Physics 2020, Fall 2011 Lab 2 page 1 of 6 University of Colorado at Boulder, Department of Physics Circle your lab day and time. Your name: Tue Tue Tue Wed Th Th Th Fri TA name: 10-12 12-2 2-4 12-2 10-12 12-2 2-4 12-2 Lab 2: Electric Fields INTRODUCTION In class we have learned about electric charges and the electrostatic force that charges exert on each other. Another way of looking at this is to recognize that every charge creates an electric field all around it. When a second charge is placed in the electric field, it feels a force due to the field – but the field from the original charge is always there, whether or not it is acting on any other charges. The electric field is a vector quantity; thus, it has a magnitude and direction at each point in space. The magnitude of the electric field at a point in space is the magnitude of the force that would be exerted on a test charge of 1 Coulomb that was placed at that point. The direction of the electric field gives the direction of the force on that test charge. Although it may not be obvious to you why this is a useful way of thinking about the interaction of two simple charges, splitting the problem into charges that produce an electric field and other charges that interact with this field turns out to be a very powerful technique when many charges are involved or when the geometrical arrangement is complicated. In today’s lab, we’ll explore the electric fields around charges. The goal is to gain some intuition about the electric fields and to learn how to add electric fields. PART I: ELECTRIC FORCES AND FIELDS A. In the lab you should have a number of diagrams, printed on paper and transparency foil, of the electric field around a single point charge (either positive or negative). Describe as many details about the field patterns for both the positive and negative charges as you can notice. In general, how does the electric field at a point in space relate to the electric force on a charge placed at that point?Physics 2020, Fall 2011 Lab 2 page 2 of 6 University of Colorado at Boulder, Department of Physics B. In each of the figures below, draw a vector representing the net force felt by the dark-colored charge. (To indicate the force on an object, draw a force vector arrow coming out of that object.) Draw the vectors to scale, so that longer arrows represent larger forces. Assume all single charges have the same magnitude.Physics 2020, Fall 2011 Lab 2 page 3 of 6 University of Colorado at Boulder, Department of Physics C. In each of the figures below, draw a vector representing the electric field at the dots. Draw the arrows to scale, so that longer arrows represent stronger field.Physics 2020, Fall 2010 Lab 2 page 4 of 6 University of Colorado at Boulder, Department of Physics PART II: SUPERPOSITION OF ELECTRIC FIELDS In case more than one electric field is present, the principle of superposition is used to find the total electric field at a given point in space. This just means that at any given point, the total electric field is equal to the vector sum of the electric fields produced by each charge: A. Overlay a transparency foil over a paper diagram, so that you can see two sets of electric field vectors – one set from each of two point charges. Describe the total electric field surrounding the charges if a positive charge is placed exactly on top of a negative charge. What is this similar to (something found in nature)? B. Using a pair of electric field vector diagrams for two charges of opposite sign, offset the transparency relative to the paper by some even number of grid spacing and lay a piece of tracing paper over the whole thing. On the tracing paper, mark the location and sign of the charges. At each grid point, draw an arrow which represents the total electric field at that point. Answer the questions in the table below for opposite signs. C. Repeat part B for two charges of the same sign. Answer the questions in the table below for same signs. OPPOSITE SIGNS SAME SIGNS Where is the field the strongest? Where is it the weakest? Does it go to zero anywhere? Are there any other noticeable details?Physics 2020, Fall 2010 Lab 2 page 5 of 6 University of Colorado at Boulder, Department of Physics PART III: SUPERPOSITION OF ELECTRIC FIELDS (REVISITED) Go to the simulation web-site (http://phet.colorado.edu/simulations) and select “Physics.” Next, select “Electricity, Magnets and Circuits” and then run the “Charges and Fields” simulation. Here you can place positive and/or negative charges wherever you like on the screen. A. Drag a positive charge from the red box to near the left side of the screen. Check the “Show E-field” box only. Notice that the strength of the field is indicated by the darkness of the arrows, rather than the arrow length as on your paper. Now drag a negative charge to near the right side of the screen. Is the electric field pattern similar to what you drew on paper? If not, why not? B. Repeat part A, but now with two charges of the same sign. Describe the differences between the two cases. PART IV: ELECTRIC FIELD HOCKEY – THE CHALLENGE Go to the simulation web-site (http://phet.colorado.edu/simulations), select under “Physics” the section “Electricity, Magnets and Circuits” and then run the “Electric Field Hockey” simulation. The aim of the game is to direct the black (positive) charge into the goal by placing positive (red) and/or negative (blue) charges on the screen. You can drag the charges from the boxes near the top side of the screen. Place one (or several) charges wherever you like on the screen. (Hint: Switch on the “field” button to view the electric field generated by the charges). Once you believe you have set up your configuration of charges, press the “start” button and see if you score a goal. Now try again after pressing “clear.” It is likely that you will score on the “practice” level, but here is the challenge: Will you be able to score on levels 1 to 3 as well? Discuss and list the configuration of charges needed to direct the charge in the goal. What do you have to change if the black charge is negative?Physics
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