joys of heat flow.


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Radiation or Conduction or Both?
     There are two basic ways that energy can be transferred from one place to another through the heat flow process - radiation and conduction.
     Radiation is a type of energy that can travel through space. It doesn't need matter to conduct it from place to place. It can travel through a vacuum, no trouble. It can also travel through air, no trouble.
     When you stand near hot molten lava, the heat you feel on your skin is mostly radiant heat. This type of heat doesn't need air to travel through. Even if you were standing in a vacuum (no air) you would feel the heat (except you'd be unconscious, or worse, from lack of air).   The sun radiates electromagnetic radiation into space
       The sun is a big hot ball that radiates energy into space. A tiny portion of the sun's energy "hits" the earth.
  Sun power energizes earth.
      Almost all of the energy from the sun that travels 93 million miles in 8 and 1/2 minutes through the vacuum of space is radiant energy. When you stand outside on a sunny day feeling the warm rays, remember that only 8 and 1/2 minutes ago it left the sun. Scientists call it electromagnetic radiation. Infrared, ultraviolet, and visible light are examples of electromagnetic radiation that can transfer energy from one object to another object.

     Conduction is the type of heat flow that results when things are actually touching. Energy traveling as heat by conduction needs matter to flow through. If you touch a hot object the heat is conducted by physical contact with your skin. The energized atoms in the object transmit their energy to the atoms in your hand. If you are standing in cold air, the heat from your body flows from the molecules in your body into the cold air molecules that are touching your body. If you are floating in cold water, the heat flows from your body into the cold water molecules that are touching your skin. If you fry vegetables on a stove you are relying on conduction to cook your vegetables. Heat from the flame flows through the metal by conduction, into and through the cooking oil by conduction, and into and throughout the vegetables by conduction.
     Conduction cannot travel through a vacuum because in a vacuum there are no atoms or molecules making contacting with other atoms or molecules. Something made of atoms or molecules has to touch something else made of atoms or molecules in order for there to be conduction.

Heat flow from a wood stove.
     Heat from a wood stove is radiated to cooler surfaces like walls, floors, ceilings, furniture, and people.
Energy as heat is also conducted into the air and circulated naturally by a process called convection from the hot metal surfaces to the air surrounding the stove.
Power vs Energy
Some of us get power and energy mixed up. We shouldn't. It's easy. Here are some vague explanations to make it clearer:

Energy, as you know, is that certain something that makes things happen. The more of it you have, the more you can make happen. The chemical energy stored in 10 gallons of gasoline (petrol) will move your car a lot further than 1 gallon of gasoline.

Power measures or describes how fast those energy things happen. It is a measure of how fast energy is being converted from one form to another.

A unit or measure of power is the Watt.
A 100 Watt light bulb uses 100 joules of energy every second, because 1 Watt = 1 Joule per second. Every second that light bulb is converting 100 joules of electrical energy to 100 joules of heat and light (mostly heat - that's why it burns to touch one).

Another well known unit of power is the horsepower.
It is based on a lesser known unit of energy called the foot pound force (ft-lbf).
If you move something by pushing on it with a 1 pound force for a distance of 1 foot, you have done 1 foot pound of energy.

Someone once actually hooked some horses to some weights and pulleys and decided that the average horse could generate about 550 foot-pounds of work every second. Energy divided by time again. That is power. He decided to call that 1 horsepower.

So 1 hp equals 550 foot pounds of energy per second. Every second that poor horse is lifting 550 pounds through a distance of 1 foot, or 1 pound through a distance of 550 feet, or 5.5 pounds through a distance of 100 feet, or 55 pounds through a distance of 10 feet. You try it sometime.
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