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Chapter 21

Electric Fields. Chapter 21. Electric Fields. A charge creates an electric field around it in all directions A second charge placed in that field will interact. Compare this to the gravitational field. Electric Fields -- Continued.

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Chapter 21

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  1. Electric Fields Chapter 21

  2. Electric Fields • A charge creates an electric field around it in all directions • A second charge placed in that field will interact. • Compare this to the gravitational field.

  3. Electric Fields -- Continued • A satellite interacts with the gravitational field without touching the planet. • Likewise, an electron interacts with the positive field created by the nucleus of an atom.

  4. Let’s say you want to measure the electric field at different points around a charge. • Always use a positive test charge, q. • Draw vector lines showing size and direction of force. These are called “force lines”

  5. Field Lines • Once the force lines are connected, they become “field lines” and they go to infinity. • They always indicate the force on a positive test charge (so they begin at a positive charge and end at a negative charge).

  6. Field Lines -- Continued • The spacing between the field lines shows the strength of the field • The magnitude of the electric field is a vector measuring the force per unit charge.

  7. Van de Graaff Generator • + charge builds up on comb at bottom. • Charges move up insulated belt to top. • Charges spread around sphere at top (why?).

  8. Energy & Electric Potential • Remember gravitational potential energy from Newtonian mechanics? • The larger the distance of something from Earth the more potential energy it has. • If there were no gravity, there would be no GPE (the object would not “want” to fall to earth)

  9. BB Bowling Ball • Bowling ball has more GPE because… • Could say that each BB-sized portion of the bowling ball has the same potential as the BB • So PE/mass = “potential”. We don’t use this in Newtonian mechanics but we do when talking about electricity. h

  10. Now consider an electric field… B A • It would take energy to move a + charge from A to B. • Amount of energy required to move charge is proportional to Battery

  11. Electric Potential Difference • Remember that work done on an object = F●d • In the electric world…need to know the “electric potential difference” (∆V) which is the work required to move a charge from one point to another divided by the magnitude of the charge. Units:

  12. Electric Potential Difference -- continued • Imagine you have a negative charge with a positive test charge nearby

  13. Electric Potential Difference -- continued • Which direction does the positive test charge want to move? • If you moved the test charge away from the negative charge would you be doing work on it?

  14. Voltage vs. Volts • The change in electric potential (electric potential difference) is called the “voltage” • We can only measure the difference between electric potential (thus the ∆ in ∆V). • The units for voltage are volts

  15. Imagine two oppositely charged plates and the electric field they would create • Except at the ends, the field would be the same between the plates. • Move a test charge a distance, d, against the field direction. • W=F●d

  16. Remember that ∆V = W/q = Fd/q = (F/q)d • F/q is electric field strength so… The potential difference in a uniform field is field strength ●distance.

  17. Millikan’s Oil Drop Experiment • Fine, charged oil particles were sprayed between two charged plates. • Electric field was adjusted so the particles were suspended between the plates

  18. Millikan -- continued • Millikan used E=∆V/d to determine the electric field strength.

  19. Millikan’s Results • Millikan • Millikan is credited with finding

  20. Electric Fields Near Conductors • Remember that electrons are free to move around on a conductor. • Electrons will move around a conductor until they are as far apart as possible. • If a closed, metal conductor (i.e. car) becomes charged, the charges are all concentrated on the outside of the conductor and the electric field inside the conductor is zero.

  21. Charges on Irregular Surfaces • On an irregular surface, electrons will be closer together on the “pointy parts” so they will become more concentrated. • If that part gets close to another object, a discharge may occur (and a spark created).

  22. What is the point (get it?) of a lightning rod?

  23. Capacitors • Provide a way to store electrical energy. • Imagine taking two conducting plates and charging them oppositely (pushing electrons from one to the other).

  24. Put an insulating layer in between to keep the electrons from jumping from one plate to the other. • If a wire is placed between the two of them, electrons will flow until the two plates are charged equally. Insulator

  25. Capacitance = net charge on one plate/potential difference between the plates.

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