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The Radiometer: Using Inquiry to Teach Energy Conversions

Grades: 5-8
Author: Sandy Van Natta


Module Description

A radiometer is made from a glass bulb from which much of the air has been removed. Inside the bulb is a low friction spindle and a rotor with several lightweight metal vanes spaced equally around the axis. When the bulb is exposed to certain energy sources, the rotor turns.

This module allows participants to investigate the motion of the rotor by designing their own experiments and drawing their own conclusions based upon their observations and data. They will then be asked to identify the energy conversions taking place that allow potential energy to be converted into the kinetic energy of the turning rotor.

This module also leads participants through the process of designing an experiment so that they can direct their own students to do the same. As a final part of this module, teachers develop a plan to implement what they have learned in their classrooms.


  • Participants will identify different forms of energy
  • Participants will form hypotheses relating to the motion of the rotor in a radiometer
  • Participants will write testable questions and design simple procedures to test their questions
  • Participants will identify several forms of energy which can cause the motion of the rotor
  • Participants will identify the energy conversions taking place in their experiments
  • Participants will design a lesson implementation plan based on energy conversions to be used in their own classrooms


One radiometer per group (or Crookes Radiometer) - available from most science supply houses for about $10.00 a piece

Have as much of the following available for testing as possible:

  • Flashlight
  • Incandescent light source
  • Laser pointer
  • Magnets
  • Hair dryer
  • Magnifying glass
  • Colored cellophane or plastic
  • Ice
  • Mirror
  • Sunlight
  • Protractor
  • Drinking Straw

*Note: only one or two of the above items are needed for testing since each group will be testing different variables



This stage focuses the participant's attention on a topic and poses a problem for the participant to explore in the next phase of the learning cycle.

  1. Divide your participants into groups of 4. Give each group a radiometer and ask them to look at it and make observations. Avoid saying the name "radiometer" when handing out the devices.
  2. Ask them to think about what it might do and how it might work. Some of your participants will already know what the instrument is and/or how it works. Ask these participants to allow other members of their group to put forth their ideas before the remaining group members "give" them the answers. Even if some participants know that light is supposed to affect the radiometer, ask all members to think about all the "things" that might make it work.
  3. Ask participants to form hypotheses that might explain how they can get the rotor inside the radiometer to turn. (You may receive such hypotheses as: amount of light, type of light, angle of light, heat, magnetism, and wind, etc. may cause the radiometer rotor to turn) You may want to list these hypotheses on the board. Make sure the participants realize that as they list their hypotheses, they are also identifying the variables that may affect the outcome of their experiments.

Assessment: This is an on-going process throughout the learning cycle. The participants should respond orally in the Engage stage as well as throughout the cycle. Try to encourage all participants to take part in your discussion.


  1. Ask the groups how they can discover which hypothesis, or hypotheses, might explain their observations. (What you are really asking the teachers to do here is design experiments to test each hypothesis individually)
  2. Show the participants the materials you have pre-assembled for testing. With these materials in mind, ask the groups which factor or variable they wish to test or change. (They may choose type of light, heat, wind, etc.) Assign one variable to test to each group.
  3. If the teachers change this variable, ask them what they will want to observe in response to their change. (Teachers should observe what happens to the rotor.)
  4. Based on the responses to steps 2 and 3 ask teachers in each group to write a testable question that will allow them to set up an experiment to test their hypothesis. (Note: this language may be new to the participants, so lead them and give an example or two.) They can be given the prompt: "How does ________ affect ______? (An example of a testable question for this experiment would be: "How does the type of light affect the motion of the rotor?" or "How does a change in temperature affect the motion of the rotor?" Ask the participants what they would want to keep the same throughout the experiment so that any changes in the movement of the rotor can be accredited to the variable they changed. (Some things teachers might list would be: location in the room, temperature of the room, amount or type of lighting already in the room, or the angle at which light strikes the radiometer.)Depending on what each group is changing, all factors listed above should be kept constant except for their tested variable)
  5. Ask teachers to write a simple procedure to test their question. Any reasonable procedure should be accepted from participants.
  6. Ask the groups to share their procedures. If all procedures seem reasonable, allow the groups to actually conduct their experiments, make observations, and record their observations.
  7. Have all groups share their observations then ask the group as a whole to draw conclusions based on their observations. (Most likely the teachers will find that light and a change in temperature cause the rotor to turn. The angle, type of light, and intensity of light, as well as the amount of heat will also have an affect on the rate of turning.)

Assessment: Monitor the groups' work to be sure that they are recording data, discussing findings, and answering the posed problem.


In this phase, participants use the data they have collected, whether observational or numerical, to draw conclusions relating to the motion of the rotor. However, you may first want to ask the participants about the steps they just went through to design and set up an experiment. (How to write a question, how to identify variables, how to write a procedure, etc.)

  1. Call the teacher's attention to the fact that they had to change one variable and then observe the response of the rotor during the course of their experiment. The variable changed by the participant is known as the independent variable. The dependent variable was what they observed or measured in response to that change. (The PPD should emphasize this terminology to all teachers by first, asking for other examples of independent and dependent variables- time vs speed, days vs growth, etc. Point out one way a teacher may choose to have students in his/her own classroom remember these terms is that the dependent variable depends on what change they make.)
  2. There were certain factors that were kept the same in each experiment. These factors are known as controls.
  3. Ask the teachers how an experiment differs from an activity. (If you had simply told your participants in the beginning why the rotor of the radiometer turned, this would have been conducted as an activity. If a phenomenon falls short of the participant generating a testable question, then the experience of the participant stays at the activity level. Experiments are based upon testable questions drawn from a hypothesis. Numerical or observational data is collected and conclusions are drawn that are supported by their data.)

You may now wish to ask the participants what factors they discovered affected the motion of the rotor.

  1. Accept any answers than can be supported by their observations and data. Both radiant and thermal energy can be converted into the motion of the rotor. Magnetic energy has no affect on the rotor unless the vanes are made from steel/iron.
  2. Ask participants to describe the energy conversions they experienced during their experimentation. (For example: light (radiant energy) to mechanical energy (spinning rotor) or heat (infrared) to mechanical.) Participants may also infer that heat energy is also produced by the spinning of the rotor itself since during work, some useful energy is converted to heat. (Work is being done here since a force is causing the rotor to move a certain distance) Participants should also refer to the changes from potential to kinetic energy.

At this point you may want to discuss the history of the rotor. A brief background is given in the Explanation of the Science section.

Assessment: During the Explanation phase, the participants can be evaluated by being asked questions that assess the participant's comprehension of vocabulary and concepts. Ask participants how they would relate this activity to the teaching of energy conversions in their own classrooms. Ask them how the use of inquiry in this activity compares to other methods of instruction used in their classrooms. Encourage all participants to participate in the discussion.


Allow the participants to design additional experiments with the radiometer using the materials at hand or other materials they might have in their classrooms if additional questions are raised during your discussion. Have them suggest other ways of using inquiry to teach energy conversions to their own students. Give time for participants to complete the lesson implementation sheet as a first step to moving toward classroom implementation.

Assessment: Check the lesson implementation plans for faithfulness to the lesson just experienced. If possible, visit the participants' classes for onsite assessment.


Every facet of our lives involves energy and energy conversions. For example, chemical energy in the food we consume is converted into the energy needed by our bodies to move (kinetic energy) as well as keep our bodies warm (thermal energy). Since energy is constantly changing from one form to another, we often take these conversions for granted.

The radiometer is a simple devise that allows the participants to determine the effect of different types of energy (thermal, radiant, magnetic, etc.) on the motion of the rotor. Testable questions can be asked, simple experiments performed, observations made, and conclusions drawn in a relatively short amount of time. Participants will have fun conducting simple experiments while, at the same time, identifying and discussing excellent examples of energy conversions. This is an excellent inquiry activity that allows the participants to determine the path of their own learning experiences. They can then use the ideas presented here as a model for inquiry lessons in their own classrooms.

Science Standards

Science as Inquiry:

Content Standard A: As a result of activities in grades 5 – 8, and 9-12, all students should develop:

  • Abilities necessary to do scientific inquiry

Physical Science Standards:

Content Standard B: as a result of their activities in grades 5-8, all students should develop an understanding of:

  • Transfer of energy

Content Standard B: as a result of their activities in grades 9-12, all students should develop an understanding of:

  • Interactions of energy and matter

Professional Development:

Standard A: Professional development for teachers of science requires learning essential science content through the perspectives and methods of inquiry. Science learning experiences for teachers must:

  • Involve teachers in actively investigating phenomena that can be studied scientifically, interpreting results, and making sense of findings consistent with currently accepted scientific understanding.
  • Build on teacher's current science understanding, ability and attitudes.

Standard B: Professional development for teachers of science requires integrating knowledge of science, learning, pedagogy, and students; it also requires applying that knowledge to science teaching. Learning experiences for teachers of science must use inquiry, reflection, interpretation of research, modeling and guided practice to build understanding and skill in science teaching.

Best Teaching Practices

  • Discussion
  • Inquiry
  • Learning Cycle
  • Hands-on/Minds-on Learning

Time Frame

This activity should take about 45 minutes to complete


Purchase the radiometers in advance unless they are already available in your science department. Assemble the "testing" materials (lights, magnets, etc.) in a tub to set out in the front of your classroom


Handle the radiometers with care since they are made of glass. No other special precautions are needed.


Have participants suggest ways of teaching energy conversions in their own classrooms.

Explanation of Science

The radiometer was invented in 1873 by the chemist Sir William Crookes. The radiometer is made from a glass bulb from which much of the air has been removed to produce a partial vacuum. A rotor with usually four lightweight metal vanes spaced equally around the axis sits on top of a low friction spindle. The vanes are either polished or white on one side and black on the other. When exposed to light, either artificial or natural, or infrared radiation, the vanes turn. Even the heat from a nearby hand can be enough to cause the rotor to turn. The more intense the energy source, the faster the spinning. The dark sides retreat from the radiation source and the light sides advance. Cooling the radiometer causes rotation in the opposite direction.

Crookes first believed that light radiation pressure on the black vanes was turning the rotor around just like water in a mill. However, there was a problem with this explanation. Light falling on the black side should be absorbed and light falling on the white side should be reflected. The net result would be twice as much radiation on the white side as the black. If that were the case, the rotor was spinning the wrong way.

So what causes the rotor to spin? Since so little gas remain inside the radiometer compared to the trillions of air particles outside the radiometer, the air remaining inside can move about more freely. The dark side of the rotor vanes absorb more energy than the light side and a temperature difference develops between the vanes. The difference between the temperature of the warmer black side and the cooler white side causes the gases to creep along the surface of the vanes. The faster gases from the black side strike the edges of the vanes at an angle with more force than the molecules from the cold side. This causes the radiometer to spin. If you cool the glass quickly in the absence of a strong light source by placing ice on the glass, it turns backwards. This is because the black sides give off more heat and cool more quickly than the lighter sides.


None available for this module.


Have participants describe the energy conversions witnessed in daily life such as a light bulb attached to a battery, falling water turning a paddle wheel, or sunlight allowing plants to grow. Have them relate the energy conversions to the Law of Conservation of Energy.

Lesson Implementation Template

Download Lesson Implementation Template: Word Document or PDF File


When dividing participants into groups, try to be sensitive to gender, ethnic and religious backgrounds. Try to make groups as heterogeneous as possible.


None available for this module.


With thanks to educators affiliated with the University of Iowa whose creative ideas in a teacher workshop inspired this module

Crookes radiometer,

How Does a Light-mill Work?