BACKGROUND:
As introduced in the Pre Lab, energy waves pass through different
substances at different speeds and frequencies. Seismic waves demonstrate
this in several ways. First, both P-waves and S-waves go faster through high
density rocks like granite than they do through low density materials like
soil or sand. Second, when these waves pass through unconsolidated (loose)
soil or sediment, the waves slow down and are amplified; wave height
increases. Finally, S-waves cannot travel through liquids. The molecules in
a liquid "slide past each other" too easily to transmit the
shearing motion of S-waves. In this lab the students will experiment with
the transmission of energy through different substances. They will not work
with seismic waves directly’ the experiments they conduct are direct
analogs for their behavior.
The behavior of seismic waves as they travel through the Earth has been
used to determine the Earth's interior structure. Through careful study of
seismograms, scientists have found that seismic waves change as they travel
through the Earth. From this work, they have determined that the Earth has a
three-part internal structure.
From its center to the surface, the Earth consists of core, mantle, and
crust. The core is composed primarily of two metals, iron and nickel. It has
two parts, the outer core where the metal is liquid (not like milk, more
like thick honey) and the inner core, which is solid. The mantle surrounds
the core, and is composed of very dense silicate minerals. Most of the
mantle is a mush of crystals and a little magma (molten rock), perhaps
similar to a snow cone in texture (but MUCH hotter!). The upper part of the
mantle is solid. It is coupled with the crust the outermost layer of the
Earth, to make the plates.
PROCEDURE:
- Set up the lab materials for each student or student group. Explain to
the class that energy waves travel through different substances in
different ways. A good analogy which might be familiar to the students
is the way that sound travels in air versus underwater. Sound seems
distorted underwater because it travels faster through this denser
medium.
Explain to the students that they will observe the transfer of energy
through different substances. The reactions they will record are caused
by the differences between the substances and the amount of energy they
will apply to the experiment. Stress that the experiments should be done
SLOWLY and CAREFULLY.
- Have the students complete the lab.
- Have the students try to determine the conclusion independently, then
have a group discussion to ensure they understand the correct answer.
Stress that their experiment is analogous to the behavior of seismic
waves inside the Earth.
ANSWERS TO LAB QUESTIONS: Exercise I. When you hit the penny, energy
is transferred from you (the energy source) to the penny. As the penny
hits another penny it again transfers energy, but there is not a total
transfer of energy. The penny will slow down. It is important for
students to somehow show that when the first penny hits the second the
energy, i.e., speed changes. They can answer the question several ways,
as long as they describe a change in the energy pattern. The same is
true for the nickel, except the change should look different from with
the penny to another penny. This is analogous to seismic waves traveling
between two types of rock, where a change in (wave) energy also takes
place. Here are suggested answers to each question:
- straight, energy transfer slowed down a little
- straight, nickel much slower
- the direction of pennies goes at an angle
- direction of nickel goes at an angle, slower than in #3
- energy is not as great as when nickel was used.
The direction of waves can change if the transfer of energy forces the
energy to go into several directions.
In Exercise II and III, the students should recognize that the same energy
goes through different substances differently. They can see the difference
in Exercise II (one fluid will have bigger waves than the other). In
Exercise III students can hear the difference. For the conclusion students
should realize that different substances allow energy waves to travel
differently. They should also understand how geologists use seismic waves
to study the interior of the Earth, expressing the information presented
in the Background in their own words.
