10-1: Temperature and Thermal Equilibrium Objectives: Relate temperature to the kinetic energy of atoms and molecules. Describe the changes in the temperatures of two objects reaching thermal equilibrium. Identify the various temperature scales, and be able to convert from one scale to another.

Defining Temperature We often associate temperature with how hot or cold an object feels when we touch it. Our sense of touch serves as a qualitative indicator of temperature. However, this sensation of hot or cold also depends on the temperature of the skin and therefore is misleading. In general, energy must be either added to or removed from a substance to change its temperature.

Temperature is proportional to the kinetic energy of atoms and molecules The energies associated with atomic motion are referred to as internal energy, which is proportional to the substance’s temperature. The symbol U stands for internal energy, and ∆U stands for a change in internal energy.

Thermal Equilibrium the state in which two bodies in physical contact with each other have identical temperatures If you put a warm can of soda in a cold bucket of water, eventually they will become the same temperature. Thermal equilibrium is the basis for measuring temperature with thermometers.

Matter expands as its temperature increases When the temperature of a substance is increased, its molecules jiggle faster and normally tend to move farther apart. This results in an expansion of the substance. Gases generally expand or contract much more than liquids. Liquids generally expand or contract more than solids. In general, if the temperature of a substance increases, so does its volume. This phenomenon is known as thermal expansion.

The extreme heat of a July day in Asbury Park, New Jersey, caused the buckling of these railroad tracks.

Expansion Joints If sidewalks and paving were laid down in one continuous piece, cracks would appear due to expansion and contraction. To prevent this, the surface is laid in small sections, separated by a small gap that is filled in with a substance such as tar. On a hot summer day, expansion often squeezes this material out of the joints.

Different materials expand at different rates. Dentists use material with the same expansion rate as teeth. Aluminum pistons of an automobile engine are smaller in diameter than the steel cylinders to allow for the much greater expansion rate of aluminum. Steel with the same expansion rate as concrete reinforces the concrete. Long steel bridges often have one end fixed while the other rests on rockers that allow for expansion.

This gap is called an expansion joint, and it allows the bridge to expand and contract.

Almost all liquids will expand when they are heated. Ice-cold water, however, does just the opposite! Water at the temperature of melting ice—0°C (or 32°F) — contracts when the temperature is increased. As the water is heated and its temperature rises, it continues to contract until it reaches a temperature of 4°C. With further increase in temperature, the water then begins to expand. The expansion continues all the way to the boiling point. Expansion of Water

The graph shows the change in volume of water with increasing temperature. Expansion of Water

A given amount of water has its smallest volume—and thus its greatest density—at 4°C. The same amount of water has its largest volume—and smallest density—in its solid form, ice. (The volume of ice at 0°C is not shown in the graph.) After water has turned to ice, further cooling causes it to contract. Expansion of Water

MEASURING TEMPERATURE The most common thermometers use a glass tube containing a thin column of mercury, colored alcohol, or colored mineral spirits. When the thermometer is heated, the volume of the liquid expands.

Temperature units depend on the scale used The temperature scales most widely used today are the Fahrenheit, Celsius, and Kelvin (or absolute) scales. The Fahrenheit scale is commonly used in the United States. The Celsius scale is used in countries that have adopted the metric system and by the scientific community worldwide. T F = 9/5*T C T C = 5/9(T F -32)

What are the equivalent Celsius and Kelvin temperatures of 50.0°F?