Lecture 2: Zero law of thermodynamics

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Diana McCoy
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1 Lecture 2: Zero law of thermodynamics 1. Thermometers and temperature scales 2. Thermal contact and thermal equilibrium 3. Zeroth law of thermodynamics 1. Thermometers and Temperature scales We often associate the concept of temperature with how hot or cold an object feels when we touch it. The temperature is a measure of the average translational kinetic energy associated with the disordered motion of atoms and molecules. Thermometers are devices that are used to measure the temperature of a system. All thermometers are based on the principle that some physical property of a system changes as the system s temperature changes. Some physical properties that change with temperature are: (1) the volume of a liquid, (2) the dimensions of a solid, (3) the pressure of a gas at constant volume, (4) the volume of a gas at constant pressure, (5) the electric resistance of a conductor, and (6) the color of an object. A temperature scale can be established on the basis of any one of these physical properties. A common thermometer in everyday use consists of a mass of liquid usually mercury or alcohol that expands into a glass capillary tube when heated (Fig. 2). 1

2 Figure 1: As a result of thermal expansion, the level of the mercury in the Thermometer rises as the mercury is heated by water in the test tube. In this case the physical property that changes is the volume of a liquid. Any temperature change in the range of the thermometer can be defined as being proportional to the change in length of the liquid column. The thermometer can be calibrated by placing it in thermal contact with some natural systems that remain at constant temperature. One such system is a mixture of water and ice in thermal equilibrium at atmospheric pressure. Temperatures are measured in one of the three standard temperature scales (Celsius, Kelvin, and Fahrenheit). To define a temperature scale, two fixed points should be chosen. The interval between these two points is called the fundamental interval. The fixed points are usually chosen to be the melting point of ice and the boiling point of water. Ice point is the temperature at which pure ice melts at 0 0 C Steam point is the temperature at which pure water boils under normal conditions at C. On the Celsius temperature scale, this mixture is defined to have a temperature of zero degrees Celsius, which is written as 0 C; this temperature is called the ice point of water. Another commonly used system is a mixture of water and steam in thermal equilibrium at atmospheric pressure; its temperature is 100 C, which is the steam point of water. Once the liquid levels in the thermometer have been established at these two points, the length of the liquid column between the two 2

3 points is divided into 100 equal segments to create the Celsius scale. Thus, each segment denotes a change in temperature of one Celsius degree. The Celsius, Fahrenheit, and Kelvin Temperature Scales The relation between Celsius, Fahrenheit, and Kelvin Temperature Scales is: 3

2. Thermal contact and thermal equilibrium To understand the concept of temperature, it is useful to define two often-used phrases: thermal contact and thermal equilibrium.

If the objects are at different temperatures, energy is exchanged between them, even if they are initially not in physical contact with each other.

4 2. Thermal contact and thermal equilibrium To understand the concept of temperature, it is useful to define two often-used phrases: thermal contact and thermal equilibrium. Imagine two objects that are placed in an insulated container such that they interact with each other but not with the environment. If the objects are at different temperatures, energy is exchanged between them, even if they are initially not in physical contact with each other. We assume that two objects are in thermal contact with each other if energy can be exchanged between them by these processes due to a temperature difference. Thermal equilibrium is a situation in which two objects would not exchange energy by heat or electromagnetic radiation if they were placed in thermal contact. Thermal contact Objects are in thermal contact if heat can flow between them. Thermal equilibrium Objects that are in thermal contact, but have no heat exchange between them, are said to be in thermal equilibrium. 4

5 3. Zeroth law of thermodynamics Let us consider two objects A and B, which are not in thermal contact, and a third object C, which is our thermometer. We wish to determine whether A and B are in thermal equilibrium with each other. The thermometer (object C) is first placed in thermal contact with object A until thermal equilibrium is reached, as shown in Figure 1 (a). Figure 1 The zeroth law of thermodynamics. (a) and (b) If the temperatures of A and B are measured to be the same by placing them in thermal contact with a thermometer (object C), no energy will be exchanged between them when they are placed in thermal contact with each other (c). From that moment on, the thermometer s reading remains constant, and we record this reading. The thermometer is then removed from object A and placed in thermal contact with object B, as shown in Figure 1(b). The reading is again recorded after thermal equilibrium is reached. If the two readings are the same, then object A and object B are in thermal equilibrium with each other. If they are placed in contact with each other as in Figure 1(c), there is no exchange of energy between them. 5

6 We can summarize these results in a statement known: as the zeroth law of thermodynamics (the law of equilibrium): If objects A and B are separately in thermal equilibrium with a third object C, then A and B are in thermal equilibrium with each other. The "zeroth law" states that if two systems are at the same time in thermal equilibrium with a third system, they are in thermal equilibrium with each other. Zeroth law of thermodynamics can be enables us to define temperature. The temperature is the property that determines whether an object is in thermal equilibrium with other objects. 6

Dr.Salwa Alsaleh fac.ksu.edu.sa/salwams

Dr.Salwa Alsaleh fac.ksu.edu.sa/salwams Dr.Salwa Alsaleh Salwams@ksu.edu.sa fac.ksu.edu.sa/salwams What is Temperature? It is the measurement of the AVERAGE kinetic energy of the particles of matter. Temperature We associate the concept of temperature