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Teacher’s Guide How Electrons Move Overview Students explore how electrons create fields and are influenced by fields. Students begin by learning how to use vectors as a concise way of visualizing force fields. Then students learn how to interpret electric field representation and how to use charged particles to deflect the motion of an electron. Students also learn how to design a uniform field that will accelerate an electron along a straight path. Finally, students explore the movement of electrons in magnetic fields. Learning Objectives Students will be able to: Know the dependence of the magnitude and direction of an electrostatic force on the distance between charged objects and their charges ( Understand Coulomb's Law). Understand the concept of electric fields created by charged particles. Determine how the motion of an electron is affected by electrostatic forces. Explain how the movement of electrons in magnetic fields is related to charge, mass, and velocity and magnetic field intensity. Prerequisite Knowledge Students should already have a basic understanding of: Charge Coulomb’s Law Vector Magnet Background and resources The article “General Students’ Misconceptions Related to Electricity and Magnetism” by Christian Raduta from the physics department at The Ohio State University discusses on page 10 how students typically see the electric and magnetic fields as having a static nature. It is important to notice whether or not your students think whether or not a field exists in a space and applies forces on charges, and/or whether they think it does not change even when a new charged particle enters a region. This conception should change after working with the models in the activity (but it may not). URL: http://arxiv.org/pdf/physics/0503132 A simple explanation of magnetic fields can be found in the following video http://www.youtube.com/watch?v=uj0DFDfQajw In the following video natural magnetic fields are revealed as chaotic, ever-changing geometries as scientists from NASA's Space Sciences Laboratory excitedly describe their discoveries. http://www.animateprojects.org/films/by_date/2007/mag_mov/1/ Approximate time for lesson completion: 60 minutes Activity Answer Guide Page 1: No questions. Page 2: 1. What is the relationship between the magnitude of the force (the length of the arrow) and the distance between two charged particles? (b) 2. What is the relationship between the direction of the electric force (the direction of the arrow) and the charges of the two particles? (d) 3. Place a snapshot here with an annotation showing where you think the hidden charge is in the model above. Page 3: 1. Did you guess incorrectly about what particles were present in any of the four examples above? If so, explain what mistakes you made and why you think you made them. In some cases I guessed correctly and in others incorrectly. When I guessed incorrectly, it was because I was not able to detect when a neutral particle was present. Otherwise, the direction of the force field vectors made it possible to guess where and what type of charge was present. 2. Electric field vectors all pointing towards a single point indicates what? (b) 3. Which of these subatomic particles would NOT have an electric field? (c) Page 4: 1. Place the snapshot of your winning game here Snapshot solutions will vary. The invisible charge is somewhere in the area of the red circle. (Note: There are many possible good images that could be used here. This is just one example.) 2. Describe what you needed to do in order to win the game. 4. Explain why you think the hidden particle is at the position you marked. I had to carefully arrange the attraction and repulsion forces that would guide the electron’s path to the target. I had to try to avoid the obstacles. Using all negative particles shows us that the hidden charge must also be negative because the visible particles are repelled by the similarly charged hidden particle. The red circle shows the area where all the force vectors are being repelled from. 3. Prediction: Do you think the electron in the Electron Cannon Game would travel on the same path if you changed all of the charges by the same amount? (c) 4. Was your prediction correct? Explain what happened when you increased or decreased the amount of charge on the stationary particles. field and it would travel straight (along the field lines). Yes, my prediction was correct. When I changed all the charges by the same amount, the interaction of the electron with the charges closest to it significantly changed the path of the electron. 1. The picture to the right was formed by placing a bar magnet under a piece of paper and then sprinkling iron filings on top. Describe the relationship between the pattern formed and the magnetic field created by the magnet. Page 5: Page 6: The iron filings line up along the magnetic field created by the magnet. Page 7: 1. Place an image of an electric field that moves an electron in an approximately straight line here. 2. Which of the following would NOT increase the average strength of the electric field within the box in the above model? Note that some of these options can be tested with the model. (c) 3. Describe what you needed to do in order to get an electron to move in a straight line and explain how it works. I needed to create a wall of opposite charges on each side. This created a uniform field in the middle so I could put a charge anywhere in that 1. Place a snapshot of your attempt to match Challenge A here. 2. Place a snapshot of your attempt to match Challenge B here. 2. In which of the following setup of electric field or magnetic field can a single charged particle possibly stay motionless? (a) (d) 3. Which of the following can move an electron in a straight line? (d) 4. Which factors affect the strength of an electric field? (d) 3. If you have a magnetic field pointing out of the screen, moving electrons, which are negatively charged, will (d) 4. Which of the following would make a charged particle traveling clockwise in a magnetic field change direction and start moving counter-clockwise? (c) 5. Which of the following images was created by gradually decreasing the field intensity from a large positive value to a small positive value? (b) Page 8: 1. If you have two charged particles positioned one above the other, which of the following is NOT possible concerning their force vectors? (a) (b) (d) 5. Mass spectrometry is used to determine which elements are in a substance. Some mass spectrometers use a magnet like the one modeled above. Using what you observed in the model above, explain how a magnet can be used to sort ions of different mass. Ions of different masses and same charge have different deflections after moving across the same magnetic field. Heavier ones are not deflected as much as lighter ones, so you can sort the ions by mass. Because each element has atoms with a certain mass you can tell what elements are present if each ion is a single atom. 6. On Page 1, you learned about the discovery of the electron. Thomson measured the mass/charge ratio of the electron and excluded the possibility that the particle was an ion. Based on the model above about mass spectrometry, can you explain how the mass/charge ratio can be measured? The assumption is that all the ions have the same charge, so you can tell the relative masses of the ions by seeing how much they are deflected by the magnetic field. Lighter ions are deflected more than heavier ones. An electron is so much lighter than any other single atom that it was apparent to Thomson that the particles he saw in the cathode ray tube could not be atoms, but something smaller. 7. Run the model to the left. Use the "Reverse charge" button to reverse the charge on the green particle. Can you explain why the green particle moves in a straight line, regardless of its charge? The green particle is exactly between the two other charges, and is given some initial velocity in the vertical direction. Because it is exactly between the two other charges it feels exactly the same attraction or repulsion from those charges, so it continues to move in a straight vertical path. It doesn’t matter what the charge is in the middle if both charges outside of it are the same charge and distance from it. The middle particle will feel the same force from the other particles.