Current & Resistance

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Current & Resistance
2014-06-15 13:36:33
Chapter Nineteen Twenty
H Physics
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  1. current
    rate at which charges move through cross section of wire or given medium; movement of electric charge
  2. SI unit for current
    1 ampere A = 1 coulomb/sec c/s
  3. charge carriers
    positive or negative charges in motion
  4. conventional current
    • current consisting of positive charges that goes in direction of electric field, from positive to negative
    • represents the effect of the actual motion of the charge carriers (equivalent conventional current of negative is in opposite direction; same direction for positive)
  5. drift velocity
    net velocity of the charge carrier moving in an electric field, taking all of their collisions and zig zags into account
  6. how a current is created
    • potential difference is applied across conductor
    • charge carriers have to be "pumped" through the circuit; work has to be done on them to maintain v
    • electric field force sets the electrons in movement
    • charges collide with atoms in the device and convert electrical potential energy to kinetic energy to move
  7. how batteries/generators maintain a potential difference
    they convert energy into electrical energy, this energy is supplied to charge carriers
  8. energy conversion in batteries vs. generators
    • batteries convert chemical energy to electrical energy
    • generators convert mechanical energy to electrical energy
  9. direct current
    • charges move only in one direction, potential difference is fixed
    • generated by batteries and generators
  10. alternating current
    • terminals of the source of the electrical potential are always changing signs, no net motion
    • only generated by generators
  11. point of conductor
    surface perpendicular to the motion of the charges
  12. circuit
    charge flows through a closed path and returns to its starting point
  13. power
    rate at which work is done; rate of energy transfer
  14. power of a resistor
    amount of heat produced when current flows through (joule heating: heating of the resistor)
  15. electromotive force emf
    max potential difference of a device
  16. terminal voltage
    • actual potential difference between the terminals 
    • less than the emf because of the internal resistance of the battery (caused by collisions between charges and atoms within battery)
  17. resistance
    opposition of the motion of charge through a conductor, quantified by the geometric characteristics of the material through which the charge is moving
  18. resistance SI unit
    1 ohm = 1v/amp
  19. resisitivity and temperature relationship
    • usually resistivity and temperature increase together
    • for semi-conductors, there is an inverse relationship
  20. Ohm's Law
    • ratio of a potential difference applied across a conductor to the current through the conductor is constant
    • graph of current vs. potential difference is linear with a constant slope equal to the inverse of the resistance
  21. ohmic
    materials that follow Ohm's Law; constant resistance over a wide range of potential differences
  22. potential difference and current relationship
    • higher potential difference means greater current
    • directly related
  23. length effect on resistance
    shorter objects have less resistance; longer objects have greater resistance
  24. cross-sectional area effect on resistance
    greater area has less resistance; smaller area has greater resistance
  25. temperature effect on resistance
    lower temperatures have less resistance; higher temperatures have more resistance
  26. relationship between current and resistance with constant potential difference
    inverse relationship; current decreases when resistance increases
  27. short circuit
    • has very little resistance so more energy gets dissipated, lacks a resistor 
    • usually the circuit is cut off from a capacitator so recharging does not occur; burns out quickly
    • low resistance increases current, can cause circuit to overheat and catch fire
  28. series circuit
    provide a single conducting path without junction; only one path for the charge to follow
  29. parallel circuit
    connected with junctions providing separate conducting paths; some charged go one way, others go the other
  30. schematic diagram
    graphic representation of an electric circuit with symbols representing circuit components
  31. electric circuit
    path through which charges can be conducted; set of electrical components connected so that they provide complete paths for charge movements
  32. load
    element that dissipates energy, increases the resistance and is a type of resistor (e.g. light bulb)
  33. simple circuit
    has a source of potential difference and electrical energy (e.g. battery) and a load (e.g. light bulb)
  34. closed circuit
    closed-loop path for electrons to follow, path from one battery terminal to the other is complete, switch is closed to allow steady current flow
  35. open circuit
    incomplete path and no charge flow and no current, switch is open, circuit does not work
  36. potential difference across a load and terminal voltage
    • potential difference across a load equals the terminal voltage
    • conservation of energy says that the energy gained must equal the energy dissipated in a charge's trip in a circuit
  37. watt conversion
    1 w= 1 vxa
  38. battery drawing
    ---| |--- longer side is positive, shorter side is negative
  39. relationship between parallel and series circuit
    inversely proportional
  40. equivalent resistance
    • a single resistor can replace a group of series or parallel resistors; represents total current found by using individual resistance values
    • for series circuits, add the sum of the individual resistances
    • for parallel circuits, do the same with inverses
  41. current in series/parallel circuits
    • constant throughout series circuits (conservation of charge, current is the same throughout all resistors)
    • proportioned across each resistor in parallel circuits
  42. voltage in series/parallel circuits
    • proportioned across each resistor in series circuit 
    • constant throughout each resistor in parallel circuits (sides of bulbs are connected to common points, so voltage across is the same)
  43. do series/parallel circuits require all elements to conduct?
    • series require all elements to conduct
    • parallel do not require all elements to conduct; provides separate alternate pathways
  44. relationship between equivalent resistance and individual resistances in parallel circuits
    inversely related; low equivalent resistance can be created with a group of high resistances in parallel
  45. complex circuits
    consist of series and parallel circuits