PHYS 1405 Conceptual Physics I Heat Transfer

Transcription

1 PHYS 1405 Conceptual Physics I Heat Transfer Leader: Skeptic: Recorder: Encourager: Materials Part 1 Air convection apparatus, candle, flash paper, matches/lighter Part 2 LabPro, Laptop, stainless temperature probe, Hot plate, Conductometer, Liquid crystal conduction strip, beaker, 15 ruler Part 3 Radiation cups with lids, LabPro, Laptop, 2 x stainless temperature probes, High intensity lamp (100 ml graduated cylinder by sink for general use) Introduction When we discuss heat transfer, it is customary to divide it into three mechanisms, conduction, convection, and radiation. In this activity we examine some examples of each of these types of heat transfer Part 1 Convection In this first procedure, we will visualize convection currents in air. The apparatus consists of a closed box with a window and two glass chimneys. A candle will be burning under one of the chimneys. 1. How will the temperature of the air directly above the candle compare to the surrounding air? 2. What will happen to the density of the air directly above the candle? Explain. 3. If the density of the air decreases, what will happen to the air directly above the candle? Explain. 4. If the air above the candle rises, what must other air near the candle do? Light the touch paper so that it smokes. Hold the smoking touch paper above the chimney that the candle is not under. The smoke will flow with the air and thus show the movement of the air. 5. Describe the movement of the air through the apparatus. 6. Does this agree with what you expected? Explain.

2 Part 2 Conduction i) Temperature Gradient Heat is a flow of energy from a hotter object to a cooler one. Metals are good conductors of heat (and electricity) so heat can be transferred through metals by conduction efficiently. During heat conduction by a metal, is the temperature the same throughout the metal? We will examine that question in this procedure. First, record what you think the answer to that question is. P1. Prediction: Is the temperature the same throughout a metal when it is conducting heat? Explain your reasoning. Fill the beaker with water from the tap. Hold the liquid crystal conduction strip in your hand. Once the strip has warmed up, it should display different colors, depending on its temperature. Make sure that the LabPro is connected to the Laptop and has power. Connect a temperature probe to CH 1 of the LabPro and start up LoggerPro. LoggerPro should automatically detect the temperature probe and give a read out of the temperature on the screen above the graph. If it doesn t, contact your instructor. Use the temperature probe to measure the temperature of the water in the beaker and then firmly hold the temperature probe in your fist and measure the temperature inside your fist. Also, measure the width of the conducting strip in cm. Water Temperature = Temperature inside fist = Width of strip = Place one of the legs of the strip into the beaker of water and hold the other tightly in your fist. After a few minutes, a pattern of color should emerge on the liquid crystal strip covering the metal strip. Questions 1) Describe the pattern. Be specific and include details about color. The liquid crystals reflect different colors of light based on temperature. Warmer temperatures will reflect blue and green light and cooler temperatures red. 2) Which end of the strip is warmer? 3) In which direction does the heat flow? 4) Does the liquid crystal strip indicate a constant temperature in the metal or a varying temperature? 5) If the metal was at a constant temperature, could heat flow through it? Explain.

3 6) The temperature gradient is the difference in temperature between the two ends, divided by the width of the strip. Determine the temperature gradient across the strip. Don t forget units. ii) Comparison of conduction in different metals. You have been provided a rectangular pan with 5 holes on the side. The holes have stoppers in them each with a different metal. This device is called a conductometer. On the end of the rods are strings with a small weight which are attached by wax. We will fill the pan with water and then heat it on a hot plate. We will detect the heat conducting in the rods by it melting the wax and the weights dropping. Several factors can affect heat conduction including the length of the rod and its cross sectional area. In this case both length and area are the same for each of the rods. The question we want to investigate is, do all metals conduct heat at the same rate? P2. Prediction: Will all the weights drop at the same time or will some drop before others? If some will drop before others make a prediction as to the order they will drop. Fill the pan with water above the level of the stoppers. Place it on the hot plate and turn on the hot plate. Arrange it so that the weights will not fall onto the burner. This procedure will take a while, so you might use your time more efficiently by having one person from your group keep an eye on it, while others continue the lab. Have someone observe the weights at all time and note if they all fall together or at different times. If they fall at different times then note by metal the order in which they fell. Questions 1. Did all of the weights drop at the same time? 2. If not then list the metals in the order that the weights did drop. 3. Explain how you can use your answer to 2 to rank the metals by how good of conductors they are. 4. Based on your observations, list the metals in the order of how good of conductors they are.

4 Part 3 - Radiation In this lab, we will explore how the temperature of objects that absorb light differently change temperature when they are illuminated by a strong light source. 1. Set Up Fill each can with 100 ml of water. Place the lid on the can and insert the temperature probes through the hole on the lids into the water in the can. Connect the probe in the black can to CH 1 on the LabPro and the probe in the silver can to CH 2. Start LoggerPro. The program should automatically detect the two temperature probes. If it doesn t, contact your instructor. (Note that if LoggerPro is already running from a previous procedure, then clicking on the New icon, should cause it to detect the two temperature probes.) Click on the Experiment menu and then click on Sampling Adjust the data collection time to 15 minutes. 2. Data Collection Plug in and turn on the lamp. Arrange the lamp and the cans so that the lamp is about 6 from the cans, and illuminates the cans evenly. The cans should not touch. Click on the collect button. After 7.5 minutes, turn off the lamp while still collecting data. Do not stop data collection when you turn off the lamp. Continue until the data collection ends. Data Analysis Examine your graphs. 1) Which of the cans seemed to go up the fastest in temperature? 2) Which of the cans seemed to go up slowest in temperature? 3) Which of the cans seemed to go down the fastest in temperature once you turned the light off? 4) Which of the cans seemed to go down slowest in temperature once you turned the light off? 5) If an object rises in temperature rapidly, it will go down in temperature. 6) If an object rises in temperature slowly, it will go down in temperature. 7) Which has a higher emissivity, the black can or the silver one? Explain. 8) Did the can with the higher emissivity go up faster or slower in temperature?

5 9) Did the can with the higher emissivity go down faster or slower in temperature once the lamp was turned off? Circle the correct choice from the italicized pair. 10) An object with a higher emissivity will absorb more/less radiation than an identically shaped object with a lower emissivity. 11) An object with a higher emissivity will emit more/less radiation than an identically shaped object with a lower emissivity. 12) If you wished to block heat transfer by radiation, should you use an object with a high or a low emissivity? Explain.