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Download Chapter 13 Heat and Temperature
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Key Terms Temperature Thermometer Absolute Zero Heat What does Temperature Indicate? Temperature is a measure of how hot or cold something is. A measure of the average kinetic energy of the particles in an object. Temperature and Energy All particle in an object are constantly moving. These moving particles have kinetic energy which is energy in motion. If we take the average of the kinetic energy it turns out that it is proportional to the temperature of the object. Common thermometers rely on expansion Thermometers are based on the principle of objects expanding when heated and contracting when cooled. Thermometers use colored alcohol that expands as their temperature increases and contracts as their temperature falls, because of energy exchange. Why do thermometers no longer use mercury? Thermometers can use different methods To measure temperature we can use liquids, metals, or electric currents. Liquid thermometers measure only a certain temperature range because if the temp is to high the liquid boils and to low it freezes. Metal thermometers work by using two different types of metals that have different expansion and contraction rates. Digital thermometers measure changing currents. Changes in temperatures cause electric currents to change. Fahrenheit and Celsius are common scales used for measuring temperature Fahrenheit (F) water freezes at 32F and boils at 212F Most countries use the Celsius (or centigrade) scale Celsius (C) water freezes at 0C and boils at 100C. The difference between the two points is divided into 100 equal parts, called degrees Celsius (C) Converting between the two different measure is possible by using conversion equations Fahrenheit Temperature = (1.8 x C) + 32 Celsius Temperature = (F - 32)/1.8 The Kelvin scale is based on absolute zero Absolute zero is the temperature that all energy in an object is zero. The energy of an object cannot go any lower. Absolute zero is the basis for another temperature called the Kelvin scale. Zero Kelvin or 0 K, is absolute zero. In the Kelvin scale there are no negative temperature values. To convert from the Celsius to Kelvin you add 273 to the object’s temperature in degrees Celsius Kelvin Temperature = Celsius + 273 Relating Temperature to Energy Transfer The feeling associated with temperature difference results from energy transfer If you hold an ice cube the energy of the ice cube is less then the energy of your hands. When the molecules of your hand come into contact with the ice, they cause the ice molecules to speed up increasing their kinetic energy. Temperature changes indicate an energy transfer Heat is the energy transferred between objects that are at different temperatures. The transfer of energy always takes place from a substance at a higher temperature to a substance at a lower temperature. Because temperature is an indicator of the average kinetic energy of internal particles, you can use temperature to predict which way energy will be transferred. Key Terms Thermal Conduction Convection Convection Current Radiation Specific Heat Methods of Energy Transfer The transfer of heat energy from a hot object can occur in three ways conduction, convection, or radiation Conduction involves objects in direct contact Thermal conduction - the transfer of energy as heat through a material Convection results from the movement of warm fluids Convection is the movement of matter due to differences in density that are caused by temperature variations. This is possible only if the substance is a fluid, either a liquid or a gas, because particles within solids are not as free to move Heated fluids have convection currents Convection current is the vertical movement of air currents due to temperature variations Radiation does not require physical contact between objects Energy that is transferred as electromagnetic waves is called radiation Radiation differs from conduction and convection in that it does not involve the movement of matter Radiation is therefore the only method of energy transfer that can take place in a vacuum, such as outer space. Conductors and Insulators Conductors are materials which energy can be easily transferred as heat. Insulators are materials which energy cannot be easily transferred as heat. Energy transfers through particle collisions Gases and liquids are extremely poor conductors because their molecules are far apart. Some solids such as rubber and wood do not conduct energy well. For this reason they are used as insulators. Metals, such as copper and silver, conduct energy very well. Metals are better conductors than nonmetals Specific Heat Specific heat describes how much energy is required to raise an object’s temperature. Specific heat is a characteristic physical property, which is denoted by ( c ). Key Terms Refrigerant Heat Engine How is heating or cooling your house possible? By machines that transfer energy as heat from one place to another. Two principles about energy account for this transfer The first principle is that the total energy used in any process - whether that energy is transferred as a result of work, heat, or both - is conserved. This principle of conservation of energy is called the first law of thermodynamics. The second principle is that energy transferred as heat always moves from an object at a higher temperature to an object at a lower temperature . Heating Systems Work can increase average kinetic energy. The process that involves energy transfer by work are called mechanical processes. Some of the energy from food is transferred as heat to blood moving throughout the human body Heated water or air transfers energy as heat in central heating systems Hot-water, steam, and hot-air systems heat buildings by circulating heated fluids to each room Solar heating systems also use warmed air or water Reptiles bask in the sun to raise their body temperature An active solar heating system moves solar-heated water through pipes and a heat exchanger There are two types of solar heating systems 1. Active solar heating systems - they require extra energy from another source, such as electricity. 2. Passive solar heating systems - this type of system requires no extra energy Usable energy decreases in all energy transfers Insulators minimizes undesirable energy transfers Cooling Systems In all cooling systems, energy is transferred as heat from one substance to another, leaving the first substance with less energy and thus a lower temperature . Cooling systems often use evaporation to transfer energy from their surroundings Refrigerant is a material used to cool an area or an object to a temperature that is lower than the temperature of the environment. Condensation transfers energy to the surrounding Heat pumps can transfer energy to or from rooms A heat pump is a refrigeration unit in which the cooling cycle can be reversed Heat Engines Heat engines are machines that transforms heat into mechanical energy or work. The two main types of heat engines internal combustion engines and external combustion engines - are named for where combustion takes place. Internal combustion engines burn fuel inside the engine Not all internal combustion engines work alike In an internal combustion engine, only part of the potential chemical energy is converted to mechanical energy.
