University of Virginia Physics Department Introduction to Transfer of Heat by Radiation A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.6
• PS.7

Objectives

Students will

• learn the distinction between conduction heating and radiation heating;
• learn that different colored materials absorb different amounts of heat by radiation.

Motivation for Learning

Demonstration: The Radiometer

 

 

Materials

A vacuum bulb that contains a rotating glass piece with attached fins that are painted white on one side and black on the other. These devices (called radiometers) are relatively inexpensive and produce an amazing amount of angular velocity when light is shone upon them. Radiometers can be obtained from most science suppliers like Edwards, Carolina Biological, etc.

Procedure

1. Show the radiometer to the students. Discuss how it is made.
   a) The inside of the bulb is sealed so that wind cannot spin the vanes.
   b) A frictionless pivot allows the vanes to turn easily.
   c) Each vane has a white and black side and is oriented in the same way, so each vane is white on one side, and black on the other.
2. Ask the students if it is possible to have the vanes spin without touching it.
3. Turn on a bright light next to it and watch it start to rotate. Put a piece of heavy paper in-between the bright light and the bulb and it will start to slow down. Take it away and it speeds up. Ask the students why they think it is doing this. Is the light heating something? The air inside? The wings? The light hits either side of the vanes equally, so why is it spinning?
4. Tell the students what is going on in the experiment. The most important concept is this: the light is transferring energy to the radiometer through the air. The photons, or "particles" of light, possess momentum that is transferred to the vanes upon collision. The atoms on the black side vibrate more when these collisions take place and push off the molecules in the air within the radiometer. Since more photon momentum is absorbed by the black side, the top rotates with the black sides receding from the light source. Thus, the light is heating up the black sides more than the white sides. To relate this to an everyday situation, ask which gets hotter on a sunny day, a black t-shirt or a white t-shirt.
5. Note that this is transfer of energy by radiation, not conduction. Conduction heating is simply applying heat directly to a sample by a flame or burner. Heat is transferred through the air to the sample from molecule to molecule by successive collisions until these molecules collide with the sample and heat it. No electromagnetic radiation (of significant value) is incident on the sample in conduction heating.

Background Information

The purpose of this activity is to show the transfer of energy from radiation energy (light) to thermal energy to kinetic energy (movement), and that these are all forms of energy, the amount of which is conserved throughout the system. The student activity will focus on the transfer of radiation (light) to thermal energy. Light can be described as either photons or particles, and electromagnetic waves that travel through space carry energy with them. When these waves/particles hit an object, they can either reflect from the object (if it has a metallic or white finish), or they can be absorbed into the object (if the object has a darker color). The qualitative way to tell if the rays or photons are being absorbed is to look at the object and observe what is reflecting off the object. If the object is reflecting every bit of light that hits it, it should look like a mirror. If it is absorbing a particular color of light, then the other colors are reflecting off it. This describes why a black object absorbs more waves since we don't see anything reflecting. White is defined as all colors mixed together, so white objects are in fact reflecting all colors at the same time. The quantitative way to measure this absorption of energy is to apply electromagnetic radiation to several objects, measure their relative temperature increases, and then compare the temperatures. The ones with higher temperature gains have absorbed more radiation energy than the ones with lower temperatures.

Answers to student activity questions

1. The black container experiences the largest heat increase because it absorbs the greatest light energy.

2. The white container heats up the least because the incident photons are reflected rather than absorbed.

Student Activity

To print out the Student Copy only, click here.

Materials (for each group of three)

• 2 tin cans, one spray painted matte black and the other spray painted matte white.
• 2 long thermometers
• 4 Styrofoam tops that can press onto the tops of the jars and have a hole in the middle for the thermometers
• 1 jug of water at room temperature (about 24°C)
• 1 stopwatch or timer
• 2 small desk lamps, each with 100 Watt bulbs
  
  Procedure

1. Pour the same amount of water in each of the two cans (about 200 mL), put the lid on, put the thermometer in, and place the cans in front of the lamps as shown. Measure the original temperature and write it on the data sheet. The initial temperatures of the liquids should be the same.
   
   
2. Turn on the lamps and start the stopwatch. Read the temperature of the thermometers each minute and write it down on the data sheet. After 20 minutes stop the experiment.
3. On the graph paper, graph the temperatures versus time for both cans. Make sure to remember what container each data point came from!

To print out the Data Sheet only, click here.

Data Sheet

Questions

1. Which container experienced the larger temperature increase? Why?

2. Which container had the smaller heat increase? Why?

Extensions

Repeat the activity with one or more of the following adaptations.

• Use different types of containers.
• Put the cans in the sun instead of using the lamps.
  
• Use sand in the cans instead of water.
  

Assessment

Data sheet and questions to be completed during the laboratory.