Temperature CHAPTER 14 Heat and Temperature The temperature of a substance is proportional to the average kinetic energy of the substance s particles. As the average kinetic energy of the particles in an object increases, the object s temperature increases. To measure temperature, we use the fact that most substances expand when their temperature increases. Temperature (cont d) Thermometers use the expansion of liquids to measure temperature. Most thermometers contain colored alcohol or mercury. As the temperature increases, the liquids expand. As the temperature decreases, the liquids contract. Temperature Scales The Fahrenheit, Celsius, and Kelvin temperature scales are commonly used for different applications in different parts of the world. The units on the Fahrenheit scale are called degrees Fahrenheit (ºF). Most countries other than the United States use the Celsius scale. One degree Celsius is equal to 1.8 degrees Fahrenheit.
The Kelvin Scale Temperature Scales (cont d) The theoretically lowest temperature that can be recorded is called absolute zero, -273.15ºC. At absolute zero, the kinetic energy of an object would be zero. Absolute zero is the basis of the Kelvin temperature scale. 0 Kelvin is absolute zero. To convert a Celsius temperature to Kelvin, simply add 273 to the temperature. Converting temperature between scales is very useful. Converting Fahrenheit to Celsius TF = 1.8 x TC + 32 Converting Celsius to Fahrenheit TC = (TF - 32)/1.8 Practice Problem 1 The highest temperature ever recorded in Earth s atmosphere was 57.8ºC at Al-Aziziyah, Libya, in 1922. Express this temperature in degrees Fahrenheit and in Kelvin. Practice Problem 2 Express the melting point of gold (1064ºC) in degrees Fahrenheit and in Kelvin.
Practice Problem 3 Express the temperature of a winter day at the North Pole (-40ºC) in degrees Fahrenheit and in Kelvin. Practice Problem 4 A normal body temperature is 98.6 F. What is this temperature in degrees Celsius? Kelvin? Relating Temperature to Energy Transfer When you touch a piece of ice, it feels very cold. When you step into a hot bath, the water feels very hot. We compare the temperatures of two objects that are making direct contact with one another. When two objects that are at different temperatures are touching, energy will be transferred from one to the other. The energy transferred between the particles of two objects, because of a temperature difference is called heat. Methods of Energy Transfer Heat energy can be transferred in three ways: conduction, convection, and radiation. Conduction takes place when objects that are in direct contact are at unequal temperatures. Convection occurs due to the energy transfer resulting from the movement of warm fluids. Radiation does not require physical contact between objects. We give off electromagnetic waves as infrared radiation.
Conductors and Insulators Specific Heat A conductor is a material through which energy can be easily transferred as heat. An insulator is a material that transfers energy poorly. Heat energy is transferred through particle collisions. What makes a substance a good or poor conductor depends in part on how much energy is required to change the temperature of the substance by a certain amount. Specific heat is used to describe how much energy is required to raise an object s temperature. For all substances, specific heat is a characteristic physical property. Specific heat (c) is the amount of energy required to raise the temperature of 1 kg of that substance by 1 K. Specific heat can be used to figure out how much energy it takes to raise an object s temperature. Energy = specific heat x mass x temperature change E = cm T Problem 1 How much energy must be transferred as heat to 200 kg of water in a bathtub to raise the water s temperature from 25ºC to 37ºC? Problem 2 How much energy is needed to increase the temperature of 755 g of iron from 283 K to 403 K?
Problem 3 How much energy must a refrigerator absorb from 225 g of water to decrease the temperature of the water from 35ºC to 5ºC? Problem 4 During an experiment, a piece of aluminum foil is heated from 27 C to 98 C. If the foil absorbed 344 J of energy, what is the mass of the foil? Problem 5 How much energy is required to raise the temperature of 5.0 g of silver from 298 K to 334 K? Laws of Thermodynamics The first law of thermodynamics states that the total energy used in any process is conserved, whether that energy is transferred as a result of work, heat, or both. Energy = work + heat The second law states that the energy transferred as heat always moves from an object at a higher temperature to an object at a lower temperature. Entropy is a measure on the disorder of a system.