Transcript Document
Project PRISE Connie Walker, National Optical Astronomy Observatory Convection • Perhaps the first thing that most people say is "heat rises". While not wrong, what you should say is "hot air rises" or "hot water rises". • Anything fluid - that is a gas or a liquid - will tend to change density with changes in temperature. • For example, if heated, air decreases in density. The surrounding air is cooler and denser. This makes it heavier, so it falls beneath the hot air, forcing it upwards. This is convection. Convection in rooms • Radiators heat rooms in some houses. This is a bad name for them - as they give off heat mainly by convection! • The convector heater warms the air in contact with it. This becomes less dense, and rises. • The ceiling forces this air to circulate as shown, warming the air around it. • Finally, when the air has cooled, it falls downwards, completing the cycle. Convection in rooms • Radiators heat rooms in some houses. This is a bad name for them - as they give off heat mainly by convection! • The convector heater warms the air in contact with it. This becomes less dense, and rises. • The ceiling forces this air to circulate as shown, warming the air around it. • Finally, when the air has cooled, it falls downwards, completing the cycle. Convection in rooms • Radiators heat rooms in some houses. This is a bad name for them - as they give off heat mainly by convection! • The convector heater warms the air in contact with it. This becomes less dense, and rises. • The ceiling forces this air to circulate as shown, warming the air around it. • Finally, when the air has cooled, it falls downwards, completing the cycle. Convection in rooms • Radiators heat rooms in some houses. This is a bad name for them - as they give off heat mainly by convection! • The convector heater warms the air in contact with it. This becomes less dense, and rises. • The ceiling forces this air to circulate as shown, warming the air around it. • Finally, when the air has cooled, it falls downwards, completing the cycle a convection “current”! Convection Everywhere • Convection currents occurs wherever fluids are being heated. Here are some more examples: Water in kettles Soup in saucepans Water in a hot water tank • Convection even occurs in the Sun! QuickTime™ and a YUV420 codec decompressor are needed to see this picture. Convection Currents in the Sun QuickTime™ and a YUV420 codec decompressor are needed to see this picture. Clouds & Convection • Pyrocumulus clouds http://www.youtube.com/watch?v=czYzu3OIjmY • Cumulus buildup over the San Francisco Peaks http://www.youtube.com/watch?v=WUkbd_omElE Convection in Our Atmosphere and Oceans • Weather forecasters show how convection currents are formed when warm and cold air masses meet in the atmosphere. • Convection currents are responsible for warm water currents that occur in oceans. • This activity demonstrates convection currents in a very colorful way: • http://wm.kusa.gannett.edgestreams.net/news/114295 2694628-03-21-06-spangler-inversion.wmv Convection in a Liquid • Energy can be transported through a fluid (liquid or gas) by convection currents.. • The temperature of the water near the flame increases. This water expands and so its density is less than the water surrounding it. The higher temperature water therefore "floats" upwards transferring energy through the liquid. Pie-Pan Convection • Fill the pie-pan between 1/2 to 3/4 full with tap water and squeeze in about 2 tablespoons of hand soap. • Gently stir the soap and water; try not to create bubbles. Stir until the soap is mixed throughout the solution. • To see convection currents and fluid flow more easily, darken the soapy solution by adding a few drops of dye. Liquid hand soap or shampoo with a pearly or metallic appearance and glycol stearate, glycol distearate or glycerolstearate. (Softsoap & Walgreen's Liquid Soap) Convection in a Fishtank • Add a few drops of red and blue food coloring to a fishtank of water that is sitting on a bowl of ice at one end of it & a bowl of hot water at the other. • After only a few minutes the warmer, therefore less dense, red water rises above the colder, & denser, blue water. • This is a perfect illustration of what happens when a warm front meets a cold front. Convection Experiments Convection Currents Experiment in water: http://www.youtube.com/watch?v=nD5NHjdxRlY Convection Currents Experiment in air: http://www.youtube.com/watch?v=pivQmIR7nDw Lava Tube: http://www.youtube.com/watch?v=vQucpUWV1HM or http://www.youtube.com/watch?v=vQucpUWV1HM How to make a lava tube: http://www.youtube.com/watch?v=GAJG8jh2SJA Teaching Tips • Establish that convection is the primary method of heat transfer in fluids, both liquids and gases. • Ask students to give examples of where convection currents might be found, and why. • If the older students have studied plate tectonics, prompt them to recall convection currents in the Earth's mantle. Convection Summary • Convection occurs in gases and liquids. Hot fluids rise, cold fluids fall. • Convection currents occur because heat is lost from the rising fluid, cooling it down. • The whole fluid will rise in temperature as a result of mixing caused by convection currents. The END (for now)