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Transcript
Kreutter: Electric Fields 1 Observe the bottle: Draw a picture of the bottle. What explanation can you come up with for the behavior of the stuff in the bottle? Another good question—what is the stuff in the bottle? Did You Know? Electric Field Lines An E field can be represented with E field lines. They have the following properties. The direction of the E field line shows the direction of the E field at that point. E field lines are representations only; they help us visualize the effects charged objects will have on other objects within the space that surrounds them. While we can feel effects of an E field, the lines themselves are imaginary. E field lines start on positively charged objects and end on negatively charged objects. The arrows point in the direction a positive charge would go (the direction of the force a positive charge would experience if it were located at that point). The density of the lines near that point represents the strength of the field at that point. More lines closer together mean the E field is stronger at that point. An electric field whose lines are parallel to each other is called uniform electric field. 1.1. Diagram Jeopardy. E field vectors due to one or more electrically charged objects are shown below. Indicate with circles, including the + or – signs, the charged source objects creating the field. a. b. Kreutter: Electric Fields 1 1.2 Reason. . a) Can E field lines ever cross? b) What is the direction of the E field directly between two positive equal charged particles? Explain why this happens. Draw an example. c) What is the direction of the E field directly between two negative equal charged particles? Explain why this happens. Draw an example. Kreutter: Electric Fields 1 Electric field Hockey On the computers, go to http://phet.colorado.edu/en/simulation/electric-hockey. Answer the questions that follow. The simulation is also available on the portal under Academic>HSN>ClassProjects>Handouts>Kreutter.Physics>electrichockey_en a) Set the difficulty to “practice”. When you drag a positive charge out of the bin on the top right and put it on the hockey table, which direction is there an arrow pointing from our black test charge? Why does it point in this direction? b) Drag a Negative charge out of the bin and place it on the field. You will notice the arrow that comes from the test charge points towards this new charge placed on the table. How does the arrow’s length depend on the distance between the source charge and the test charge? c) Position the source charges so you can shoot the test charge into the goal. Press start and try it out. d) With a single positive and negative charge on the table separated by about an inch, click the box that says “Field.” Describe the direction the clear/white arrows on the table point. What do you think these “Field” arrows represent? e) Using a few combinations of charges, try to score goals on all three of the difficulty settings. (Don’t let the puck leave the table) Kreutter: Electric Fields 1 1.3 Draw E field lines for the electric field created by the source charged objects described in the table that follows. A) A positively charged object B) A point-like positively charged object with twice the magnitude of charge as in part A. C) A point like negatively charged object D) A point-like negatively charged object with twice the magnitude of charge as in part C. E) Two positively charged point objects of equal magnitude charge, separated by a distance “d” F) Two negatively charged point objects of equal magnitude charge, separated by a distance “d” G) A small negatively charged object and a small positively charged object of equal magnitude charge separated by a distance “d” H) A small positively charged object and a small negatively charged object with twice the magnitude of charge separated by a distance “d” Kreutter: Electric Fields 1 1.4 Represent and Reason a) Circle the correct sign of the charge for the particles represented in A through I. b) Examine the charges G, H, & I. Which of the charged objects has a bigger magnitude? c) What would happen to the E field lines if you kept adding identical charged objects to the left of G and to the right of I? Draw out a diagram of the E -field lines if 6 charged objects are added to the ends of G and I. 1.5 Represent and reason. Imagine that a small, positively charged object moving toward the top of the page enters an electric field with the lines shown below. a) Sketch on the illustration an approximate path the object follows as it moves through the field. The initial direction of the object as it enters the field is sketched on the picture. b) Do those lines represent the path the object will follow? What do they represent? v qtest Kreutter: Electric Fields 1 1.6 Reason A hollow metal box is placed in a uniform electric field. Negatively charged electrons in the metal can move freely. a) Indicate the electric charge distribution in the metal due to the external electric field. b) Draw electric field lines caused by this induced charge distribution on the surface of the box and discuss the magnitude of the total E field inside the box. Compare the density of these lines with the lines of the external electric field. c) Discuss how your reasoning for parts (a) and (b) helps explain why it is safe to sit in a car during a lightning storm.