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The Relationships between Temperature, Energy, and Heat As a small child, you learned to avoid objects that are "hot." You also discovered that if you forget to wear your coat outside, you can become "cold." These basic notions about hot and cold carry over into physics. Temperature depends on...

The Relationships between Temperature, Energy, and Heat As a small child, you learned to avoid objects that are "hot." You also discovered that if you forget to wear your coat outside, you can become "cold." These basic notions about hot and cold carry over into physics. Temperature depends on average kinetic energy If you touch a hot object, you feel an immediate sensation of pain. What causes the pain? It turns out that the pain is related to the speed of the particles (typically molecules) in the hot object. Experiments show that the particles in a hot object move faster than those in a cold object. This observation leads to the physics definition of temperature: Temperature is proportional to the average kinetic energy of particles in a substance.The kinetic energy of a particle changes with time as it is jostled and bumped randomly by its neighbors---only the average value of its kinetic energy is related to the temperature. The total amount of energy in a substance---the sum of all of its kinetic and potential energy---is referred to as its internal energy, or thermal energy. Thus, an object's thermal energy refers to both the random motion of its particles (kinetic energy) and the separation and orientation of its particles relative to one another (potential energy). Adding thermal energy to a system is known as heating and removing thermal energy is known as cooling. A temperature difference causes energy to flow The water-filled U-tube in Figure 10.1 provides a useful analogy for temperature and thermal energy. Think of the height of the water as the temperature and the amount of water as the thermal energy. The U-tube is in equilibrium when the height of water is the same on the two sides. Suppose you start with the water at a greater height on the right side, as in Figure 10.2. As you know, water will flow to the left side until the water levels are the same---even though there is more water on the left side to begin with. Similar behavior occurs when a hot object is placed in contact with a cold object. Thermal energy flows from the hot object to the cool object until the temperatures are the same---even if the cool object starts out with more energy. Temperature doesn't depend on the total amount of energy in an object; it depends on the average kinetic energy of the particles in the object.Energy Flow Imagine putting a hot brick in contact with a cold brick, as in Figure 10.3. When a rapidly vibrating molecule in the hot brick collides with a slowly vibrating molecule in the cold brick, the slow molecule picks up speed and the fast molecule slows down. As a result, energy is transferred from the hot brick to the cold brick. Occasionally, a molecule in the cold brick gives some energy to a molecule in the hot brick, but overall it is much more likely that molecules in the cold brick will gain energy. Thermal Equilibrium Energy transfer continues between the bricks until they reach the same temperature. When this happens, the amount of energy transferred between the bricks is the same in either direction, and we say that the system is in thermal equilibrium. To be specific, a system is in thermal equilibrium when its temperature is constant and there is no net transfer of energy. So, although the bricks exchange energy back and forth as long as they are in contact, the total energy in each brick remains the same when they are in equilibrium. Now imagine that you place a small block of metal in a large bucket of water. Will thermal energy flow between the block and the water? To answer this question, you only need to measure the temperature of each. If the temperatures are the same, then there is no net flow of thermal energy. If the temperatures are different, thermal energy flows from the warmer object to the cooler one. Nothing else matters---not the type of metal, its mass, its shape, the amount of water, whether the water is fresh or salt, and so on. All that matters is the temperature of each. This conclusion is referred to as the zeroth law of thermodynamics: The Zeroth Law of Thermodynamics Objects in contact with one another are in thermal equilibrium if they have the same temperature. Nothing else matters **Heat is the flow of thermal energy** When you put a cool pan of water on a hot stove burner, we say that you are "heating" the water. The fast particles in the burner collide with the slow particles in the pan of water and cause them to speed up. This kind of transfer of energy is referred to as *heat*. In general, heat is the energy that is transferred between objects because of a temperature difference. Because heat is energy, it is measured in joules. When we say that there is a "transfer of thermal energy" or a "flow of thermal energy" from a hot brick to a cold brick, we simply mean that the total energy of the hot brick decreases and the total energy of the cold brick increases. Thus, an object does not "contain" heat---what it does contain is thermal energy. **An object has a certain amount of thermal energy,** **and the thermal energy it *exchanges*** **Measuring Temperature** A variety of temperature scales are used in everyday situations and in physics. Let's look at the most common temperature scales and see how to convert from one to another. **The Celsius scale is used in most of the world** Perhaps the easiest temperature scale to remember is the Celsius scale, named in honor of the Swedish astronomer Anders Celsius (1701--1744). By definition, water freezes at zero degrees Celsius, which we abbreviate as 0 °C. In addition, water boils at one hundred degrees Celsius, or 100 °C. The choice of the zero level for a temperature scale is completely arbitrary, as is the number of degrees between any two reference points. In the Celsius scale, as in others, there is no upper limit to the value a temperature may have. There is a lower limit, however. In the Celsius scale the lowest possible temperature is -273.15 8C, as we shall see later in this lesson. **The Fahrenheit scale is used in the United States** The Fahrenheit scale was developed by Gabriel Fahrenheit (1686--1736), who chose zero to be the lowest temperature he was able to achieve in his laboratory. He also chose 96 degrees to be body temperature, though why he made this choice is not known. In the modern version of the Fahrenheit scale, body temperature is 98.6 °F. In addition, water freezes at 32 °F and boils at 212 °F. The Fahrenheit scale not only has a different zero point than the Celsius scale, it also has a different size for its degree. For example, 180 Fahrenheit degrees make up the span from the freezing to the boiling point of water. Only 100 degrees are needed for this span on the Celsius scale, however. Therefore, the Fahrenheit degrees are almost one-half the size of the Celsius degrees. The precise ratio is 100 180 = 5 9 To convert to a Fahrenheit temperature, *T*F, from a Celsius temperature, *T*C, we can use the following relationship: Conversion between Degrees Celsius and Degrees Fahrenheit Fahrenheit temperature = 95 1Celsius temperature2 + 32 *T*F = 95 *T*C + 32 Similarly, a conversion in the opposite direction is given by the following: Conversion between Degrees Fahrenheit and Degrees Celsius Celsius temperature = 59 1Fahrenheit temperature - 322 *T*C = 59 1*T*F - 322 The following Guided Example shows how the conversion equations are used..