s6pe-10103-ca
12/14/05
9:07 AM
MAZER
Page 27
Heat flow by conduction requires contact.
One way that energy is transferred as heat is through direct contact
between objects. Conduction is the process that moves energy from
one object to another when they are touching. The heat energy moves
from one object to another. But there is no flow of matter.
SUPPORTING MAIN IDEAS
Use this diagram to help
you take note on how
heat flows by conduction.
Recall that the atoms and molecules in matter are always moving.
The average energy of motion of particles in the warmer object is
greater than that of the particles in the cooler object. When particles
collide, or bump into each other, some of the energy of motion of
faster-moving particles is transferred to slower-moving particles.
Therefore, energy is transferred from the warmer object to the cooler
object. As long as the objects are touching, conduction continues until
the temperatures of the objects are equal.
Conduction can also occur within a single object. In this case,
energy is transferred from the warmer part of the object to the cooler
part of the object by heat flow. Suppose you put a metal spoon into a
cup of hot cocoa. Energy will be conducted from the warm end of the
spoon in the cocoa to the cool end until the temperature of the entire
spoon is the same.
Conduction transfers
energy from the cocoa
to the mug and through
the gloves to the person’s
hands.
How is energy transferred by conduction?
Some materials transfer heat flow by conduction better
than others. Materials that transfer energy easily are called
conductors. Metals are typically good conductors. You know
that when one end of a metal object gets hot, the other end
quickly becomes hot as well. Consider a saucepan that has a
metal handle. After the pan has been placed on a stove that has
been turned on, the handle might become quite hot. You would
use something to protect your hand when you held the handle.
Materials that are poor conductors are called insulators.
Some examples of insulators are wood and paper. Air is also a
poor conductor of heat energy. Many materials that are insulators have a large amount of trapped air inside them.
Consider a cup made of plastic foam. It contains many open spaces
within it that are filled with trapped air. A plastic foam cup will not
easily transfer energy by conduction. As a result, plastic foam is often
used to keep cold drinks cold and hot drinks hot.
Think about the handle of the pan mentioned above. Often, the
handle is not made of metal, a good conductor. Instead, it is made of
a material that is an insulator, such as wood or plastic. Although a wood
or plastic handle will get hot when the pan is on a stove, it takes a much
longer time for wood or plastic to get hot compared with a metal handle.
Chapter 1: Energy and Change 27
PDF