Lesson 2.3 Solids
Overview
This lesson deals with solids, density and elasticity and
the behavior of the particles that make up solids. On
completion of the lesson, you should be able to use
density data to estimate masses and volumes. You should
be able to explain elasticity and Hooke’s Law.
 MINI LAB
CHOICE
OF ACTIVITIES
1)Compare the ways in
which rubber, metal balls and marbles bounce when they
collide with each other.
2) Measure the density of
a metal cube or rectangular object
EXPERIMENT
#1 Bouncing
Purpose: To
illustrate elasticity
Equipment:
Two large marbles
Two metal balls
Two rubber balls
String – about 3 meters
Procedure:
- For each ball, cut a
piece of string roughly 50 cm long.
- Glue or fix the mid-point
of the piece of string to the ball.
- Select a pair of
similar balls and attach the ends of the string
to a supporting device in such a way that the two
balls will collide when released from opposite
sides of the support
- Observe the degrees
to which different materials bounce after
collision.
Questions
- Which material
appears to have the greatest degree of
elasticity?
EXPERIMENT
#2 Density
Purpose: To
illustrate the measurement of density
Equipment:
Rectangular-shaped block of wood, plastic or other solid
Ruler
Balance
Procedure:
- Weigh the item to be
measured
- Measure each of the
sides of the object.
- Calculate the volume
of the object
- Calculate the density
of the material
- (If possible) Compare
the density with published values.
Crystals and Amorphous
Solids
Many solids have a crystalline structure. The atoms,
molecules or ions that make up the solid have a regular
arrangement and this is multiplied many times so that a
piece of the solid has the same shape as one of the
microscopic parts of its crystal lattice. The shape of
the crystal depends on the way in which the atoms or ions
are bonded together.
Some materials like glass and many plastics have an
amorphous structure. There is no regular arrangement of
the molecules that make up the substance. They are packed
together in a random way and this affects their
properties.
Amorphous materials are usually more likely to be
transparent than crystalline materials. There are
exceptions like gemstones, but most metals have a crystal
structure.
Solid Structures
In solid structures, the atoms, molecules or ions are
held in position by bonding forces. There are sets of
forces: Forces of attraction and forces of repulsion. The
particles that make up a solid are in constant motion but
they do not move about. This happens only in liquids and
gasses. The particles that make up a solid are packed
together a bit like marbles in a bag. They get as close
together as possible.
When a force is applied, the bonds get stressed and the
particles move closer together or further apart.
Density
Particles in a solid pack together in a way that is
determined by the bonds between them. Some materials have
greater distances between the particles than others. The
density of a material depends on the weight of the atoms
and the distances between the atoms. If we look at the
periodic table, we can see that gold atoms for example
are heavier than osmium atoms. Osmium is in fact more
dense than gold because the bonds between the atoms are
shorter.
We can measure the density of a substance if we can
measure its mass and its volume.
Water
Ice has a lower density than water because the bonds
between the water molecules in ice keep the molecules
further apart than they are in water.
Elasticity
An elastic material returns to its original shape after a
stress that has been applied to it has been removed. Many
substances that we regard as elastic are not truly
elastic because they change shape slightly every time
they are distorted. Other substances like glass and steel
are highly elastic.
Balances And Springs
Many balances and springs work on the principle of
elasticity
Coiled springs distort under torsion.
Hooke’s Law: The extension of a spiral spring is
proportional to the force applied unless the force
exceeds the elastic limit of the material.
Volume of sphere = 4/3 p r 3 Where: r = radius of sphere
(m)
Example
1.1.1: Density of regular-shaped object
A rectangular aluminum
object has dimensions: 3 cm x 5 cm x 6 cm.
What is the density of
aluminum if the object’s mass is 243 grams?
Solution
Volume of solid = 3 x 5 x
6 = 90 cm3.
Density = mass / volume =
243 / 90 = 2.7 grams per cm3. (2700 kg / m3)
Example
1.1.1: Density of spherical object
A large glass marble has a
diameter of 20 mm.
What is the density of the
glass if the marble’s mass is 11.3 grams?
Solution
The radius of the marble =
10 mm = 1 cm.
The volume of the marble =
4/3 x p x r3 = 4.19 cm3.
The density of the glass =
mass / volume = 11.31 / 4.19 = 2.7 g/cm3 (or
2700 kg/m3)
Questions
- How do amorphous
materials differ from crystalline materials?
- Gold atoms have more
mass than osmium atoms. Why does 1 kilogram of
osmium occupy less space than 1 kilogram of gold?
- Pure gold has a
density of 19.3 grams per cubic centimeter. What
is the volume of a gold bar that has a mass of
100 Kilograms?
- Is it possible that
an alloy of gold, silver and osmium could have
the same density as that of pure gold?
- What is the
difference between an elastic material and an
inelastic material?
- Explain what is meant
by the elastic limit of a material.
- If a thin metal rod
is clamped horizontally at one end and a weight
is suspended from the other end, is the metal in
the upper part of the rod compressed or
stretched?
- What causes the rod
to return to its normal shape after the weight is
removed?
- Describe the
differences in the ways in which particles of a
spring are displaced when a part of the spring is
a) Under tension, b) under compression .
- A mixture of gold and
silver contains 50% gold and 50% silver by mass.
If the density of gold is 19.3 grams per cubic
centimeter and that of silver is 10.5 g/cm3,
what is the volume percentage of gold in 100
grams of the mixture?
- What is the density
of a mixture containing 50% gold and 50% silver
by mass?
- A mass of 2 kg is
suspended from a coiled spring that obeys Hooke’s
law. This causes an increase in the length of the
spring of 20 centimeters. By how much would the
length increase if the mass at the end of the
spring were increased to 3 kg?
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