Investigation: Is There Energy Transfer without Matter? (~4 hrs.)
We know that the Sun transfers energy in the absence of matter by a process called radiation. The Sun isn't the only thing that radiates - everything radiates, including things that aren't burning hot, or even glowing; your cup of coffee, you, and even an ice cube!
Image source: science.hq.nasa.gov
In our everyday, matter-filled world, it is often difficult to identify energy transfer by radiation. When you leave your car outside on a brisk but sunny autumn day (18°C), you might find that the inside of your car gets warmer and warmer. It's a complicated scenario - the sun is radiating, your car is radiating, and there's air all around it. How do radiation, conduction, and convection affect the temperature of the car?
To find clues about the effect of radiation on temperature change, let's focus on a smaller system that it's easier to control. Use the lamp and the metal clip from your kit to find evidence of energy transfer without matter.
Once again you'll be looking at temperature data to find evidence of energy transfer - by conduction, convection, and radiation. As you work, be aware of energy transferred to an object as well as energy transferred from an object. If you see that the temperature is not changing, that doesn't imply that there is no energy transfer - it indicates that heating and cooling are occurring at the same rate. The temperature data will give you clues about energy transfer, but you'll have to look carefully at a situation to uncover the whole story.
Suppose you put your hand 1 to 2 feet in front of the lamp and turn on the lamp.
- How long do you think it will take until you feel the heat?
- What do you think will happen when you turn the light off?
Set up your lamp and try it. Then use your hand to explore further how you experience energy transferred from the lamp; closer to it, farther away, above, below, and beside it.
!!CAUTION!! You are working with a hot lamp. Don't burn anything with it.
- Clamp the lamp securely to the sturdy edge of a table or a shelf, or to the back of a chair.
- Make sure that no wires are exposed
- Keep the bulb at least 8 inches from all other objects.
- Avoid touching the bulb.
- Avoid pointing the light at a nearby flammable surface.
What do you think is happening? Try explaining your observations in terms of heat transfer by conduction and convection. Do you find these explanations are adequate? Your hand is surrounded by air, but what if there were no air, just empty space, around your hand?
Work with a concept cartoon.
What's a Concept Cartoon?
Cartoon characters express different ideas about a science concept. Some are scientifically accurate and some are not but all these ideas are commonly held by learners. A concept cartoon is a useful formative assessment tool in that it motivates learners to discuss and debate and, thus, make their ideas visible.
We see three people discussing how they think heat is transferred from a lamp to their hands. In your journal: Use your experience turning the light on and off, and what you know about heat transfer by conduction and convection, to explain why you agree or disagree with each of these statements.
How does radiation affect the rate of cooling?
In Professor Tobin’s vacuum apparatus the air pressure was reduced so low that there was only a millionth of the number of molecules per cubic centimeter as there normally are in the air around us.
You were able to use your hand to explore how the lamp radiates energy to an object. It's trickier to explore how radiation transfers energy from an object. We asked physicist Roger Tobin to help us.
In his physics laboratory, Roger investigated how radiation affects the rate of cooling of a black anodized aluminum cylinder. He ran two tests to compare cooling with and without the presence of air. In each test, he heated the cylinder to about 80°C and then monitored its temperature for the following 2 1/2 hours. In his first test, the cylinder was in a vacuum, so energy could only be transferred without matter. The second test was identical to the first except that, in this case, the cylinder was surrounded by air.
How do you think the temperature of the cylinder changed in the environment where there are almost no molecules? How do you think this result compared to the temperature change of the same object cooling in air?
Common Patterns of Temperature Change
See Analyzing Temperature Graphs for more information about the patterns.
Before you look at the data Roger collected, predict: Sketch 2 curves showing change of temperature with time - curve 1: cylinder cooling in a vacuum, curve 2: same cylinder cooling in air.
Annotate each curve, identifying common patterns of temperature change, describing how you think energy may be transferred to and from the cylinder as time passes, and explaining why you think that the curve looks the way it does.
Compare your sketch to Roger Tobin's data. Any surprises? How much do you think air molecules or other matter contribute to energy transfer to and from the cylinder? How do his data affect your understanding of energy transfer?
To learn more about how energy can be transferred in a vacuum, without any material to transport it, read On the nature of light and other electromagnetic radiation by Judah Schwartz.
Meanwhile, back in the everyday world, we don't have a vacuum chamber to investigate the effect of radiation on the rate of temperature change. But, like Roger Tobin, we do have a probe and we have a metal clip. We've planned a 45 minute test to collect temperature data for both heating and cooling.
A 45 minute test to collect temperature data in the everyday world
- The probe is clamped securely onto the stand, attached by the plastic handle, and probe tip up. The metal clip is placed onto the tip of the probe.
- The the light bulb is placed 10 inches from the metal clip on the stand.
- For purposed of comparison, a second metal clip that will not be exposed to light is placed onto the tip of a second probe.
- The lamp is turned on so that light shines directly at the clip on the stand for 20 minutes.
- The lamp is turned off and data is collected for another 25 minutes.
Before you go on, predict. Sketch a prediction curve showing how you think the temperature of the two metal clips will change during the 45 minute test.
For each clip , explain why the prediction curve looks the way you've drawn it. Identify sections of the curve that show common patterns of temperature change (see the patterns in Part A). For each section:
Explain how you think that energy is being absorbed and emitted. Your metal clip is surrounded by air. What if there were no air? How much do you think air molecules or other matter contribute to energy transfer to and from the clip?
Explain what the temperature change over time curve indicates about energy change over time.
A sample explanation of a curve is provided.
Tips for running the test
(1) For important reasons of safety, you should never leave your lamp on unattended - you'll need to be in the room with the lamp the entire time you are heating the metal clip.
(2) The clip will remain very hot for some time after the lamp has been turned off - another safety issue. (You can use tongs to plunge a hot clip in cold water.)
(3) Your most important work is the thinking and reasoning about heat transfer that you do in the process of analyzing temperature data.
Run the test for about 45 minutes.
The challenge is to identify heat transfer by conduction and by convection, and to convince yourself that energy transfer by radiation must be occurring. As you run your test,
Try explaining your observations in terms of heat transfer by conduction and convection. Do you find these explanations are adequate?
Try to minimize heat transfer by uncontrolled sources (e.g., HVAC goes off, oven goes on, sun shines through the window).
As you collect temperature data, note when the light is turned on and off, and any changes in the environment (e.g., lamp moved, window opened).
As you work, you may have some new ideas to test about how energy is transferred in the lamp/air/clip system. For example, if you wonder how much energy is transferred from the lamp to the cylinder by convection, you might want to see what happens when you use a fan to blow away the air around the lamp, or you might want to compare what happens when the cylinder is above or below the lamp. Do a thought experiment and take notes about your ideas and, if you have time, go ahead and test them.
D. Interpret, make sense, and explain
Now the fun of data analysis begins. You can use our data or your own.
Examine each segment of the temperature change curve
Identify any of the 7 common patterns of temperature change that you find.
For each pattern you find, use your understanding of energy transfer to explain it. In your explanation, describe how you think that energy is absorbed and emitted, and how you think that the presence of matter might affect energy transfer.
Reexamine Roger Tobin's data . Note any insight or questions that it raises about the test data.
Keep an eye out for thermal equilibrium. If you find it, when did it occur and how do you explain it?
An object is said to be in thermal equilibrium when it maintains a constant temperature over a period of time. In Session 2, you saw that two systems in contact are in thermal equilibrium when they have reached a common temperature. In this case, the metal clip will maintain a constant temperature when the rate of absorbing energy is equal to the rate at which energy is emitted.
Finally, read the section in Conceptual Physics on heat transfer by radiation (pages 310-317 in Chapter 16 of the tenth edition). As you read, jot down thoughts and questions in your journal. How do these ideas relate to your investigation? What new questions does the reading raise?
If you would like to learn more about the wave nature of radiation, read Electromagnetic Waves.
Write a report about your work this week. How does radiation affect the rate of temperature change of the metal clip? What evidence have you found that there is energy transfer without matter?
In your report include your analysis of the graph from Part D.
How well does the evidence support your predictions? Explain any changes in your thinking about heat transfer.
F. Heat Transfer in Your Life
This week, look at the world around you focusing on radiation in your surroundings. Use the Heat Transfer in Your Life section of your journal to record information about at least three examples. For each example you choose, make a diagram and annotate it using arrows and words to describe where energy is transferred by radiation. Note where there may be energy transferred by conduction and convection as well.
For each example how do you think that thermal equilibrium is reached and maintained?