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Unformatted text preview: the average separation distance increases as energy (temperature) increases. (b) Comparison of the bondenergy curves for two materials:
material A has a deeper and more symmetric bondenergy well so that the locus of midpoints on the constanttemperature line segments changes less rapidly for material A. This means that th (A)
th (B). line segments and connecting their midpoints, we see that the average atomicseparation
distance increases as the temperature increases. For many materials this relationship
can be represented over a limited temperature range by the linear equation
xe x0
th x0 T T0 (2.5–4)  v v where xe is the equilibrium spacing at temperature T , x 0 is the equilibrium spacing at the
reference temperature T0 , and t h is the coefﬁcient of linear expansion.
Figure 2.5–2b shows bondenergy curves for two materials with approximately the
same equilibrium separation distance. The curve for material A is nearly symmetric in
the vicinity of x0 while that for material B is highly asymmetric. The midpoints of the
constanttemperature line segments for each curve show that the bond length changes
more rapidly with temperature for material B than A. Since t h is a measure of the
normalized change in dimensions with temperature, the magnitude of t h increases as the
bondenergy curve becomes more asymmetric. We can combine this observation with
another, that deeper energy wells tend to be more symmetric, and predict that materials
with high bond energies— those with deep and symmetric wells—should have low t h
values. This prediction is supported by the data in Table 2.5–1. The elements with higher
melting temperatures have stronger primary bonds and lower coefﬁcients of thermal
expansion.
It is important to recognize that the relationships between the bondenergy curve and
macroscopic properties developed in this section show general trends. They are extremely
helpful in understanding and predicting relative differences in properties between different materials. However, the constant...
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This note was uploaded on 02/25/2013 for the course PHYS 2202 taught by Professor Sowell during the Spring '10 term at Georgia Institute of Technology.
 Spring '10
 sowell
 Physics, The Crucible

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