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Key Question: What is the relationship between heat, temperature, and energy? * Temperature is NOT the same as thermal energy. Thermal energy is energy stored in materials because of differences in temperature. The thermal energy of an object is the total amount of random kinetic energy for all the atoms in the object. Remember, temperature measures the random kinetic energy of each atom. Imagine heating a cup of coffee to a temperature of 100°C. Next think about heating up 1,000 cups of coffee to 100°C. The final temperature is the same in both cases but the amount of energy needed is very different. Heat flows from the hot coffee to the cooler air in the room. Heat is what we call thermal energy that is moving. The joule (J) is the unit of heat (or thermal energy) used for physics and engineering. The calorie is a unit of heat often used in chemistry. The specific heat is the quantity of heat it takes to raise the temperature of one kilogram of material by one degree Celsius. The temperature of gold rises quickly compared with water because its specific heat is much less than the specific heat of water. Heat energy (J) Specific heat (J/kgoC) Q = mC(T) Mass (kg) Change in Temperature (oC) One kilogram of water is heated in a microwave oven that delivers 500 watts of heat to the water. One watt is a flow of energy of one joule per second. If the water starts at 10°C, how much time does it take to heat up to 100°C? Energy loss is equal to energy gain. Key Question: What is temperature? * All thermometers are based on some physical property (such as color or volume) that changes with temperature. A thermistor is a device that changes its electrical resistance as the temperature changes. A thermocouple is another electrical sensor that measures temperature. Temperature measures the kinetic energy per atom due to random motion. Random motion is motion that is scattered equally in all directions. In pure random motion the average change in position is zero. When the temperature gets down to absolute zero, the atoms are said have the lowest energy they can have and the temperature cannot get any lower. Technically, we believe atoms never stop moving completely. Figuring out what happens when atoms are cooled to absolute zero is an area of active research. The three most common phases of matter are called solid, liquid, and gas. At temperatures greater than 10,000 K the atoms in a gas start to break apart. In the plasma state, matter becomes ionized. When thermal energy is added or subtracted from a material, EITHER the temperature changes, or the phase changes. Generally the phase change occurs and then temperature rises The melting point is the temperature at which a material changes phase from solid to liquid. Melting occurs when the kinetic energy of individual atoms equals the attractive force between atoms. The heat of fusion is the amount of energy it takes to change one kilogram of material from solid to liquid or vice versa. Heat energy (J) Q = mLf Heat of Fusion (J/kg) Mass (kg) How many joules of energy does it take to melt a 30 gram ice cube at 0°C? The boiling point is the temperature at which the phase changes from liquid to gas. Just as with melting, it takes energy for an atom to go from liquid to gas. The heat of vaporization is the amount of energy it takes to convert one kilogram of liquid to one kilogram of gas. Heat energy (J) Q = mLv Heat of Vaporization (J/kg) Mass (kg) A steam iron is used to remove the wrinkles from clothes. The iron boils water in a small chamber and vents steam out the bottom. How much energy does it require to change one-half gram (0.0005 kg, or about half a teaspoon) of water into steam? Greatest Energy Gas Liquid Least energy Solid If a material is moving from solid to liquid, is heat energy gained or lost? gained If a material moves from a gas to a liquid, is energy gained or lost? lost