Chapter 15 Temperature, Heat, and Expansion Although the temperature of these sparks exceeds 2000ºC, the heat they impart when striking my skin is very small. Temperature and heat are different concepts. Temperature Heat Specific Heat Capacity 1
Temperature All matter is composed of continually jiggling atoms or molecules. Whether the atoms and molecules combine to form solids, liquids, gases depends on how fast they are moving. By virtue of their motion, the molecules or atoms in matter possess kinetic energy. When a solid, liquid, or gas gets warmer, its atoms or molecules move faster. They have more kinetic energy. The average kinetic energy of the individual particles is directly related to how hot something feels. The quantity that tells how warm or cold an object is with respect to some standard is called temperature. 2
Different temperature scales Most thermometers measure temperature by means of the expansion and contraction of a liquid, usually mercury or colored alcohol. Fahrenheit and Celsius scales on a thermometer Three different temperature scales: Celsius thermometer Fahrenheit thermometer The Kelvin scale: this scale is calibrated not in terms of the freezing and boiling points of water, but in terms of energy itself. The number 0 is assigned to the lowest possible temperature- absolute zero, at which a substance has absolutely no kinetic energy to give up. 3
Understanding temperature Temperature is related to the random motion of atoms and molecules in a substance. More specifically, temperature is proportional to the average translational kinetic energy of molecular motion (motion that carries the molecule from one place to another). Molecules may also rotate or vibrate, with associated rotational or vibrational kinetic energy - but these motions do not directly affect temperature. 4
Example - understanding temperature True or false? Temperature is a measure of the total kinetic energy in a substance. Answer: False. Why? Temperature is a measure of the average (NOT total!) translational kinetic energy of individual molecules in a substance. For example, the temperatures of 2 liters of boiling water and 1 liter of boiling water are the same. This is because the average translational kinetic energy per molecule is the same in each. However, the total molecular kinetic energy in 2 liters of boiling water is twice as much as in 1 liter of boiling water. 5
Heat Heat: The energy transferred from one thing to another because of a temperature difference between the things is called heat. The direction of spontaneous energy transfer is always from a warmer thing to a neighboring cooler thing. Understanding heat: It is important to point out that matter does not contain heat. Heat is energy in transit from a body of higher temperature to one of lower temperature. Measuring heat: heat is the flow of energy from one thing to another due to a temperature difference. Heat is a form of energy. It is measured in joules or calories. 6
Internal energy Internal energy: the grand total of all energies inside a substance, including the translational kinetic energy of jostling molecules and energy in other forms in a substance. A substance does not contain heat - it contains internal energy. When a substance absorbs or gives off heat, internal energy in the substance increases or decreases. 7
Distinguish between temperature and heat Temperature is a measure of the average translational kinetic energy of the individual molecules in a substance. How much heat flows depends not only on the temperature difference between substances but on the amount of material as well. Look at the example on the left: the temperature of the sparks is very high, about 2000ºC. This means that the energy per molecule is very high. However, because there are few molecules per spark, the total energy transferred (the heat) each spark impart when striking our skin is safely small. 8
Example - understanding heat and temperature Although the same quantity of heat is added to both containers, the temperature increases more in the container with the smaller amount of water. Suppose you apply a flame to 1 liter of water for a certain time and its temperature rises by 2 ºC. If you apply the same flame for the same time to 2 liters of water, by how much will its temperature rise? Hint: think about temperature is a measurement of the average translational kinetic energy of individual molecules in a substance. Answer: its temperature will rise by only 1 ºC. Although the same quantity of heat is added to both, there are twice as many molecules in 2 liters of water and thus each molecule receives only half as much energy on the average. 9
Distinguish between temperature and internal energy There is more internal energy in the larger container filled with warm water than in the small cup filled with hot water. For things in thermal contact, heat flow is from the substance at a higher temperature to a substance at a lower temperature, but not necessarily a flow from a substance with more internal energy to a substance with less internal energy. Having more internal energy does not necessarily mean having a higher temperature, and vice versa. 10
Specific heat capacity Why do some foods remain hotter much longer than others do? The filling of hot apple pie may be too hot to eat even though the crust is not. Different materials require different quantities of heat to raise the temperature of a given mass of the material by a specified number of degrees. Specific heat capacity: the quantity of heat required to change the temperature of a unit mass of the substance by 1 degree. 11
Understanding specific heat Because water has a high specific heat capacity, it takes more energy to warm the water than to warm the land. We can think of specific heat capacity as thermal inertia. Specific heat capacity is like a thermal inertia since it signifies the resistance of a substance to a change in temperature. For example: water has a high specific heat capacity. A relatively small amount of water absorbs a great deal of heat for a correspondingly small temperature rise - water is a very useful cooling agent. Water also takes a long time to cool - hot-water bottles were employed on cold winter nights. 12
Example - understanding specific heat capacity Desert sand is very hot in the day and very cool at night. What does this tell you about its specific heat capacity? Sand has low specific heat capacity and it is easy for sand to change its temperature. - desert sand warms quickly in the morning sun and cools quickly at night. 13
Homework 7 (03/05/04) Chapter 15, P. 303, Exercises: 5, 13, 15. 14