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ELECTRIC CURRENTS

ELECTRIC CURRENTS. Example Problem.

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ELECTRIC CURRENTS

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  1. ELECTRIC CURRENTS

  2. Example Problem A parallel-plate capacitor is held at a potential difference of 250 V. A proton is fired toward a small hole in the negative plate with a speed of 3.0 x 105 m/s. What is its speed when it emerges through the hole in the positive plate? (Hint: The electric potential outside of a parallel-plate capacitor is zero). Slide 21-26

  3. Capacitance A capacitor consists of two conductors that are close but not touching. A capacitor has the ability to store electric charge.

  4. Capacitance Parallel-plate capacitor connected to battery. (b) is a circuit diagram.

  5. Capacitance When a capacitor is connected to a battery, the charge on its plates is proportional to the voltage: The quantity C is called the capacitance. Unit of capacitance: the farad (F) 1 F = 1 C/V

  6. Charging a Capacitor Slide 21-37

  7. Capacitance The capacitance does not depend on the voltage; it is a function of the geometry and materials of the capacitor. For a parallel-plate capacitor:

  8. Dielectrics A dielectric is an insulator. The molecules in a dielectric tend to become oriented in a way that reduces the external field.

  9. Dielectrics and Capacitors This means that the electric field within the dielectric is less than it would be in air, allowing more charge to be stored for the same potential. Q= CV Slide 21-39

  10. Dielectric Constant With a dielectric between its plates, the capacitance of a parallel-plate capacitor is increased by a factor of the dielectric constantκ: Slide 21-40

  11. Storage of Electric Energy A charged capacitor stores electric energy; the energy stored is equal to the work done to charge the capacitor. If we start with zero charge at V=0, the work we do charging the capacitor is Q Vav Vav = 0 + V = ½ V 2

  12. Storage of Electric Energy The energy density, defined as the energy per unit volume, is the same no matter the origin of the electric field: The sudden discharge of electric energy can be harmful or fatal. Capacitors can retain their charge indefinitely even when disconnected from a voltage source – be careful!

  13. Storage of Electric Energy Heart defibrillators use electric discharge to “jump-start” the heart, and can save lives.

  14. Problem: (21-42) • Two uncharged metal spheres, spaced 15 cm apart, have a capacitance of 24.0 pF. How much work would it take to move 12.0 nC of charge from one sphere to the other?

  15. Electric Current Electric current is the rate of flow of charge through a conductor: (18-1) Unit of electric current: the ampere, A. 1 A = 1 C/s.

  16. Example Problem The discharge of the electric eel can transfer a charge of 2.0 mC in a time of 2.0 ms. What current, in A, does this correspond to? Slide 22-14

  17. Electric Current A complete circuit is one where current can flow all the way around. Note that the schematic drawing doesn’t look much like the physical circuit!

  18. Electric Current In order for current to flow, there must be a path from one battery terminal, through the circuit, and back to the other battery terminal. Only one of these circuits will work:

  19. Electric Current By convention, current is defined as flowing from + to . Electrons actually flow in the opposite direction, but not all currents consist of electrons.

  20. Properties of a Current Slide 22-12

  21. Conservation of Current Charge is conserved It is not stored at junction Slide 22-15

  22. Checking Understanding • Rank the bulbs in the following circuit according to their brightness, from brightest to dimmest. • The greater the current, • the brighter the bulb • A  B  C  D • A  B  C  D • A  D  B  C • B  C  A  D Slide 22-16

  23. Answer • Rank the bulbs in the following circuit according to their brightness, from brightest to dimmest. • The greater the current, • The brighter the bulb • A  B  C  D • A  B  C  D • A  D  B  C • B  C  A  D Slide 22-17

  24. Checking Understanding The wires below carry currents as noted. Rate the currents IA, IB and IC: Slide 22-18

  25. Answer The wires below carry currents as noted. Rate the currents IA, IB and IC: Slide 22-19

  26. Batteries The potential difference between the terminals of a battery, often called the terminal voltage, is the battery’s emf. E Slide 22-20

  27. Batteries in series • 1.5 V 3 V • 1.5 V • Like two escalators one above the other. This doubles the height of the escalator.

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