Conceptual Physics

18.23 - Summary

Temperature can be thought of simply as what thermometers measure.

Thermometers rely on physical properties that vary with temperature in a reliable,

reproducible way.

Different scales are used to measure temperature. The Fahrenheit scale is

commonly used to measure temperature in the United States, but the Celsius scale

is used in most of the rest of the world, and the Kelvin scale is used in science.

Reference points such as the freezing and boiling points of water define a

temperature scale and how to convert between scales. Another important (though

ultimately unreachable) temperature point is absolute zero, where molecules are at

their minimum energy state. This is 0 K on the Kelvin scale, or í273.15°C.

Heat is the flow of energy between objects resulting from a difference in

temperature. Like work, heat changes the energy of an object, but we cannot say

that an object has a certain amount of heat, any more than we could say it contains

a certain amount of work. Heat can change the internal energy of a system. Heat is

commonly measured in joules, or sometimes in calories or British Thermal Units. A

food Calorie is actually a kilocalorie, or 1000 calories.

Objects change in length and volume when their temperature changes, in a process

called thermal expansion. The change in length of an object depends on the

coefficient of linear expansion of the material from which it is made, which is

represented by the symbol Į. Its change in volume depends on the coefficient of

volume expansion, represented by ȕ. Water has the unusual property that it

expands with decreasing temperature from 4°C to 0°C; this is why ice floats.

Specific heat is a property of a material, not an object, that relates the amount of

heat to the change in temperature for a unit of mass of the material.

The transformation of an object from one state (solid, liquid, or gas) to another is a

phase change. Phase changes require heat to be either added or taken away. The

latent heat of a material is the energy consumed or released per kilogram during a

particular phase change. The latent heat of fusion is the heat flow during a change

between solid and liquid; the latent heat of vaporization is the heat flow during a

change between liquid and gas.

Ways in which heat is transferred include conduction, convection, and radiation.

Conduction is the flow of thermal energy directly through a material without motion

of the material itself. Thermal resistance, sometimes called the R-value, determines

the rate of heat transfer by conduction. Convection transfers heat by the bulk movement of molecules in a gas or liquid. Radiation transfers

energy by means of electromagnetic waves. Every object emits electromagnetic radiation.

TK = TC + 273.15

TC = (5/9)(TF – 32)

Thermal expansion

ǻL = LiĮǻT

ǻV = ViȕǻT

Specific heat

Q = cmǻT

Latent heat

Q = Lfm

Q = Lvm

Thermal conduction

Pc = Q/t

R = L/k

(^354) Copyright 2007 Kinetic Books Co. Chapter 18