How does the application of heat affect materials?
As I begin the unit I would like to deal with the basic concept of convection which is the primary concept that ties the unit together. It is crucial for students to understand that when something is heated such as air it is less dense than cooler air so it rises. This kind of rising and falling is at the heart of our weather. Similarly temperature differences in the ocean result in ocean currents which mix Earth's huge mass of water. Likewise, the huge layer of rock called the mantle rises and falls, driving plate tectonics and causing earthquakes and volcanoes. Fluid motion occurs in solids, liquids, and gases. The mantle flows over a long time but it is not (primarily) molten; it is solid. The term "convection" is appropriate to all these situations and the notion of heated water rising up and falling back down is the type of cyclical movement they need to grasp.
Applying heat to water and air makes them expand. There are a couple of often
used demonstrations that can make this concept more tangible to children. One is to stretch the opening of a balloon over a flask of water. Heat the flask or place it in warm water and students will see that the balloon will expand due to the expansion of air within the flask. In response to students questions about why this happens I have found it useful to tell them that all matter such as water and air is made of small particles called molecules. At a cool temperature the molecules stay close together and move very little. However when heat is applied the molecules react by pulling away and expanding outward and their movement quickens. As the molecules spread out the air becomes less dense and so it rises.
Another good illustration of this is to have students pretend that they are molecules. As they stand together in a group side by side they would be simulating molecules at room temperature. Remember that the molecules are always in motion. Suggest to the children that they are now being heated up. As molecules they would begin to move round increasing their speed and widening the space between their neighbors and themselves. If the heating source were then removed the molecules (students) would begin to slow down and get closer to one another until they were standing next to one another. 1
What makes convection work is heat energy. Energy is the fuel that makes things happen. Energy is partially defined as the ability to do work. It can be found in a number of different forms. It can be chemical energy, electrical energy, heat (thermal) energy, light energy, or nuclear energy.
There are two types of energy: stored energy (potential) and moving energy (kinetic). If we have a battery we have potential energy. If we use that energy to power a flashlight we now have kinetic energy. Energy can be transferred into another sort of energy but it cannot be created and it cannot be destroyed. Energy always existed in one form or another. Heat is a form of energy and it can move in three ways: conduction (from hot to cold by vibration of molecules; convection (from warm to cool by fluid movement) and radiation (energy carried by light or electromagnetic radiation) which is how the Earth receives the energy from the Sun. 2
What is convection?
In the most basic term convection is the transfer of heat from a high temperature object to a low temperature object. According to scientific laws heat cannot pass from a cold object to a warm object. Convection takes place because heated materials rise because of their lower density and cool materials fall. A heated fluid will rise and then fall to be reheated again. Of course the rising and convection cannot occur without gravity. Sometimes fluids can become trapped in the cycle of heating, rising, cooling, and then falling and when that happens the circulation is called a convection cell. This pattern of rising and falling air is constantly happening in and around the Earth. Students can see this by heating some water and throwing in some rice or small macaroni. The heated water wants to expand and so rises up to the top. It then cools and falls back down toward the heat source below the bottom of the pot. Convection is critical in driving the Earth's weather. Convection is able to take some of the excess heat from the Earth's surface and send it into the higher levels of the atmosphere where it is released, and this helps keep the earth's temperature stable. Without convection the temperature of the Earth would be an average of 125º F rather than the normal 59ºF.3 Convection in the atmosphere and inside the Earth plays an important role in keeping the temperature on the Earth within a range that is capable of supporting human life.