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Journal 3/3/17 Electric fields are similar to gravity. But how are they different? Objective To learn how the electric field is different from gravity Tonight’s Homework pp 445: 11, 12, 15, 16, 19 Notes on Electric Shielding / Capacitors Let’s start today with a setup. Imagine we have a hollow metal sphere. This sphere is charged with a bunch of negative charges. Inside, we have a positive charge. Let’s draw the field lines for this setup. Notes on Electric Shielding / Capacitors Let’s start today with a setup. Imagine we have a hollow metal sphere. This sphere is charged with a bunch of negative charges. Inside, we have a positive charge. Let’s draw the field lines for this setup. Notes on Electric Shielding / Capacitors Let’s start today with a setup. Imagine we have a hollow metal sphere. This sphere is charged with a bunch of negative charges. Inside, we have a positive charge. Let’s draw the field lines for this setup. Notes on Electric Shielding / Capacitors You’ll notice that outside the sphere, we have a lot of lines pointing in to the negative charges. It doesn’t seem like the positive charge has had any effect outside the sphere. Notes on Electric Shielding / Capacitors Let’s do another setup that’s similar. This time we place our positive charge outside the sphere. Where do the field lines inside go? (draw them!) Notes on Electric Shielding / Capacitors Let’s do another setup that’s similar. This time we place our positive charge outside the sphere. Where do the field lines inside go? (draw them!) We can’t! There is no way to draw the lines inside so they don’t exist! Notes on Electric Shielding / Capacitors There are two conclusions we can draw from these examples: 1) From a distance, a charged metal shell will appear exactly the same as a single point charge. 2) If we have a metal shell, the area inside and the area outside are completely independent of each other electrically. Charges on the inside cannot be detected outside and vice versa. Notes on Electric Shielding / Capacitors This leads us to what we saw in our lab a few days ago. If we create a metal cage around an object, we get this same effect. Any charges we place inside can’t “communicate” with the outside. The first person to really realize the potential of this was Michael Faraday in 1836. He coined these “Faraday Cages”. These cages have tons of use. The FBI headquarters has metal lining the entire building. This prevents any spy devices from seeing computer information from outside. Notes on Electric Shielding / Capacitors Let’s look at one last setup for today. What kind of electric field do we get in this example where we have 2 charged sheets of metal? Notes on Electric Shielding / Capacitors Let’s look at one last setup for today. What kind of electric field do we get in this example where we have 2 charged sheets of metal? You’ll note that our electric field between the sheets is constant. It’s uniform. If we pack more and more charge on to something like this, we can create a device called a capacitor. Notes on Electric Shielding / Capacitors Not THAT kind of capacitor. (If none of you get this, then Mr. C. is officially getting old) Notes on Electric Shielding / Capacitors Capacitors have the ability to store electrical energy. How? As we add charge to the plates, the electric potential between them increases. We’re making a steeper “hill to valley” of potential. We measure capacitance as charge over voltage. How much charge we have and how strong of a potential it creates. The more potential we can create with less charge, the more efficiently we’re storing electrical energy. Equations Potential on Parallel Plates E V=E●d V V: Potential difference in Volts E: Electric field strength in N/C d: Distance between the plates in meters S This equation tells us how strong of a potential difference we can find between two charged, parallel plates of metal. Equations Capacitance E C=q/V V C: Capacitance in Farads (1 C/V) q: The strength of the charge on one plate in Coulombs. V: The potential difference between the plates in volts. S This equation tells us how efficiently electrical energy is stored between two charged plates. Exit Question Do you think a stronger capacitor could be made with 3 charged plates instead of 2? a) yes b) no