Data Table: Investigation 11
Material
Time (minutes:seconds) for each wax dot to melt
Wax dot 1
Wax dot 2
Wax dot 3
Wax dot 4
solid steel rod
solid brass rod
solid glass rod
solid aluminum
rod
hollow brass rod
hollow aluminum
rod
Data Table: Investigation 11
Material
solid steel rod
solid brass rod
solid glass rod
solid aluminum
rod
hollow brass rod
hollow aluminum
rod
Time (minutes:seconds) for each wax dot to melt
Wax dot 1
Wax dot 2
Wax dot 3
Wax dot 4
Making Sense of Energy
Scientists and engineers have a word, conductivity, which is used to
describe how well heat energy moves through a solid. Sometimes the
phrase thermal conductivity is used to be sure that there is no confusion
with electrical conductivity. If a material has a high conductivity, then heat
energy can move through it faster than a material having a low conductivity.
Like density, the conductivity is a property of a substance, and is not a
property of an object. For example, the conductivity of a steel nail is the
same as the conductivity of the steel hull of a huge aircraft carrier. Objects
like frying pans and woodstoves are designed to transfer heat energy, so
they will be made from materials that are good conductors. Other objects
like winter gloves are designed to slow down heat transfer, and are made
from materials that are poor conductors. Thermal insulators (poor thermal
conductors) are good at keeping hot things hot and also keeping cold things
cold. The thermal conductivity of a substance is an important property that
must be considered when using the substance to make products.
We have seen that some materials conduct heat energy more
effectively than other materials. Metals, as a general rule, are good
conductors, and glasses are poor conductors. As poor as glasses are at
conducting heat energy, they are far from being the worst conductors.
There are materials; like wood, foam products, and some fabrics; that are
much poorer thermal conductors than glass. These products are so poor at
conducting heat energy, they are called thermal insulators, and they are
used to help keep heat energy from moving. If it is desirable to keep heat
energy from escaping a container, thermal insulators are used to make the
walls or line the walls of the container. The container can be as small as a
thermos bottle designed to hold hot coffee, or as large as a house. In both
cases, the plan is to keep heat energy from leaving the container. It is
impossible to completely stop the heat energy from conducting out, but
good thermal insulators slow down conduction dramatically, keeping the
inside of the container warm or even hot.
1. Which substance tested is the best conductor of heat energy, and
which is the poorest conductor?
2. Is there a small difference or a large difference in the conductivity of
metals and glass? Explain.
3. How can good insulators help keep the inside of a container cold
when it is hot outside, but also keep the inside of a container hot
when it is cold outside?
4. Did the wax drops take the same time to melt on the hollow bar as
they did on the solid bar that was made from the same material? If
the answer is NO, does energy pass through a hollow bar faster or
slower than it does through the solid bar of the same material?
5. Can you explain why heat energy moves differently through the
hollow bars than it did through the solid bars?