electric fields unit 4

  1. people handling microchips wear antistatic clothing and work in rooms filled with antistatic flooring explain why
    • damage to microchip by charge 
    • potential difference between chip and body will create an electric field 
    • flow of charge that could damage inside the chip
    • antistatic clothing prevents build up of charge on person 
    • antistatic flooring prevents transfer of charge
  2. there are only two kinds of charge
    positive and negative
  3. charge of the same kind
    repel each other
  4. charge of different kinds
    attract each other
  5. the amount of charge is called the
    quantity of electricity
  6. quantity of positive charge is a
    positive number
  7. quantity of negative charge
    negative number
  8. charge cannot be
    created or destroyed but moved from one object to another or from one part of a body to another
  9. total amount of charge in the body in the transfer process
    remains the same
  10. this is called the
    law of conservation
  11. electrons and protons carry the
    same amount of different charge
  12. quantity of charge e is given on
    data sheet
  13. quantity of all charged objects is equal to
    either e or its multiple therefor the quantity of charge e is called the elementary charge
  14. Columbus law
    the magnitude of F the electrical force exerted by one point charge on another point charge is directly proportional to the magnitudes q1 and q2 and inversely proportional to the square of the distance r between the charges
  15. F =
  16. where k is constant and is
    1/4pie0 where e0 is the permitivity of free space
  17. f is often called the
    electrostatic force
  18. regardless of whether the forces are attractive or repulsive they are directed along
    the line between the charges and have equal magnitude
  19. each charge exerts a force of magnitude F on the
  20. what are the units for electric field strength
    • Jm-1C-1
    • JA-1s-1m-1
    • NA-1s-1
  21. define the electric potential at a point in an electric field
    work done per unit charge on a small positive test charge in moving the charge from infinity to the point
  22. if got parallel plates with +3v at top and +1v at bottom the electric field direction is
  23. an ion travels horizontally and enters a uniform vertical electric field the horizontal component of the velocity of the ion is
    unaffected by the electric field
  24. the electric field that exists at a point is the
    electrostatic force experienced by a small positive test charge placed at that point divided by the charge itself
  25. electric field is a
    vector and its direction is the same as the direction of the force F on a positive test charge
  26. SI unit of electric field
  27. it is the .... in the environment that create an electric field at a given point
  28. the field exists in a sense that
    whenever a positive or negative charge is placed at that point the field exerts a force on the charge
  29. any charge in the environment contributes to
    electric field that exists at a point
  30. to determine the net electric field it is necessary to
    determine the various contributions separately and then find the vector sum of the contributions
  31. the electric field produced by a point charge q can be obtained in general terms from Columbus first law , magnitude of force on positive test charge =
    • F = kq0q/r2
    • where q0 is the positive test charge
  32. divide by q0 to obtain magnitude of electric field
    E = kq/r2
  33. so electric field doesn't depend on
    test charge
  34. electric field disregards
    whether charge is negative or positive
  35. electric field lines go from
    positive to negative
  36. of q is positive then
    E is directed away from q
  37. if q is negative then E is
    directed towards q
  38. in an electric field when moving positive test charge from point a to point b the work done is equal to
    difference between the electric potential energy at B and that at A
  39. W =
    EpEb - EpEa
  40. the work done to move the charge from a to b depends on
    the magnitude of the charge
  41. W =
    qx V
  42. V is the
    electric potential energy per unit positive test charge
  43. SI unit of electric potential is
    JC-1 = volt
  44. a positive test charge accelerates from a region of
    higher potential to a region of lower potential
  45. conversely a negative test charge accelerates from a
    region of lower potential to a region of lower potential
  46. electric potential at a distance r from a point charge is
    V = kq/r
  47. the previous expression for v assumes the potential is
    zero at infinite distance away from the charge
  48. see diagram from notes in the electric field E the work done required to move a negative test charge from a to b is W=
  49. where F is the
    magnitude of the applied force directed to the left in opposition of the electric force
  50. where r is the
    distance between a and b
  51. E =
  52. V =
  53. W/Q =
  54. W =
  55. if the test charge is moved at a constant speed the magnitude of the applied force equals
    the magnitude of the electric force but in the opposite direction so F = -EQ
  56. the minus sign in the above equations indicates E is
    in the direction of decreasing potential
  57. an alternate unit for electric field E is
  58. comparison of electric fields and gravitational fields 
    • gravitational force exists between any two objects with mass
    • electric force exists between any two objects with charge 
  59. magnitude of force 
    • newtons law of universal gravitational constant F = GMm/r2
    • G is the universal gravitational constant 
    • Newtons law of gravitation for the force between two point masses is an inverse square law - F Ø 1/r2
    • coloumbs law F = 1/4pie0 x q1q2/r2 where e0 is the permitivity of free space 
    • coloumbs law for the electric force between two point charges is an inverse square law F Ø r2 

  60. direction of force 
    always attractive 

    can be attractive or repulsive 
  61. field strength 
    is the gravitational force per unit mass g = GM/r, so field around a mass is proportional to the quantity of mass and inversely proportional to the square of the distance between the centre of mass M and the point in space where the field is measured 

    is the electric force per unit charge , E = 1/4pie0 x Q/r so field around charge is proportional to the quantity of charge and inversely proportional to the square of the distance between the centre of charge Q and the point in space where the field is being measured 
  62. potential in radial field 
    V = -GM/r , gravitational potential energy is the work done per unit mass on a small mass in moving the mass from infinity to that point . gravitational potential is zero at infinity . 

    V = 1/4pie0 x Q/r , electric potential is the work don per unit charge on a positive test charge in moving the charge from infinity to that point . electric potential is zero at infinity . 
  63. line of force or field line 
    path of free test mass in the field 

    path of free positive test charge in a field 
  64. unit of field strength 
    NKg-1 or ms-2

    NC-1 or Vm-1
  65. unit of potential 

    JC-1 or V 
  66. uniform fields 
    g is the same everywhere , field lines parallel and evenly spaced 

    E is the same everywhere , field lines parallel and evenly spaced 
Card Set
electric fields unit 4