Field of dreams, the REAL reason you are safe in your car in a lightning storm!

Transcription

1 Field of dreams, the REAL reason you are safe in your car in a lightning storm! 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. 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 at a point is in the same direction as the E field at that point. E field lines start on positively charged objects and end on negatively charged objects. The density of the lines near that point represents the strength of the field at that point. An electric field whose lines are parallel to each other is called uniform electric field. 1.2 Reason. Examine the definition above and answer the following questions on E field lines. 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.

2 Electric field Hockey (Electric field hockey is a property of the University of Colorado) On the computers, go to Answer the questions that follow. 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? Is the relationship linear? Why does it exhibit this pattern? 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)

3 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 magnitude of charge as in part A. C) A point like negatively charged object D) A point-like negatively magnitude of charge as in part C. E) Two positively charged point objects of equal magnitude charge, separated F) Two negatively charged point objects of equal magnitude charge, separated 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 magnitude of charge separated

4 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 charged objects of the same magnitude 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. d) What happens to the E-field arrows as more and more charged objects are added? 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 q test

5 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.