(EGY) Heat Transfer Lesson

Heat Transfer

From early childhood, you have been able to tell by touch if an object is hot or cold. If you want to heat an object, you bring it in contact with a hot object. Heat transfer should not be a new concept to you. In this lesson, we are going to look beyond the basic facts of heat transfer into what is happening at the molecular level during heat transfer. To see a demonstration of the three types of heat transfer, watch the video below. As you watch the video, listen for descriptions of conduction, convection, and radiation. In addition, watch for the direction that heat transfer occurs (hot to cold or cold to hot).

Did you enjoy looking through the lens of a heat-sensitive camera?  Do you know the definitions for conduction, convection, and radiation? For more examples of each type of heat transfer, participate in the following activity. Be sure to note various examples of each kind of heat transfer. Also, be sure to note that heat always flows from hot to cold. When two objects of different temperatures come in contact, heat will be transferred until both objects are the same temperature. This is called thermal equilibrium.

In summary, you should be able to define and provide examples of each type of heat transfer.

  • Conduction is the transfer of heat energy between materials that are in direct contact with each other. The process of conduction can be felt in the handle of a metal spoon that has been placed in a bowl of hot soup. The hot soup transfers heat to the end of the spoon; the heat is then transferred through the spoon to the handle. The rate of heat transfer depends on the type of material. Good conductors, such as metals, conduct heat rapidly. Insulators, such as wood or plastic, conduct heat very slowly.
  • Convection is the transfer of heat energy by the mass movement of fluids containing heated particles. Fluids are materials that can flow. Liquids and gases are examples of fluids. When particles of a fluid are heated, the particles move farther apart, causing the fluid to expand. This movement of heated particles creates convection currents. Home heating systems force heated air into rooms by way of convection currents. These currents heat the colder air in the room.
  • Radiation is the transfer of heat energy through electromagnetic waves. These waves originate from accelerated charged particles. Electromagnetic waves travel through matter or through empty space. Heat transfer through empty space is unique to radiation. Both conduction and convection require a medium or matter to transfer heat energy. Since the space between the Sun and Earth is essentially a vacuum, the heat energy from the Sun is transferred to Earth only by radiation.

Now that you know the definitions for conduction, convection, and radiation; let's look at what is happening at the molecular level to cause heat transfer.   Let's begin by looking at what happens to a potato when it is put into the oven.   In the picture, see what happens when a cold potato turns into a hot potato.   The difference between hot and cold is all about the movement of molecules. When you heat a substance—a solid, a liquid, or a gas—what you're really doing is adding energy to its molecules.   Simply put, temperature measures the movement of molecules. The hotter the substance, the faster its molecules are moving.    

image of cold molecules coming out of potato with hot molecules going into potato

People say anybody who can boil water can learn to be a cook. What actually happens to the water—or any liquid—when it's heated?   When a liquid, such as water, is heated the molecules in the water (H ₂ O) zoom around faster and faster.   Imagine you're in a cold room and you turn on an electric space heater.   What's happening to the molecules in the air as the room gets warmer?     Air is a gas—and gas molecules react to heat in the same way that liquid and solid molecules do: they start jumping around and bumping into their neighbors.   What is the definition of "heat"?   Heat is the transfer of kinetic energy between two objects due to a temperature difference between the two.

Now we know that heat is all about the movement of molecules. Let's take a closer look at that idea in the kitchen. Scientifically speaking, cooking is a process of transferring heat to food. Imagine we're making a recipe that begins: "Pre-heat a saucepan." That's easy. To warm the pan, we set it on a burner (the heat source) and turn on the power. While we're waiting for the pan to heat, we can see one form of heat transfer in action: conduction, a way of transferring heat through solid substances. First, the hot burner directly transfers, or conducts, heat to the metal pan. Then as the molecules in the pan start vibrating, they bump into nearby molecules and get them moving faster and faster. Pretty soon the whole pan is hot.

Conduction image

Continuing, convection is a way that heat is transferred through liquids (such as water) and gases (such as air). Let's put a pot of water on the stove and watch that happen.  As we heat the pot (through conduction), the water molecules on the bottom start moving faster and faster. As the water on the bottom gets hotter, it rises to the top, while cool water sinks to the bottom. Why do hot liquids and gases rise while cool ones sink? This is because as the particles are heated, they begin to move more rapidly and spread out.  As the particles spread out, it becomes less dense causing it to rise.  The particles in cool liquids and gases move slower causing them to condense and move closer together.  As they move closer together, it becomes more dense causing it to sink.  This cycle of hot air rising and cool air sinking repeats itself over and over. This circulation of hot and cold is called convection. We can see convection at work not only in our kitchens but also in the ocean and in the atmosphere.

Convection image with two areas circulating heat forcing cold out

A third way heat can be transferred from one substance to another is through radiation. Don't let that word scare you! Radiation is a form of electromagnetic energy, like light waves and radio waves that are all around us. And we've found ways to put that energy to good use in the kitchen. Radiant heat waves deposit energy into molecules directly—not through the process of conduction or convection.

image of radiation cooking potato with many heat waves going into potato

Now it is time to test to see what you know. Take the following self-quiz and see how well you have understood conduction, convection, and radiation.

Directions: Test your knowledge of the three types of heat transfer (conduction, convection, and radiation).  

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