Physics 2020 Lab: Electric Fields and Potentials page 1 of 8 University of Colorado at Boulder, Department of Physics Lab: Electric Fields and Potentials 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. In today’s lab we’ll explore the electric fields around charges and the electric potential produced by charges. The goals are to gain some intuition about the electric fields and potentials surrounding individual charges, and to learn how to add electric fields and potentials from multiple charges. PART I: ELECTRIC FORCES AND FIELDS You should have a number of diagrams, printed on paper and transparency foil, of the electric field distribution and the electric potential in the space around a single point charge (either positive or negative). Let’s start with the electric field vector plots. Describe as many details about the field patterns as you can notice. Describe both the positive and negative charges. 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 Lab: Electric Fields and Potentials page 2 of 8 University of Colorado at Boulder, Department of Physics 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 Lab: Electric Fields and Potentials page 3 of 8 University of Colorado at Boulder, Department of Physics In each of the figures below, draw a vector representing the electric field at the dot. Draw the arrows to scale, so that longer arrows represent stronger fields.Physics 2020 Lab: Electric Fields and Potentials page 4 of 8 University of Colorado at Boulder, Department of Physics PART II: SUPERPOSITION OF ELECTRIC FIELDS 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. Is this similar to any situation 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 spacings), 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. Where is the field the strongest? Where is it the weakest? Does it go to zero anywhere? Are there any other noticeable details? C. Repeat part B for two charges of the same sign. 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 Lab: Electric Fields and Potentials page 5 of 8 University of Colorado at Boulder, Department of Physics PART III: SUPERPOSITION OF ELECTRIC POTENTIALS In the previous section, you used the principle of superposition to find the total electric field at a number of points. 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: ...=++rrrtot 1 2EEE The same principle applies to electric potential (which is also called voltage). Namely, the total electric potential at any given point is equal to the sum of the electric potentials produced by each charge: Vtot = V1 + V2 + ... (Remember that electric potential is a scalar, not a vector.) A. Now using the electric potential diagrams, overlay a transparency foil over a paper diagram, so that you can see two sets of equipotential curves – one set from each of two point charges. Describe the total electric potential surrounding the charges if a positive charge is placed exactly on top of a negative charge. Does this make sense? B. Using a pair of electric potential diagrams for two charges of opposite sign, offset the transparency relative to the paper (by some even number of grid spacings), and lay a piece of tracing paper over the whole thing. On the tracing paper, mark the location and sign of the charges. Find an intersection between two curves, and mark the total electric potential at that point on your tracing paper. Repeat this for 10-15 points, or enough so that your paper contains sufficient information to roughly describe the potential surrounding the two charges. Find any dots that have the same total charge, and connect them to create equipotential curves (curves where every point on the curve has the same potential). Where is the potential the highest? Where is it the lowest? Does it reach zero anywhere? Are there any other noticeable details?Physics 2020 Lab: Electric Fields and Potentials page 6 of 8 University of Colorado at Boulder, Department of Physics PART IV: SUPERPOSITION OF ELECTRIC POTENTIALS (REVISITED) Go to the “Charges and Fields” simulation web-site (your TA will write the address on the board). Here you can place positive and/or negative charges wherever you like on the screen, and see the resulting potentials. A. Start with one charge: Drag a positive charge from the red box to the middle of the screen. Make sure that the check-boxes on the lower-right panel are unchecked. If you were to plot equipotential lines around this single charge, what would they look like? B. Now drag the “equipotential” box to various places on the screen and press the “equipotential” button. This will draw an equipotential contour line that has the same electric potential as the location of the crosshairs. C. Now try two charges: Drag a second positive charge from the red box to a location an inch or two away from the first charge. All of the previously-drawn equipotential lines should disappear – this is because the potentials around the charges are now
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