EDWARD R. MURROW HlGH SCHOOL
Teacher: Dr. D. Roemer
Class: SP24 (G band)
Topic: Mass-energy relationship
Date of Observation: 6/9/95
Date of Conference: 6/12/95
Dear Dr. Roemer,
This lesson followed a pre-observation conference that you and I held on Wednesday, 6/7 during J band in my office.
As students entered the room they took a copy of a worksheet from the front of the room. (see attached). At the top of this sheet was "Aim: What is the mass-energy relationship? Lesson 93" You began the band by asking, ''Are there any questions about 92, 91 or 90?" When a student indicated that you hadn't finished lesson 92, you indicated that lesson 93 didn't depend on lesson 92 so it was not necessary to go over it at that time. You went on to demonstrate the effect of radioactive decay by tearing a piece of paper in half and stating that after each half life period, half the paper would remain. You wrote the decay equation, "(238)U(92) ----(234)Th (90) +(4)He(2)" on the board. You explained how the equation illustrates conservation of charge and conservation of nucleons. You then said, "This doesn't mean that mass stays the same. In this process mass is not conserved. One way to see to see this is through the discovery of positrons."
You described the operation of a cloud chamber, using the chalkboard to illustrate the tracks of particles through the fog when an electric field was introduced. You said that in 1930 a new particle called a positron was discovered using a cloud chamber and you drew its track, describing how its track differed from the tracks of the other particles. You asked, "Any questions so far?" The only question asked was about radioactive half-life, to which you responded, "I thought we did that already, but we'll review it a later time.'' You then went on to describe another experiment with a different set of tracks. You asked, "If you were a physicist, how would you interpret this set of tracks? After several student responses you explained that the gamma ray was splitting into a positron and an electron. A student asked, "How come if didn' t happen before?" to which you responded that you can only see this occasionally and another student asked, "What's the difference between a photon, a positron, and a proton?" You told the students to discuss in pairs the question,"Is mass conserved?" while you took attendance. After several minutes you asked, "Did anybody come up with an answer?" and "How many groups felt that mass was conserved?", "Eddie, what did your group decide?" to which he wanted to know which question from the sheet you were asking him about . You asked the class, "What's the mass of the photon?", "What's the mass of the electron?", ''What's the mass of a positron?", and "Is mass conserved?" You then stated, "I've just proven that there is no conservation of mass."
You went on to question the conservation of energy in the same process. You compared the energy of the photon of light to the kinetic energy of the electron and positron that were produced. You told the students that the kinetic energy or the particles could be determined by using the expression 1/2 mv(2), You told the class the kinetic energy or each particle was 50 ev while the energy of the photon was 1,000,000 ev. You indicated that a new form of energy was invented to account for the difference and it was called ''rest energy". When a student asked if the positron and electron could join back to form the gamma ray you described this process as annihilation. You rewrote the formula for the energy of each particle as mc(2) +1/2 mv(2) and worked it out to be .51 Mev. A student questioned the use of mc(2) to find the missing energy.
You went on to compare the mass of the deuterium atom with the masses of the particles composing the atom stating that the mass of the atom was less than the mass of the particles that make it up. You defined the difference as the "mass defect" and explained that the mass was being used to bind the nucleons. You defined "binding energy" and wrote the formula: (mass defect) x c(2) = binding energy. When a student asked why why the entire mass of the atom wasn't used to find the binding energy, you answered that "We really don't understand where the binding energy comes from". The students thought the mass defect was the difference between the two isotopes of hydrogen, but you corrected them by redefining the mass defect.
During our post-observation conference we discussed the following aspects of this lesson:
1. This lesson, as your earlier ones, was one that was teacher dominated. This can be illustrated by the following:
a.When students make comments in your class they speak quietly and direct their remarks to you. Although you state that you would like everyone in the class to hear the comments of other students, you do nothing to assist this. Students behind the person speaking cannot hear what is being said. You say that you are aware of this but will not do anything to encourage students to speak in a loud voice or to speak to each other. You can do this by asking students to repeat their questions and by asking students to respond to each others' questions but you feel this is typical of a "standard teacher-dominated lesson" and will have no part of it. You are fearful that this will dissuade students from participating in your class. From the lessons I have observed, participation cannot fall far. Students speak only to you and you respond to them individually. Does this not set you up as the center of the classroom?
b. You draw conclusions for the students. For example, after discussing the decay of U-238, you told the class that although mass seemed to be conserved, it wasn't. If this wasn't apparent then why not allow a contradiction to develop and give the students a chance to explain. If the decay of U-238, as wntten on the board, indicated a conservation of mass, leave it. Describe the experiment with the cloud chamber and ask, "How do these two results contradict each other?" Allow the class to discover that mass is not conserved before you tell them. You state that this would take too long. You have learned that the easiest way is the fastest way, therefore, you tell the students what they need to know, you give them an example, and you tell them again. By you stating, "I have just proven that there is no conservation of mass." it does not follow that anybody understood what you were showing them. Asking several students in the class to generalize from the evidence by asking, "How can we relate this information to the Law of Conservation of Mass", would have allowed you to find out if you have in fact proven anything to the class. The students have to be able to make the connections in their own minds. Having you state that you have proven something does not mean they believe or understand you.
You repeated this when you described the mass defect to the class and tried to relate it to binding energy. After presenting the evidence to the class, the difference in masses between the mass of a deuterium particle and the mass of its separated particles, you explained that the mass was being used to bind the nucleons together. Why not ask the students where they thought the missing mass was? You did not even stop to ask if they understood that there was mass missing. One of the students thought that you were comparing the mass of a deuterium atom to a normal hydrogen atom.
You have stated that presenting information in this manner would be confusing to students. Physics is a logical science that is centered around a series of equations. According to you, everything in the universe is clear and there is nothing to discover. Since we know that this is not so, that the current set of equations cannot account for many phenomena why not let the students experience the fun of discovery.
2. Your questioning is poor. You began the class by asking "Are there any questions about 92, 91 or 90?" A better way to review prior learning is for you to compose a question that would require students to use knowledge they gained in a previous class. This would tell you not only if they had any questions but also if they understood the material. Frequently, when asked if they have questions, students will not respond, whether they have questions or not. In this lesson, since you began by referring to the decay of U-238 you could have asked the students what fraction of the material would be remaining after a given amount of time. This would have reviewed the material on half-lives and directed the lesson into the discussion of conservation of mass. In our post-observation conference, you indicated that none of this was necessary since "your review for them".
You frequently stopped during the lesson to ask. "Are there any questions?" If this is supposed to serve as a medial summary, it is a poor way to check on student's understanding When students do not understand and do not ask, you blame the students for their failure to apply themselves. As I wrote above, questions that are prepared beforehand and included in your lesson plans would allow you to ask meaningful questions that could check on how well students are following your lesson. For example, when discussing the cloud chamber experiment and pointing out the different types of tracks the particles take through the fog, instead of asking, "Are there any questions?" you could ask, "How can we identify the type of particle that is present in the fog, by its track?" You might want to use an analogy of a footprint that helps investigators discover what type of organism walked passed an area.
The major "pivotal" question that you asked during the lesson, "Is mass conserved?" called for nothing more than a yes or no response by the students. In fact, although you had given students the opportunity to discuss the answer to the question while you took attendance, how long does it take to discuss yes or no. You asked for no reasoning, no follow up, no proof using the evidence you had presented. Several students you called on had no response, and one, Eddie, thought you were referring to a question from the worksheet you distributed. Other questions, such as , "What's the mass of the photon?", "What's the mass of the electron" , and "What' s the mass of a positron ask the students to do little more than read a number the board or off of their reference tables.
3. Your lesson had no final summary. You simply thanked the class and dismissed them. The fact that they did not get up to leave and seemed stunned was in part due to the fact that you were in the middle of explaining the concept of binding energy. A good final summary question would have been. "Why have scientists come to believe that mass and energy are really forms of the same thing?" In our conference you said that you do not feel a final summary is necessary because in physics one lesson does not end but only leads into the next one. Students have a.chance to review the material when they get home and do their homework questions. How well they do on their homework questions is the feedback you require. Your statement that one lesson leads into the next is particularly interesting in light of your remark to the class regarding their comment that you did not finish lesson 92. To paraphrase what you had said, "Lesson 92 is not necessary for Lesson 93."
4. At the conclusion of the lesson 1 asked you for a copy of your lesson plan which I have attached. This is nothing more than a page of notes. The only question on it was "Is mass conserved?" There were no provisions for medial or final summaries, no homework assignments, and no aim. A lesson plan should represent your strategy for teaching the material to be covered in the lesson. Your page of notes is nothing more than the material you want to cover.
This lesson must still be rated as unsatisfactory. You made little effort to involve the students in your lesson. In fact, except for three of the students in the class, you did not refer to the students by name during the lesson. You explained that you feel it is intimidating to students to call them by name. You presented evidence and then you drew the conclusions. You asked few questions and those that you asked were poorly phrased. You had no medial or final summary. You assigned no homework, although you indicated that the students understand that any questions on their "student lesson plan" is their assignment. Unfortunately, I have seen very little change in your lessons since you arrived at Murrow.
You have made it clear by your comments that the supervisory suggestions that have been given to you during your year at Murrow are not useful. You believe that questioning is a weak teacher' s tool for ''filling in time'', that students should be responsible for their own learning. You made the comment that once the students are given your prepared "student lesson plan" they have all the questions they will need in order for them to learn the material. When I commented that if that were the case, you could walk into the room, distribute your sheet of questions and dismiss, you replied that this would be the ideal case.
Assistant Principal Supervision
I have received a copy and read this report.
I understand that a copy will be placed in my file.