PET. Evaluating Magnetism Models Introduction. MSE Unit M Extension B. Next Gen

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1 Evaluating Magnetism Models Introduction Previously you developed and tested your initial model of magnetism. You probably found that your initial model could explain some phenomena (e.g., the two-ended nature of a magnetized nail), but not explain other phenomena (e.g. cutting a magnetized nail anywhere along its length produces two pieces of nail that are each twoended). An important skill in science is to be able to evaluate a model by deciding what it can and cannot explain. (This assumes that is communicated in way that you can understand it!) If it can explain all the observations that have been made, then it should be considered good. However, if there are some observations that it cannot satisfactorily explain, then it is in need of some revisions. You will practice this skill in this extension assignment by evaluating models proposed by other students in previous classes at various stages of their model development. On the following slides, you will be presented with observations made by some groups of students in previous classes and the models they proposed as a result. Your task is to decide whether the model is good, or whether it needs revising, by examining whether or not it can be used to explain their observations (not whether you agree with it or not). The first model (Group 1) will be used as an illustration of how to perform this evaluation. Then you will do the same yourself for the models for Groups 2 and 3.

2 Evaluating Magnetism Models Model of Group #1 Group 1: This group performed some experiments and made the following observations: a) An unmagnetized nail neither attracts nor repels paper clips. Both ends of a magnetized nail attract paper clips. b) The N-Pole of a magnetized nail is repelled by the N-pole of a bar magnet, and is attracted to the S-Pole of the magnet. The opposite is true for the S-Pole. c) Both ends of a magnetized nail can pick up three small paper clips. d) After dropping a nail in water, the magnetized nail is still magnetized. e) The point end of a nail can be made into a N-pole by rubbing it from head to tip with the S-pole of a bar magnet. The group s model pictures for the unmagnetized and magnetized nail are shown below, together with a brief description of their model. Description: Inside the nail there are equal numbers of separate N and S magnetic particles that can move around when attracted or repelled by a magnet. In the unmagnetized nail, they are all jumbled up randomly. When the nail is rubbed with one pole of a bar magnet, all the unlike-particles are attracted to the magnet and dragged to one end of the nail, whereas all the like-particles are repelled to the other end.

3 Evaluating Magnetism Models Model of Group #1 On the next three slides, we describe how this model could be used to explain the observations the group made. Description: Inside the nail there are equal numbers of separate N and S magnetic particles that can move around when attracted or repelled by a magnet. In the unmagnetized nail, they are all jumbled up randomly. When the nail is rubbed with one pole of a bar magnet, all the unlike-particles are attracted to the magnet and dragged to one end of the nail, whereas all the like-particles are repelled to the other end. a) An unmagnetized nail neither attracts nor repels paper clips. Both ends of a magnetized nail attract paper clips. The unmagnetized nail is the same at both ends (a mixture of N and S), whereas the magnetized nail is different at each end (all N at the tip and all S at the head). In the unmagnetized nail, the effects of the N s and S s cancel out so neither end can effect paper clips. In the magnetized nail, the areas where either the N s or S s are bunched, enhancing their effect, so they can attract paper clips or any ferromagnetic object.

4 Evaluating Magnetism Models Model of Group #1 Description: Inside the nail there are equal numbers of separate N and S magnetic particles that can move around when attracted or repelled by a magnet. In the unmagnetized nail, they are all jumbled up randomly. When the nail is rubbed with one pole of a bar magnet, all the unlike-particles are attracted to the magnet and dragged to one end of the nail, whereas all the like-particles are repelled to the other end. b) The N-Pole of a magnetized nail is repelled by the N-pole of a bar magnet, and is attracted to the S-Pole of the magnet. The opposite is true for the S-Pole. In the magnetized nail, each end is different (all N s at the tip and all S s at the head). So the tip is the North Pole of the nail, and the head is the South Pole. Since like Poles repel and opposite Poles attract, then the tip end of the nail is repelled by the N-pole of the bar magnet and attracted to the S-Pole. c) Both ends of a magnetized nail can pick up the same number of small paper clips. There are equal numbers of magnetic particles at each end of the magnetized nail, so each end will be equally strong.

5 Evaluating Magnetism Models Model of Group #1 Description: Inside the nail there are equal numbers of separate N and S magnetic particles that can move around when attracted or repelled by a magnet. In the unmagnetized nail, they are all jumbled up randomly. When the nail is rubbed with one pole of a bar magnet, all the unlike-particles are attracted to the magnet and dragged to one end of the nail, whereas all the like-particles are repelled to the other end. d) After dropping a nail in water it is still magnetized. The magnetic particles are inside the nail so they do not get washed off. e) The point end of a nail can be made into a N-pole by rubbing it from head to tip with the S-pole of a bar magnet. Rubbing in this way attracts all the N-particles to the point end and repels all the S-particles to the head end. The end with all the N-particles is the N-pole of the magnetized nail. Conclusion: Because this model can be used to explain all the observations made by the group they should consider it good (at least for now).

6 Evaluating Magnetism Models Model of Group #2 Group 2: A second group of students made some of the same observations as Group 1, with a slight difference for observation (c): a) An unmagnetized nail neither attracts nor repels paper clips. Both ends of a magnetized nail attract paper clips. b) The N-Pole of a magnetized nail is repelled the N-pole of a bar magnet, and attracted by the S-Pole of the magnet. The opposite is true for the S-Pole. c) The head end of a magnetized nail picked up four small paper clips, while the tail end only picked up three. d) After dropping a nail in water, it is still magnetized. From (c), Group 2 concluded that the head end had a stronger magnetic effect. Here is their model. Description: There are no magnetic entities associated with the unmagnetized nail, but when a magnet is rubbed along the surface of the nail, it deposits small N and S entities (like dust particles) loosely along the surface, as shown above. On the following slides, you will be asked if this model can explain the observations made by Group 2.

7 Evaluating Magnetism Models Model of Group #2 Description: There are no magnetic entities associated with the unmagnetized nail, but when a magnet is rubbed along the surface of the nail, it deposits small N and S entities (like dust particles) loosely along the surface, as shown above. Can this model explain the observations that an unmagnetized nail neither attracts nor repels paper clips; the magnetized ends of a nail attract paper clips; that the tip of the magnetized nail is repelled by a magnet s N-pole and attracted by the S-pole, and that the opposite is true for the head? A) Yes, the model can explain the observations. B) No, the model cannot explain the observations. Feedback: Choice A is correct. The model shows that for the magnetized nail the tip end has only N s and is thus a N- pole, and hence will be repelled the N-pole of the magnet (like poles repel), while the head end has only S s and is the S-pole, and hence will attract the S-pole (opposite poles attract). Either end, because it has a magnetic pole, it will attract paper clips, or any unmagnetized objects with iron in them. It can also explain why an unmagnetized nail does not attract or repel paper clips ( or other unmagnetized ferromagnetic objects), since there are no entities on the nail.

8 Evaluating Magnetism Models Model of Group #2 Description: There are no magnetic entities associated with the unmagnetized nail, but when a magnet is rubbed along the surface of the nail, it deposits small N and S entities (like dust particles) loosely along the surface, as shown above. Can this model explain the observation that the head end of the magnetized nail picked slightly more paper clips than the point end? A) Yes, the model can explain the observation. B) No, the model cannot explain the observation. Feedback: Choice A is correct. The model can explain that the head end of the magnetized nail picks up more paper clips than the point end, because the head end contains more S entities (13 in the diagram) than the point end contains N entities (10).

9 Evaluating Magnetism Models Model of Group #2 Description: There are no magnetic entities associated with the unmagnetized nail, but when a magnet is rubbed along the surface of the nail, it deposits small N and S entities (like dust particles) loosely along the surface, as shown above. Can this model explain the observation that, after dropping a nail in water, it is still magnetized? A) Yes, the model can explain the observation. B) No, the model cannot explain the observation. Feedback: Choice B is correct. Unlike the previous model, in this one all the entities are on the surface of the nail, rather than inside. Since the magnet deposited them there when it slid across the nail, presumably these entities can also be washed off. Thus, this model cannot explain the observation that a wet nail remains magnetized.

10 Evaluating Magnetism Models Model of Group #3 Group 3: A third group of students made some of the same observations as Group 1, but in addition did an experiment in which they cut a magnetized nail exactly in half and found that each half of the cut nail behaved like it was itself two-ended. Here is the model they proposed for the magnetized nail after seeing this: (Their diagram for the unmagnetized nail showed an equal number of randomly distributed N and S particles, similar to Group 1 s initial diagram.) Description: Inside the nail, there are equal numbers of separate N and S magnetic particles that can move around when attracted or repelled by a magnet. In the unmagnetized nail, they are all jumbled up randomly. When the nail is rubbed with one pole of a bar magnet, the magnetic particles arrange themselves as shown in the diagram. On the following slides, you will be asked questions about this model.

11 Evaluating Magnetism Models Model of Group #3 Description: Inside the nail, there are equal numbers of separate N and S magnetic particles that can move around when attracted or repelled by a magnet. In the unmagnetized nail, they are all jumbled up randomly. When the nail is rubbed with one pole of a bar magnet, the magnetic particles arrange themselves as shown in the diagram. Can this model explain Group 3 s observation that, after cutting the nail exactly in half, each half of the nail behaves like it is two-ended? A) Yes, the model can explain the observation. B) No, the model cannot explain the observation. Feedback: Choice A is correct. The model can explain how each half of the nail could still be two-ended because, if the nail is cut exactly in half, the half with the point would have a South Pole on the point end, and a North Pole on the cut side. The half with the head would have a South Pole on the cut end, and a North Pole on the end with the head. Each of the four poles would be of equal magnetic strength, as well.

12 Evaluating Magnetism Models Model of Group #3 Description: Inside the nail, there are equal numbers of separate N and S magnetic particles that can move around when attracted or repelled by a magnet. In the unmagnetized nail, they are all jumbled up randomly. When the nail is rubbed with one pole of a bar magnet, the magnetic particles arrange themselves as shown in the diagram. Suppose a magnetized nail were cut into ¼ and ¾ pieces. Would Group 3 s model predict that those pieces would be one-ended or two-ended? Choose the answer that best matches your expectation. [A one-ended nail would be one in which both ends behave the same; that is, they are either both N-poles or both S-poles.] A) The model would predict that both pieces would be two-ended. B) The model would predict the short (¼) piece would be one-ended (South Pole if the point end, North Pole if the head end), and the larger (¾) piece would be two-ended. C) The model would predict that both pieces would be one-ended. Feedback: Choice C is correct. In Group 3 s model, if you cut off the last quarter of either end of the nail, the smaller piece would have only S entities (point end) or N entities, so it would be one-ended. The remaining ¾ piece would have entities of only one type on either end. For example, if the ¼ piece contains the point, the ¾ piece would have N entities on both the head end and the cut end. Hence, the ¾ piece would also be one-ended.

13 Evaluating Magnetism Models Quiz Take the extension quiz that your instructor has made available for you in your institution s Learning Management System. Keep this homework open in a separate browser window until you finish answering all the questions.

14 Quiz Question 1. 1) Consider the model from a different group, similar to but not the same as Group 2 s model: Description: There are no magnetic entities associated with the unmagnetized nail, but when a magnet is rubbed along the surface of the nail, it deposits small N and S entities (like dust particles) loosely along the surface, as shown. Which of the following observations made by some other groups would be difficult to explain (or could not be explained) using this model? (Choose all that apply.) A. The magnetized nail behaves like a two-ended (bar) magnet. B. Cutting the magnetized nail in half produces two pieces that each behave like two-ended magnets. C. Cutting the nail anywhere along its length produces two pieces that each behave like two-ended magnets. D. Each end of the magnetized nail can pick up the same number of paper clips. E. When cut in half, each end of each half of the nail (four ends in all) can pick up the same number of paper clips. F. After dropping the magnetized nail in water and removing it, the wet nail is still magnetized. Feedback: Observations C, E, and F would be hard to explain using this group s model. Cutting the nail in half would produce two, two-ended pieces (B), but cutting into about 2/3 and 1/3 pieces would produce two one-ended pieces, so observation C couldn t be explained. If cut in half, in this model the cut ends on both halves would be weaker (fewer N or S particles) than the head or tip ends, so observation E couldn t be explained. Finally, if the N and S entities are deposited like dust particles, then like dust, they would likely wash off in water, so observation F couldn t be explained.

15 Quiz Question 2 2) Now consider a group who found that, when they cut a magnetized nail into either halves or ¼ and ¾ pieces, the pieces of the magnetized nail still behaved as if they were two-ended. Description: Inside the nail, there are equal numbers of separate N and S magnetic particles that can move around when attracted or repelled by a magnet. In the unmagnetized nail, they are all jumbled up randomly. When the nail is rubbed with one pole of a bar magnet, the magnetic particles arrange themselves as shown in the diagram. Which of the following observations made by some other groups would be difficult to explain using this model? (Choose all that apply.) A. The magnetized nail behaves like a two-ended (bar) magnet. B. Cutting the nail anywhere along its length produces two pieces that each behave like two-ended magnets. C. Each end of the magnetized nail can pick up the same number of paper clips. D. When cut in half, each end of each half of the nail (four ends in all) can pick up the same number of paper clips. E. After dropping the magnetized nail in water and removing it, the wet nail is still magnetized. Feedback: Observation B would be hard to explain using this group s model. Although cutting the nail in half or into ¼ and ¾ pieces would produce two two-ended pieces, cutting the nail into 1/8 (say, cutting off the tip between the first set of S-entities and the first set on N-entities on the left) and 7/8 pieces would produce two one-ended pieces.

16 Quiz Question 3 This group s model is similar to the last group s model. In this case, the group performed experiments in which they cut a number of magnetized nails into two pieces of various lengths and found that both pieces were always two-ended. Here is their model for the magnetized nail: Description: Inside the nail there are equal numbers of separate N and S magnetic particles that can move around when attracted or repelled by a magnet. In the unmagnetized nail, these N and S particles are all jumbled up randomly. When the nail is rubbed with one pole of a bar magnet, the magnetic particles arrange themselves alternately along the length of the nail. 3) Could this model be used to explain the observation that when a magnetized nail is cut into two pieces of arbitrary lengths both pieces are always two-ended? A. Yes, according to the model diagram, anywhere the nail is cut would produce two pieces that are both two-ended. B. No, according to the diagram, there are some places where the nail could be cut and the two pieces produced would not be two-ended. Feedback: Choice B is correct. For example, slicing off just a small slice at the head would create a piece with only N particles, creating a one-ended sliver of a magnet. The remainder of the nail would also be one-ended, with both ends being South Poles.