Ch. 18 Solids

1. Ch. 18 Solids

2. Crystal Structure • Crystals– have solid, regular geometric shapes. • Crystals – may be seen by x-ray diffraction. Each type of crystal (e.g. NaCl) has its own unique x-ray pattern as seen on photographic film. • Sodium chloride – these crystals, in 3-dimensions, show an alternating pattern of sodium cations (+) and chloride anions (-).

3. Density • Density – a measure of how tightly packed a material is, of how much mass is squeezed into a given space. It is mass per unit volume. • Density = mass/volume • Both the masses of atoms & the spacing between them determine density. • If we break a chocolate bar in half, each piece has the same density. We would now have ½ the mass but also ½ the volume for each piece.

4. Density • Osmium is the densest substance on earth, even though it is less massive than gold or lead. What accounts for its great density is the tight spacing between osmium atoms within its crystal structure.

5. Definitions • One gram was originally defined as the mass of one cubic centimeter of water at 4°C. • Weight density – Weight per unit volume (Wt./Vol.). It is commonly used when discussing liquid pressure. • Specific gravity – the ratio of the mass of a substance to the mass of an equivalent volume of water. SG has no units since it is a ratio of the density of one substance to the density of water (which is 1 gram per cubic cm.). It is just some number. • * Which has greater density, 1 Kg of water or 10 Kg of water? • * Which has greater density, 1g of uranium or the planet Earth?

6. Elasticity • Elasticity – the property of a body by which it experriences a change in shape when a deforming force acts on it & by which it returns to its original shape when the deforming force is removed (e.g. a bow or a spring). • Inelastic – a property of a body by which it experiences a change in shape when a deforming force acts on it and does not change back to its original shape when the force is removed. (e.g. clay, putty, dough or lead). • The stretch of a spring is directly ~ to the applied force. Double the weight (force) to double the length the spring stretches. This relationship was noted by Robert Hooke in the mid 17th century. In equation form we have: F ~ x

7. Elasticity • Elastic limit – the distance at which an elastic material will stretch before becoming permanently distorted. Hooke’s law holds up to this point (i.e. the elastic limit). • Hooke’s law can be expressed as: F = kx in which F = force, x = distance stretched & k = a constant, which varies depending upon the material being stretched.

8. Compression & Tension • Steel is elastic; it can be compressed or stretched, can be used to make springs but also as a construction material. • A horizontal steel beam will sag due to its own weight as well as the load it carries. E.g. a steel beam with a weight at its end will be stretched at the top but compressed at the bottom. Somewhere in between there is a region referred to as the neutrallayer where the material is neither stretched (tension) nor compressed.

9. Compression & Tension • I-beams: have flanges at the top and bottom held together by a web. The flanges are at the top & bottom where stretching & compression occur the most when horizontally mounted. Strength = same as solid beam but uses less material (therefore it is cheaper to make & lighter in weight).

10. Scaling • Scaling – the study of how size affects the relationship between weight, strength & surface area. • As the size of a thing increases, it grows heavier much faster than it grows stronger (e.g. an ant vs. an elephant). • If the linear dimensions of an object are multiplied by some number, the area of the object will increase by the square of that number & its volume will increase by the cube of that number (which means its mass & weight will increase by the cube of that number). • Volume increases more rapidly than surface area, which is why it is important to wrap a baby warmly. • Similarly, crushed ice (more surface area) has a better cooling effect than a single ice cube (less total surface area) of the same mass. • See pg. 267 for examples.