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Ch. 17, Electrical Energy and Current

Ch. 17, Electrical Energy and Current. Electrical Potential Energy: Results from an interaction of two objects’ charges. How can you give a test charge lots of electric potential?. Electric Potential is a component of Mechanical Energy: ME = KE + PE gravity + Pe elastic + PE electric.

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Ch. 17, Electrical Energy and Current

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  1. Ch. 17, Electrical Energy and Current

  2. Electrical Potential Energy: Results from an interaction of two objects’ charges.

  3. How can you give a test charge lots of electric potential?

  4. Electric Potential is a component of Mechanical Energy: ME = KE + PEgravity + Peelastic + PEelectric

  5. Recall that whenever a force is used to move an object, work is done. • This is the case for electric work.

  6. Electrical potential energy in a uniform electric field • Peelectric = -qEd

  7. See figure 2

  8. Helpful Table:

  9. PEelectric is similar to PEgravity

  10. Electric Potential: • V = PEelectric/q

  11. Potential Difference: • Delta V = PEelectric/q • Hint for sign on delta V: If the charge GAINS potential energy, it will be +.

  12. V = volts = 1 J/C • As a charge of 1 C moves through a potential difference of 1 V, it gains 1 J of energy.

  13. Potential difference in a uniform electric field: • Delta V = -Ed • d = motion parallel to the electric field.

  14. Potential difference between a point at infinity and a point near a charge: • Delta V = Kc * q/r

  15. Often, earth is defined as having an electric potential of zero. • Grounding an electrical device creates a useful frame of reference.

  16. Sample Problem A, page 599

  17. Batteries: • A 1.5 Volt battery will provide a constant 1.5 V difference between the + and – terminal.

  18. A chemical reaction in the battery provides a large supply of electrons. • As 1 C of charge moves through the battery, 1.5 J of energy are given to the device.

  19. High voltage: means it has the potential to do a lot of work.

  20. A capacitor is a device that is used to store electrical potential energy. • Has many uses, including in radios.

  21. A capacitor is useful because energy can be taken from the capacitor when needed. • A parallel-plate capacitor has two metal plates separated by a small distance. • A battery or other source of potential difference is connected to the two plates.

  22. Capacitance is the ratio of charge to potential difference. • C = Q/deltaV • C = farads (F)  1 C/V

  23. Figure 5, page 602

  24. In a vacuum: • C = ε0 A/d • ε0 = 8.85 E-12

  25. A sphere’s capacitance increases as the sphere gets bigger. The earth has a massive C. • Giving charge to the earth will not change its electric potential much.

  26. A dielectric can be used to decrease the capacitance of a capacitor.

  27. Discharging a capacitor releases its charge. • Camera flashes and some keyboards use this.

  28. A capacitor stores electrical potential energy. • PEelectric = ½ Q*deltaV • Q = charge on 1 plate.

  29. There is a limit to how much energy can be stored.

  30. There is a limit to how much energy can be stored.

  31. Movement of electric charge: CURRENT. • Current is the rate at which charges pass through a certain area. • I = Delta Q / delta t • I = A (amperes, 1 C/s)

  32. Conventional current is the direction that + Charges travel.

  33. Drift velocity: • When you turn on a switch, the electrons travel in a direction opposite the electric field. • They bounce off the walls of the conductor and the vibrational energy increases the temp. of the conductor.

  34. Drift velocity is very slow, about 2.5 E -4 m/s. It would take electrons about an hour to travel 1 meter. But the signal to start moving (a changed electric field) travels down the wire at the speed of light.

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