Britannica Educational Publishing - Heat

HEAT

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Introduction by Shalini Saxena

Library of Congress Cataloging-in-Publication Data

Heat/edited by Andrea R. Field.
p. cm.(Introduction to physics)

In association with Britannica Educational Publishing, Rosen Educational Services.
Includes bibliographical references and index.
ISBN 978-1-61530-843-9 (eBook)
1. HeatJuvenile literature. I. Field, Andrea R.
QC256.H436 2013
536dc23

2011053232

Cover (burner), p. 3: ccat82/Shutterstock.com; cover (equation) EtiAmmos/Shutterstock.com; pp. iStockphoto.com/Milos Jokic; remaining interior background images iStockphoto.com/Floriana Barbu

INTRODUCTION TO PHYSICS

EDITED BY ANDREA R. FIELD

T he sting we feel after touching a hot pan, the welcome relief of a cold drink on a warm day, and the comfort of a heater as it thaws our frozen limbs are sensations that are familiar to most of usindeed, hotness and coldness texture a number of our everyday experiences. We may throw around terms such as heat and temperature casually in conversation to describe what we perceive in such situations, but physics shows us that these are not interchangeable terms. As you will discover in these pages, we must begin at the atomic level of matter to understand heat and the related concepts of temperature and thermal energy as well as the relationships between them.

In physics the term heat refers to the energy that is transferred from a warmer object to a colder one. The energy that is transferred must, of course, come from somewhere. In fact, it all starts with the atoms and molecules that compose all matter. Each of these particles is constantly moving and generating energy from this motion. This is called kinetic energy. The total amount of kinetic energy an object has is equal to its thermal energythat is, the energy that an object has stored as a result of its temperature. To illustrate, lets consider a pot of soup and a cup of soup ladled from it. The thermal energy of the pot is greater because it has more particles that are moving and creating energy.

Temperature represents the average speed of all the particles in an object: the faster its particles move, the higher its temperature will be. Unlike thermal energy, temperature does not depend on the quantity of the substance. In the case of the pot and cup of soup, the average speed of the particles may be the same in both, thus giving them the same temperature even though they differ in size.

In most instances, when heat is transferred from a warmer substance to a colder one, the temperature of the warmer substance will decrease while that of the colder one will increase. This will continue until the two are at the same temperature. At other times, however, heat transfer may not cause a change in temperature at all; rather, the molecules of the heat-absorbing substance may change and rearrange themselves in what is called a phase change. One example of this is the change of water to vapor as it is heated.

Both temperature and heat can be measured. The Fahrenheit and Celsius scales, named after Daniel Gabriel Fahrenheit and Anders Celsius, respectively, are perhaps the most commonly known and are frequently used on temperature-measuring instruments called thermometers. Heat changes in a physical or chemical reaction can be measured with a device called a calorimeter.

Anders Celsius, for whom the Celsius temperature scale is named. Science Source/Photo Researchers, Inc.

The process of heat transfer can occur through a few different methods. Conduction is heat transfer between adjacent parts of a body or through direct contact between two bodies. Convection transfers heat by movement of a heated fluid such as air or water. In contrast, heat transfer through radiation requires neither contact nor a medium to carry the energy. In this process, energy is emitted (radiated) by a heated surface and travels to another surface, where it is absorbed. Heat transfer usually involves some combination of all three of these processes.

The relationships between heat, other forms of energy, temperature, and work form the basis of thermodynamics. The three basic laws of thermodynamics describe the nature of these relationships and hold true for all biological and physical systems. Indeed, much of our understanding of the universe is informed in part by thermodynamics.

Although we may only think about heat incidentally, heat is always around us in one form or another. It is critical to all life, and our ability to tackle issues in such areas as climate change, biology, and technology rests on our application of the fundamental concepts you will encounter in this book.

H eat is so well known from our earliest childhood that we hardly think about it. A steaming bowl of soup, an active radiator, and a sauna feel hot; a book and a chocolate bar at room temperature seem less hot; and an ice cube feels cold. In everyday speech it is common to say that the soup has more heat than the book and that the ice cube has less heat than the chocolate bar. However, people often use the word heat when what they really mean is temperature or a type of energy called thermal energy.

Temperature is a measure of hotness or coldness. In physics, heat refers specifically to energy that is transferred from one thing to another because of a difference in temperature. If two objects at different temperatures are brought together, energy is transferredthat is, heat flowsfrom the hotter object to the colder one. A radiator gives off heat, warming the cooler air around it. If a person holds an unwrapped chocolate bar, his or her hands transmit heat to the chocolate, eventually melting it. It is incorrect to speak of the heat in an object itself because heat is restricted to energy being transferred. Energy that is stored in an object because of its temperature is called thermal energy.