(Walter Glogowski, Chaz Shapiro & Reid Sherman) INTRODUCTION

Transcription

1 Convection (Walter Glogowski, Chaz Shapiro & Reid Sherman) INTRODUCTION You know from common experience that when there's a difference in temperature between two places close to each other, the temperatures tend to even out over time: the hot part cools and the cool part warms. A fire can warm a whole room, not just the air right around it. You close your doors and windows in the winter because the heat wouldn't stay in your building if you left them open. So heat can obviously get transferred from one place to another. But how does this process happen? First we must picture what heat is: heat is the energy in the "random motion" of something's atoms (types of motion are described in Buoyancy Background). The atoms in a glass of hot water are moving around more quickly than the atoms in a glass of cold water. When water is hot enough, the atoms move so fast that they "escape" to become a gas (steam, a.k.a. water vapor), and when water loses enough heat, it becomes a solid (ice) whose atoms barely move at all - they simply jiggle in place. So when we talk about heat "moving" we're really talking about energetic atoms in one place transferring some of their random motion to atoms in another place. Source: brocku.ca/courses/1p93 Source: Then how does heat move? A simple way is for a fast particle to bump a nearby particle, transferring a little energy. That bumped particle will then bump into another one, and so on, until the heat energy (the motion of the particles) has been spread out. This process of energy being transferred by nearby particles bumping is called CONDUCTION, and it can happen in solids, liquids or gasses. However, FLUIDS (meaning liquids and gasses - things that can flow) have another important way of moving heat energy around. If one part of a fluid is hotter than another part, and if all the hot (fast moving) atoms get moved to a different place in the fluid, then their heat has moved with them. This process is called CONVECTION: energy being transferred by "bulk motion" of particles through a fluid (types of motion are described in Buoyancy Background). Convection cannot happen in a solid because, like the bricks in a wall, the atoms in a solid cannot move far from where they are. In liquids and gasses, however, the particles are free to move about (which is why a fluid can change its shape to match its container). If one part of a fluid is heated, where will the hot part go? For reasons that you will learn about in the Lighter Than Air day lab, heated gas or liquid tends to rise, while cooler stuff will sink. The heated 2006 Yerkes Summer Institute Convection 13

2 substance rises, moving farther away from the heat source and spreading out. In doing so, the heated fluid gives its heat to the surrounding fluid, thereby cooling off. When the heated fluid rises, something has to take its place. It can't happen that everything rises, because that would leave empty space at the bottom. As the warm material rises, some cool fluid from the top sinks and moves into the vacated space at the bottom. The cool fluid then gets heated by the heat source and repeats the cycle. So the whole picture is that warm material will move away from the heat source and warm up the cooler areas while cool fluid will move toward the heat source and get warm. In this way the heat from the heat source gets transferred throughout the fluid by convection. h PART I: WATER & FOOD COLORING In this demonstration, we will begin our exploration of the close relationship between temperature and fluid motion. We will first make predictions about what will happen, and then your job will be to watch the demonstration closely and record your detailed observations. We will fill a fish tank with tap water. We will then fill two beakers with water, one with warm water and one with cold. We will add colored dye to each beaker. If we place the beakers in the fish tank, what do you think will happen? In your lab notebook, write your predictions for:? What do you think will happen right away to the water in each beaker?? What do you think will happen over a long time?? What would be different if the warm beaker water is just a little Record your predictions! warmer than the water in the rest of the tank, than if the beaker water was a lot warmer than the tank water? Why? Now, watch as the experiment is carried out, and record your observations in your lab notebook. Make a sketch of the experiment. Did what you think would happen actually happen? Record your observations! Yerkes Summer Institute Convection 14

3 PART II: OIL & THYME The idea in this section is to make your own convection cell and study it see how it moves and what makes it move the way it does. Materials: Pyrex bread pan Vegetable oil Thyme Hot Plate Metal block 2 clamps 2 thermometers Stopwatch Ruler Ring stand Source: preparedpantry.com!!! WARNING!!! For this experiment we will be working with hot objects. The hot plate, breadpan and oil will all get very hot - take care not to touch any heated surfaces or you may be burned!! DO NOT TOUCH THE HOT PLATE, BREAD PAN, OR OIL WHEN IT IS HEATED! BE CAREFUL: ITEMS WILL REMAIN HOT EVEN WHEN THE HOT PLATE IS TURNED OFF!! Directions: Fill the breadpan about halfway with vegetable oil and mix in some thyme. Be sure to stir in thyme so that it is suspended throughout the oil rather than all being at the top or bottom. Place the metal block on the hot plate and the breadpan on top of the block so that only the center of the bottom of the pan gets heated. 1) The pieces of thyme will follow the oil as it moves. Watch the motion of the fluid as the bottom gets heated. Record your observations! In your lab notebook, record What do you observe? Sketch it! How does the motion change as time goes on? Draw a picture in your lab book tracing out the motion. 2) Now attach the thermometers with the clamps to the ring stand in such a way that one will measure the temperature of the oil closest to where it is being heated and the other will measure the oil at the edge, as far away from the heat source as can be. 3) Using a ruler and a stopwatch, measure the speed that a piece of thyme moves across the pan. While you are measuring the speed, take a measurement of both thermometers. Record the speed, the two temperatures, and the temperature difference between the thermometers in your lab book. Record your data! 4) Do not continually heat the oil, but occasionally turn the hot plate off and on, so that you are sometimes taking measurements while the oil is being heated and sometimes while it is not. (Note: it will still be hotter than room temperature because it takes a while to cool off.) Also, once or twice, stir up the oil, wait for it to settle for a couple of seconds, and take a measurement Yerkes Summer Institute Convection 15

4 Graphing: Now make three graphs in your lab notebook. In the first one, put the temperature measured at the center of the pan on the horizontal axis and the speed of the oil's motion on the vertical axis. Then for each of your measurements, plot a data point. When you have plotted all the points, look at the graph. Do you notice a clear trend to the points? Plot your data! For the second graph, do the same thing, but instead of putting the temperature at the center of the breadpan on the horizontal axis, put the temperature at the edge. In the third graph do the same again, but the horizontal axis will be the temperature difference between the center and the edge.? Which of the three graphs shows the strongest connection between the value on one axis and the value on the other? PART III: CONVECTION IN THE SUN We will use a solar telescope to take pictures of the surface of the sun. We will put a filter on the telescope so that we only see light from a specific temperature-sensitive process happening in the sun. NEVER LOOK AT THE SUN THROUGH A TELESCOPE WITHOUT A SOLAR FILTER!! The filtered image of the sun will show us a general map of the temperature of the surface of the sun. In your lab notebook, write down: A description & sketch of what you see. How does this relate to convection? Does it look like a convective cell? In the convective cells we looked at earlier, there was a heat source at the bottom of some fluid, and hot stuff rose while cool stuff sank. How is that related to what in going on in the sun?!!! WARNING!!! For this experiment we will be working with sunlight. The sun is a very bright source of light and can be very dangerous to your eyes. DO NOT STARE DIRECLY AT THE SUN! DO NOT LOOK THROUGH THE TELESCOPE AT THE SUN WITHOUT A SOLAR FILTER! 2006 Yerkes Summer Institute Convection 16

5 In your lab notebook, answer the following:? In the sun, where is the heat source?? In what direction would rising or sinking movements be?? Can you think of an analogy of where we could see convection from a similar perspective as we see the convection in the sun? PART IV: CONVECTION ALL AROUND US Here are a few more places where convection plays an important role: Weather As you probably know, the reason the earth is warm and comfortable for life rather than being an isolated ball of ice is that we are close to the sun. However, the actual heating of the air takes place almost entirely from sunlight absorbed by the earth and then released into the air as heat (infrared radiation). So the heat source of the atmosphere is the ground, at the bottom of the atmosphere, and not above us in the sky. That's why the air tends to get colder higher in the atmosphere. Mountains have snow at their peaks, not at their bases. Source: But, we know that hot air rises. So with a heat source at the bottom of a large amount of air, convection often forms in the atmosphere, just as it did in the breadpan of oil and thyme. As warm air rises from the ground, if often carries with it a large amount of water vapor. This situation happens because the water is evaporating at the ground as well, and humid air (air with a lot of water in it) is lighter than dry air. When the air rises to where it is cool, the water vapor condenses into little droplets. Water condensing high in the atmosphere in this way is how clouds form. Clouds are the peaks of gigantic convective cells! You will have a chance to further explore clouds in the night lab Weighing Clouds. Cooking What happens when you put a pot of water on the stove? The bottom of the pot gets heated very quickly, and that heats the water at the bottom of the pot. The hot water will rise to the top, letting cooler water take its place at the bottom of the pot, where it will get heated. Convection cells form in the water as the water at the bottom gets heated from the flame and water at the top loses some heat to the air above it. This convection is why when you boil a pot of liquid, the liquid seems to be bubbling. You are seeing the tops of convection cells Yerkes Summer Institute Convection 17

6 You also may have heard of convection ovens. The name is actually a little misleading, since regular ovens use convection as well. In a regular oven, the heat source is at the bottom, causing the hot air to rise; new air then is next to the bottom, gets heated, and rises. Convection will work to heat all the air in the oven, but it does not do it very efficiently, and most of the hot air stays at the top of the oven. Thus, you have to pay attention to where in the oven you put your food, because the temperature is different in different parts of the oven. A convection oven has fans in it that basically force even more convective air currents than would happen naturally. These currents even out the air temperature faster and get the hot air all over the oven rather than just at the top. Clothing You know that in winter time you don't want to go outside without much clothing on. However, the reason you feel cold is much more complex than just the air around you being cold. After all, air is a terrific insulator, and does not carry much heat away from your body. In newer, more expensive windows, there will be two panes of glass with a little pocket of air between them because that air prevents heat from leaving your house. So why doesn't the air around you prevent you from getting cold outside? Because as soon as your body warms up the air around you, that air rises and lets new, cold air move in. Your body acts as a heat source for convection in the air, and loses heat much more quickly that way! In fact, the reason clothes make you warm is that they trap air next to your body. When your body heats the air around you, instead of rising and leaving you with new, cold air to heat, the warm air gets stuck where it is, forming a warm pocket around you. Down feathers are very warm because they trap lots of air. Source:kicp.uchicago.edu/edu cation/explorers/2002winter- YERKES/ The Rest of Summer Institute Why are we talking about convection at this Summer Institute? What does the transfer of heat throughout a fluid have to do with measuring density by floating objects in liquid or building hot air or helium balloons? Well, one of the main things we will discuss in this lab is that hot gas or liquid will rise. Hot fluid rises because it has a lower density than the cooler material around it. Things of low density float on things of high density, as you will study in the Does It Float? day lab. The hot air balloon lab, Lighter Than Air, also is closely connected to convection. Convection explains how the air in the balloon gets hot, since hot air balloons put the heat source at the bottom of the balloon and let convection do the work in moving the hot air around and warming the whole balloon. You can also think of the entire balloon as a small package of air in a convective cell. As it heats, it rises up into the atmosphere, and as it cools by releasing its heat into the air around it, it falls back to the ground. IN CONCLUSION In this lab, you became an expert in convection and its effects, and discovered how convection is useful in your everyday life and in other labs at this Summer Institute. In your lab notebook, write a short paragraph summarizing what you learned about and did with convection in this lab Yerkes Summer Institute Convection 18