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EXAM III

EXAM III. Check out HW problems Ch. 20: P-2,26, Example 21.4 Ch. 22: P-1 Is the change in U e Δ U, A) positive B) negative C) zero as a positive charge moves from point labeled i to f?. + - i f. Example Problem.

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EXAM III

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  1. EXAM III • Check out HW problems • Ch. 20: P-2,26, Example 21.4 • Ch. 22: P-1 • Is the change in Ue ΔU, A) positive B) negativeC) zeroas a positive charge moves from point labeled i to f? +- i f

  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. Additional Example Problems • In the circuit shown below: • How much power is dissipated by the 12 Ω resistor? • What is the value of the potential at points a, b, c, and d? Slide 23-51

  4. Problems • In the circuit shown below: • Rank in order, from most to least bright, the brightness of bulbs A–D. Explain. • Describe what, if anything, happens to the brightness of bulbs A, B, and D if bulb C is removed from its socket. Explain. Slide 23-49

  5. Magnetism

  6. Magnets and Magnetic Fields Magnets have two ends – poles – called north and south. Like poles repel; unlike poles attract.

  7. Magnets and Magnetic Fields However, if you cut a magnet in half, you don’t get a north pole and a south pole – you get two smaller magnets.

  8. Mapping Out the Field of a Bar Magnet Slide 24-15

  9. Magnets and Magnetic Fields Magnetic fields can be visualized using magnetic field lines, which are always closed loops. Field lines go from N to S leave at N enter on S

  10. Magnetic Fields Produced by Bar Magnets A single bar magnet (closeup) A single bar magnet Slide 24-17

  11. Magnetic Fields Produced by Bar Magnets Two bar magnets, like poles facing Two bar magnets, unlike poles facing Slide 24-18

  12. Magnets and Magnetic Fields The Earth’s magnetic field is similar to that of a bar magnet. Note that the Earth’s “North Pole” is really a south magnetic pole, as the north ends of magnets are attracted to it.

  13. Magnets and Magnetic Fields A uniform magnetic field is constant in magnitude and direction. The field between these two wide poles is nearly uniform.

  14. Checking Understanding Slide 24-19

  15. Checking Understanding • Answer • B • A • D • E Slide 24-20

  16. Electric Currents Also Create Magnetic Fields A long, straight wire A current loop A solenoid Slide 24-22

  17. Electric Currents Produce Magnetic Fields Experiment shows that an electric current produces a magnetic field.

  18. Electric Currents Produce Magnetic Fields Slide 24-24

  19. Magnetic Field Due to a Long Straight Wire The field is inversely proportional to the distance from the wire: ( The constant μ0 is called the permeability of free space, and has the value:

  20. Definition of B Unit of B: the tesla, T. 1 T = 1 N/A·m. Another unit sometimes used: the gauss (G). 1 G = 10-4T.

  21. Electric Currents Produce Magnetic Fields The direction of the field is given by a right-hand rule.

  22. Checking Understanding Point P is 5 cm above the wire as you look straight down at it. In which direction is the magnetic field at P? Slide 24-26

  23. Answer Point P is 5 cm above the wire as you look straight down at it. In which direction is the magnetic field at P? D. Slide 24-27

  24. Checking Understanding The magnetic field at point P is zero. What are the magnitude and direction of the current in the lower wire? • 10 A to the right. • 5 A to the right. • 2.5 A to the right. • 10 A to the left. • 5 A to the left. Slide 24-31

  25. Answer The magnetic field at point P is zero. What are the magnitude and direction of the current in the lower wire? • 10 A to the right. • 5 A to the right. • 2.5 A to the right. • 10 A to the left. • 5 A to the left. Slide 24-32

  26. Magnetic Fields Exert Forces on Moving Charges

  27. The Force on a Charged Particle Moving in a Magnetic Field Slide 24-46

  28. Magnetic Fields Exert Forces on Currents (Moving Charges)

  29. ELECTRICITY AND MAGNETISMELECTRICITY HAS MAGNETIC EFFECTSMAGNETISM EFFECTS ELECTRICITY

  30. Right Hand Rule

  31. Force on an Electric Current in a Magnetic Field; A magnet exerts a force on a current-carrying wire. The direction of the force is given by a right-hand rule.

  32. Force on an Electric Current in a Magnetic Field; A magnet exerts a force on a current-carrying wire. The direction of the force is given by a right-hand rule.

  33. Force on an Electric Current in a Magnetic Field; The force on the wire depends on the current, the length of the wire, the magnetic field, and its orientation. 1

  34. What is direction of B • A) up • B) down • C) left • D) right

  35. Force on Electric Charge Moving in a Magnetic Field The force on a moving charge is related to the force on a current: ) Once again, the direction is given by a right-hand rule.

  36. Force on Electric Charge Moving in a Magnetic Field

  37. Solenoids and Electromagnets A solenoid is a long coil of wire. If it is tightly wrapped, the magnetic field in its interior is almost uniform: (

  38. Solenoids and Electromagnets If a piece of iron is inserted in the solenoid, the magnetic field greatly increases. Such electromagnets have many practical applications.

  39. Induced EMF When Bchanges a current is induced. The current depends on how much Bchanges and whether B increases or decreases. A changing B induces an emf

  40. Motional emf Magnetic field B is into the screen. We represent this by X If B was out of the screen we draw . Slide 25-12

  41. Induced Current in a Circuit Slide 25-13

  42. Which of the following will cause an induced current in a coil of wire? A. A magnet resting near the coil. • B. The constant field of the earth passing through the coil. • C. A magnet being moved into or out of the coil. • D. A wire carrying a constant current near the coil. Slide 25-5

  43. Answer • Which of the following will cause an induced current in a coil of wire? A. A magnet resting near the coil. • B. The constant field of the earth passing through the coil. • C. A magnet being moved into or out of the coil. • D. A wire carrying a constant current near the coil. Slide 25-6

  44. Faraday’s Law of Induction The induced emf in a wire loop is proportional to the rate of change of magnetic flux through the loop. Magnetic flux: (21-1) Unit of magnetic flux: weber, Wb. 1 Wb = 1 T·m2

  45. Faraday’s Law of Induction; Lenz’s Law This drawing shows the variables in the flux equation:

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