Physics 4C. Chapter 18: Temperature and Heat. Thermodynamics

Transcription

1 Physics 4C Chapter 18: Temperature and Heat Zeroth Law of Thermometers Constant Volume Gas Thermometer Thermal Expansion Unusual Behavior of Water thermodynamics the study of temperature, heat, and the exchange of energy between a system and its environment How do we usually judge the temperature of an object? By how hot or cold it feels. Is this reliable? No!!! For example: a metal spoon and a cardboard package of frozen peas in the freezer are both at the same temperature but the spoon will feel colder to the touch! We need a reliable and reproducible method for establishing the relative hotness and coldness of objects that is based solely on the temperature of the object. When hot and cold water are mixed together, they will eventually reach the same temperature. Energy is transferred from the hot object to the cold object. heat energy transferred between objects because of a temperature difference 1

2 Two objects are said to be in thermal contact if they can exchange heat. Two objects in thermal contact will eventually reach thermal equilibrium. The flow of heat ceases because there is no temperature difference (they have reached the same temperature). A T Zeroth Law of B zeroth law of thermodynamics if bodies A and B are each in thermal equilibrium with a third body T, then they are in equilibrium with each other This allows us to define temperature as a property of an object and use T to measure temperature. T A B Zeroth Law of If the zeroth law of thermodynamics were not true, than on object could have many different temperatures depending on what objects were in thermal equilibrium with it. How can we use object T as a thermometer? Measure some change in its physical properties. thermometric property a physical property that changes with temperature Thermometers For example, most gases, liquids, and solids expand when heated so their volume changes common thermometer temperature measured by looking at changes in the volume of mercury or alcohol thermometers are calibrated by placing them in thermal contact with objects at two calibration temperatures, usually the ice point (0 o C) and the steam point (100 o C) 2

3 Constant-Volume Gas Thermometer The constant-volume gas thermometer is the standard thermometer against which all other thermometers are calibrated. For a gas kept at constant volume, the gas pressure increases with temperature. Constant-Volume Gas Thermometer The height of the mercury in column A is kept constant so the volume of the gas is constant. The height h of the mercury in column B then gives a measure of the gas pressure and hence the temperature of the bath. Constant-Volume Gas Thermometer As the gas is cooled, its pressure decreases. The decrease in pressure is approximately linear. Constant-Volume Gas Thermometer In every case, regardless of what gas is used, the pressure extrapolates to zero when the temperature is o C. This line can be extrapolated backwards to find the temperature at which the pressure is zero. Since you can not have a negative pressure, o C represents the lowest possible temperature and is known as absolute zero. 3

4 Thermal Expansion Thermal Expansion Extreme heat caused these railroad tracks to buckle because of thermal expansion. To compensate for thermal expansion, bridges have expansion joints and one end of the bridge rides on rockers (the other end is fixed). The Unusual Behavior of Water The Unusual Behavior of Water Although most materials expand when heated, a few do not. For example, if water at 0 o C is heated, its volume actually decreases until its temperature reaches 4 o C. Above 4 o C water behaves normally, and its volume increases as the temperature increases. The unusual behavior of water is very important to aquatic life on this planet. Why? As the temperature of water decreases below 4 o C, the volume of water increases so the density of water decreases. 4

5 The Unusual Behavior of Water Because of this, ice is less dense than water and therefore floats. Because of this, ponds and lakes freeze from the top down. If water behaved like most liquids, its volume would decrease as its temperature decreased. Ice would then be more dense than water and would sink. Bodies of water would then freeze from the bottom up and many bodies of water would remain frozen all year round aquatic life as we know it could not exist! When part of a fluid (liquid or gas) is warmed, such as the air above a fire, the volume of the fluid expands and therefore the density of the fluid decreases. The surrounding fluid is cooler and more dense. Because of this, the surrounding cooler fluid exerts a buoyant force on the warmer fluid and pushes it upward. As the warmer fluid rises, the surrounding fluid replaces it. The cooler fluid, in turn, is warmed and pushed upward. Because of convection, hot air rises and creates updrafts or thermals. This process continues, and a continuous flow called a convection current is established, which transfers heat. convection the process in which heat is carried from place to place by the bulk movement of a fluid. 5

6 Examples of Currents 6