Mary Ann Horton, age 15, of Willingboro, N.J., for her question:
HOW DOES HEAT TRAVEL?
Heat is a form of energy. Heat and energy cannot be seen, but the work they do can. Heat passes from one object or place to another by three methods: conduction, convection and radiation.
Conduction is the movement of heat through a material. When heat travels by conduction, it moves through a material without carrying any of the material with it.
Convection is the transfer of heat by the movement of a heated material. Convection carries heat by circulating a heated material.
Radiation carries heat in the form of waves through space. Heat causes a wire in a heat lamp to give off waves of radiant energy called infrared rays. When these rays strike someone, their energy warms that person.
In conduction and convection, moving particles can actually transmit the heat. But heat can also travel through empty space that doesn't have any particles.
In any object, the moving atoms or molecules create waves of radiant energy. These waves travel through space in much the same way as water waves travel on the surface of a pond.
When the radiant energy strikes an object, it speeds up the atoms or molecules in that object. Much energy from the sun travels through space to the earth. These rays warm the earth's surface by radiation.
Convection occurs in liquids as well as in gases. Convection currents, for example, will form in a pan of cold water on a hot stove. As the water near the bottom warms up and expands, it becomes lighter than the cold water near the top of the pan. This cold water sinks and forces the heated water to the top. The convection current continues until all the water reaches the same temperature.
An example of conduction: a copper rod placed in a fire quickly becomes hot. Heat travels from atom to atom until it reaches the other end of the rod, but the atoms themselves do not move from one end to the other.
Insulation is a way to control the movement of heat by keeping it in or out of a place. Houses, for example, are insulated to keep the heat inside in winter and outside in summer. Three methods of insulation are used because heat can travel in any one of three ways.
Certain materials, such as plastic and wood, make good insulators against the movement of heat by conduction. This is why many pots and pans have plastic or wood handles. The metal utensil itself heats rapidly by conduction, but the handle stays cool.
The movement of heat through the air by convection can be controlled by blocking the space between a hot and cold area with "dead air." The layer of air between a storm window and the inner window, for example, acts as an insulator.
Surfaces that reflect infrared rays can insulate heat traveling by radiation. Shiny metal roofs, for example, reflect the sun's rays.