EGEE 101
Energy and the Environment

Heat Transfer

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pile of household electronics: stove, microwave, speaker, computer, TV, monitor, printer
Consumer electronics and appliances in the home. But it is missing the all-important coffee maker. One of the many devices in the kitchen that heats something.

As I stand in my kitchen, I realize that all of the various appliances only do a couple of functions: they either heat (the coffee machine, toaster oven, oven, microwave) or do work (the blender, extractor fan). For me, the absolute first thing I must have in the morning is my cup of tea! We’ll use my cup of tea to demonstrate the three modes of heat transfer; conduction, convection, and radiation. The key ingredients for a perfect cup of tea are tea and hot water. The hot water comes from a kettle which is a very simplistic boiler. I know when it has boiled — because as the steam escapes through a small hole in the spout, it whistles. Boiling water requires a great deal of energy to transform liquid water into a gas (steam).

Conduction

A  photograph of a red kettle sitting on an electric range. Steam is coming out of the spout.
Credit: Red Kettle by
New Africa / adobe.stock.com

In the case of my range, electric energy is supplying the heat. The electricity supplies a stream of electrons that whiz around in the coil at very high speed (close to the speed of light) until they encounter some resistance. Impeding the flow of electrons produces heat, and so the metal coil (on which the kettle sits) becomes hot. The first heat transfer mode to the kettle is conduction. The metal of the cool kettle is in contact with the metal of the hot heating element, and heat always “flows” from hot to cold, so there is heat transfer. The atoms in the heating element behave a little like slam dancers at a Green Day concert. They will pass on their energy to the surrounding people, making them more active, and in turn passing on their energy to another layer, and so on until the whole concert is full of crazed slam dancers.

Watch the following 1:33 minute video representation of conduction through a metal rod.

Click for a transcript of the conduction video.

Conduction is the transfer of heat energy by the kinetic motion of atoms in a substance. Remember that all atoms have motion of some sort. This is true even for atoms that make up a solid. In this animation, note that the atoms in the bar of metal are vibrating slowly due to their relatively low temperature (in real life, I might add, atoms are vibrating at tremendous speeds, so this is just a representation). Now watch what happens when I slide the bar of metal into a furnace.

Notice that as the metal heats up, the atoms at the end of the rod begin to vibrate faster. The faster-vibrating atoms transfer some of their energy to the slower adjacent atoms. Thus, the slower atoms also vibrate faster, giving them a higher temperature. This is conduction.

I should also mention that conduction works to cool an object as well. When a slower vibrating atom is next to a faster-vibrating one, we’ve seen that the slower atom takes some of the energy from the fast atom. This makes the slow atom vibrate faster BUT, it also slows down the faster-vibrating atom, cooling it. If there are MANY, MANY more “cooler” atoms than warmer ones, then the net effect of the energy transfer is to cool the warmer atoms back to a temperature equal to its surroundings.

Credit: David Babb

Some other examples of conduction include:

  • hot tea in a teacup transfers energy through a spoon to your hand as you stir in the honey

  • transfer of heat from a hot cookie sheet to the cooler cookie dough heats the dough and cooks the cookie

  • hot days heat is conducted into your home through the roof, walls, and windows

Convection

diagram of convection in a tea kettle as described below
Credit: Heidi Sporre © Penn State, CC BY-NC-SA 4.0

Convection is another way in which heat is transferred. It occurs when heat is transferred by the movement of fluids (liquids or gases). In our tea example, you’ll see that when a water molecule heats up, it has more energy, (occupies more volume) and is thus lighter and thermal gradients form where the warmer molecules rise to the top and the cooler molecules sink to the bottom. The cooler molecules are then closer to the heat source and become heated, repeating the cycle. Water molecules will also collide — passing along energy (heat).

Watch this 2:31 summary video about Convection from CentainTeed.

Click here for a transcript of the convection video.

Convection is the second mode of heat transfer. Heat transfer by convection occurs as a result of movement of liquid or gas over a surface. Wind blowing against the building is an example of a gas moving over a surface.

There are two types of convection: forced and natural.

Natural convection occurs when the movement of liquid or gas is caused by density differences. For example, we're all aware that warm air rises. That's because it has a lower density than the surrounding cool air and that's what causes a hot air balloon to rise. And of course, we know the opposite is true. Cool airdrops. In our wall example, the warm air inside the building comes in contact with the cool exterior wall. Some heat is lost to the wall, causing the interior air adjacent to the wall to cool. Since this air has a higher density, the airdrops. The warmer exterior surface of the wall heats the air next to it decreasing the air's density and causing it to rise. And as a result, the movement of the air along the surface of the wall increases the heat transfer. This type of heat transfer is called natural convection. This heating and cooling create convection loops adjacent to both the interior and exterior surfaces. Convection can also take place inside of empty cavities. One example is the movement of air in a double-pane window. In winter, air is heated on the inside surface of the window cavity causing the air to rise. The air adjacent to the outside surface cools and drops. What results is a convection loop inside the window cavity that transfers heat from the inside to the outside.

A second type of convection is forced convection. Here the movement of the liquid or gas is caused by outside forces. If the wind is blowing, the air movement across the outside of the wall will be higher increasing the rate of heat transfer. The rate of heat transfer by convection depends on the temperature difference, the velocity of the liquid or gas, and what kind of liquid or gas is involved. For example, heat transfers more quickly through water than through air.

Credit: CertainTeed

Some other examples of convection include:

  • a lava lamp
  • the breeze on a windy day

Radiation

Radiation is how the “warmth” of the sun is transferred to you when you're out or how you warm your hands in front of the fire. It is electromagnetic radiation, and it does not need a medium or direct physical contact to propagate. Thus, it can travel through space. The red color from the heating element indicates that it is so hot that it is radiating energy, and these electromagnetic waves contribute to the heating of the kettle. Some of the more expensive models of stoves use this method of heat transfer primarily. Following is a nice video about how radiation works in a building.

Caption
Click here for a transcript of the Radiation video.

Radiation is the third type of heat transfer. Radiation heat transfer is by invisible electromagnetic waves from one object to another. Heat transfers from areas of higher temperature to areas of lower temperature. One common example of radiation heat transfer is from the sun. When you walk outside on a sunny day you immediately feel the warmth from the sun, even if the air is cold. Heat from the sun is being transferred through space by radiation, in order to warm you. Radiation also plays a role in heat transfer in a building. If you stand in front of a window on a cold day, your body radiates heat to the cold surface of the window and the result is, you feel colder. Likewise, if you stand in front of a window with the sun streaming in, you feel warm as a result of the incoming solar radiation. This type of energy--solar radiation is primarily shortwave radiation. Glass is nearly transparent to the shortwave radiant energy from the sun and as a result, once sunlight enters a room, the sun's energy is absorbed by the walls and the contents of the room and is converted to heat. At the same time, the warm objects in the room also emit radiant energy.

Credit: CertainTeed

Some other examples of radiation include:

  • visible light from a candle
  • x-rays for an x-ray machine
  • microwaves from a microwave oven

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