18.6 - Internal energy

Internal energy: The energy associated with the

molecules and atoms that make up a system.

In the study of mechanics, energy is an overall property of an object or system. The

energy is a function of factors like how fast a car is moving, how high an object is off the

ground, how fast a wheel is rotating, and so forth.

In thermodynamics, the properties of the molecules and/or atoms that make up the

object or system are now the focus. They also have energy, a form of energy called

internal energy. The internal energy includes the rotational, translational and vibrational

energy of individual molecules and atoms. It also includes the potential energy within

and between molecules.

To contrast the two forms of energy: If you lift a pot up from a stovetop, you will

increase its gravitational potential energy. But in terms of internal energy, nothing has

changed. The potential energy of the pot’s molecules based on their relationship to each other has not changed.

However, if you turn on the burner under the cooking pot, the flow of heat will increase the kinetic energy of its molecules. The molecules will

move faster as heat flows to the pot, which means the internal energy of the molecules of the pot increases.

Internal energy

Energy of system’s atoms, molecules

18.7 - Thermal expansion

Thermal expansion: The

increase in the length or

volume of a material due to a

change in its temperature.

You buy a jar of jelly at the grocery store and store it

on a pantry shelf. When it comes time to open the jar,

the lid refuses to budge. Fortunately, you know that

placing the jar under hot water will increase your odds

of being able to twist open the lid.

By using hot water to coax a lid to turn, you are implicitly using two physics principles.

First, most materials expand as their temperature increases. Second, different materials

expand more or less for a given increase in temperature. The metal lid of the jar

expands more than the glass container as you increase their temperatures, effectively

lessening the “grip” of the lid on the glass. (The temperature of the lid is also likely to

increase faster, another factor that accounts for the success of the process.)

The expansion of materials due to a temperature change can be useful at times, as the

jar-opening example demonstrates. Sometimes, it poses challenging engineering

problems. For example, when nuclear waste is stored in a rock mass, heat can flow

from the waste to the rock, raising the rock’s temperature and causing it to expand and

crack. This could allow the dangerous waste to leak out. Knowing the exact rate of

expansion can help engineers design storage intended to prevent cracking.

Good engineering takes expansion into account. For instance, bridges are built with

expansion joints, like the one shown at the top of this page, that accommodate

expansion as the temperature increases.

Expansion joints allow bridge sections to expand without breaking.

Thermal expansion

Most materials expand with increased

temperature

Different materials expand at different

rates

18.8 - Thermal expansion: linear

Thermal linear expansion:

Change in the length of a

material due to a change in

temperature.

Most objects expand with increased temperature; how

much they expand varies by material. In this section,

we discuss how much they expand in one dimension,

(^338) Copyright 2007 Kinetic Books Co. Chapter 18