What is Temperature? ZWhen we think of temperature we think of how hot or cold something is. ZTemperature – the measure of the average value of kinetic energy of molecules in random motion.
ZThink of a glass of water… ZIts made of molecules that are in random motion. ZRandom = different speeds – some fast, some slow ZIf they are moving, the molecules have KE. ZTemperature measures the average value of their movement.
ZThe more kinetic energy the molecules have, the higher the temperature. ZHigh temp = faster molecules ZLow temp = slower molecules ZEx. Water molecules move faster in a cup of hot water than they do in cold water.
Thermal Expansion ZWhen molecules speed up, they move farther apart. ZThe object expands (stretches) ZWhen molecules slow down, they move close together. ZThe object contracts (shrinks)
ZSolids: ZIn the summertime power lines will sag, whereas in the winter they will be pulled more tightly. ZLiquids: ZYou probably been told never to put a glass bottle of soda in the freezer. Why? It can explode and you will have glass all over you freezer. ZGases: ZPlace a balloon in a very warm room and it will expand, then place it in a very cold room and it will shrink.
The amount of expansion or contraction depends on: ZThe type of material ZLiquids expand more than solids ZThe degree to which temperature changes ZThe greater the temperature change, the more expansion or contraction
Measuring Temperature ZThermometer – a device used to measure temperature ZConsists of a liquid such as mercury or alcohol sealed inside a narrow tube with markings symbolizing degrees. ZTemperature changes cause the liquid to rise and fall as the liquid expands and contracts.
Temperature Scales ZThere are three common scales: ZFahrenheit ZCelsius ZKelvin ZEach is divided into regular intervals
Fahrenheit Scale ZThe most common in the U.S. ZDivided into degrees Fahrenheit ( o F) ZFreezing point of water = 32 o F ZBoiling point of water = 212 o F ZSpace between boiling and freezing is 180 equal degrees.
Celsius Scale ZMost commonly used in the rest of the world ZDivided into degrees Celsius ( o C) ZFreezing point of water = 0 o C ZBoiling point of water = 100 o C ZSpace between boiling and freezing is 100 equal degrees Z o C are bigger than o F
Kelvin Scale ZThe SI Unit for temperature ZDivided into Kelvins (not degrees!) ZBoiling point of water = 373 K ZFreezing point of water = 273K ZAbsolute zero = 0 K (the lowest possible temperature an object can have!) ZKelvin and Celsius degrees are the same size!
Temperature Conversions ZFahrenheit Celsius Z o C = (5/9)( o F – 32) ZCelsius Fahrenheit Z o F = (9/5)( o C) + 32 ZPlease Excuse My Dear Aunt Sally!
Temperature Conversions ZKelvin can only be calculated if you know the object’s temperature in o C. ZK = o C + 273 Z o C = K - 273
Thermal Energy ZIs the sum of the KE and PE of all molecules in an object. ZAll molecules have PE that can be converted into KE ZThey exert attractive forces to each other ZPE increases as molecules are farther apart.
Temperature Vs. Thermal Energy ZHow hot or cold something feels does not necessarily give you enough information to determine thermal energy. ZThermal energy is related to quantity of the molecules!
Examples ZA glacier and an ice cube have the same temperature, but the glacier has much more thermal energy due to its size. ZCompare a glacier to a boiling pot of water. ZWhich has a higher temperature? ZWhich has more thermal energy?
Heat ZIs thermal energy that is transferred from one object to another when the objects are at different temperatures ZThe amount of heat that is transferred when two objects are brought into contact depends on the difference in temperature between the two objects. ZHeat continues to be transferred until equilibrium is reached (both objects are at the same temperature).
Transfer of Heat ZThermal energy always moves from warmer to cooler objects. ZHeat never flows from a cooler object to a warmer object. ZThe warmer object loses thermal energy and becomes cooler while the cooler object gains thermal energy and becomes warmer until their temperatures are equal.
ZThere are three ways heat can be transferred: ZConduction ZRadiation ZConvection
Conduction ZThe transfer of thermal energy by direct contact ZEx: The bottom of a pot on a stove is heated by direct contact between it and the heat source ZOccurs when particles in a material collide with neighboring particles. Energy is passed from molecule to molecule during these collisions.
ZOccurs most easily in solids and liquids where atoms and molecules are closer together ZAs a result, heat is transferred more rapidly by conduction in solids and liquids than in gases.
Radiation ZThe transfer of energy in the form of electromagnetic waves ZEx: You feel the warmth of the Sun when standing on the beach. ZThese waves can carry energy through empty space as well as through solids, liquids, and gases.
Convection ZThe transfer of thermal energy by the movement of molecules from one part of a material to another ZEx: water boiling in a pot on the stove ZOccurs most easily in liquids or gases where molecules can move much more easily than in a solid ZForms convection currents
Convection Current ZA continuous cycle of heating and rising, cooling and sinking ZA substance is heated which makes it less dense and causes it to rise. When it rises it cools, which causes it to become more dense and sink. The process continually repeats itself.
Natural Convection ZNatural convection occurs when a warmer, less dense fluid is pushed away by a cooler denser fluid. ZEx: Wind movement near a lake or ocean is caused when air over land is heated and becomes less dense. Denser cool air rushes in, pushing the warm air up. The cooler air is then heated by the land and the cycle is repeated. You feel the cooler air rushing in as wind.
Forced Convection ZOccurs when an outside force, such as a fan, pushes a fluid, such as air or water, to make it move and transfer heat ZEx: Computer use fans to keep electronic components from getting too hot.
Thermal Conductors ZAny material that easily transfers heat ZIf a material has some electrons that are not held tightly by the nucleus, they are free to move around and bump into each other, transferring thermal energy. ZThe best thermal conductors are metals, such as gold and copper.
Thermal Insulators ZAny material in which heat does not flow easily ZLiquids and gases are usually better insulators than solids because their particles are farther apart and cannot transfer thermal energy by conduction as easily as a solid.
ZAir is a good insulator. Many insulating materials contain air spaces. ZGood conductors are poor insulators and vice-a-versa.
Specific Heat ZThe amount of heat needed to raise the temperature of 1 kg of substance by 1 °C ZMore heat is needed to change the temperature of a material with a high specific heat than one with a low specific heat.
ZEx: Sand heats up and cools down faster than water. ZHotter than the water during the day ZCooler than the water at night
Thermal Pollution ZThe increase in the temperature of a body of water caused by adding warmer water ZSources: ZHot water produced by power plants that is released into a body of water ZRain falling on hot asphalt that runs off into waterways
ZEffects: ZContains less oxygen ZCan cause some organisms to become more sensitive to chemical pollutants, parasites, & disease ZReducing thermal pollution: ZUse cooling towers to cool warm water before it is released into waterways