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Homework Help: Radial Electric Field Conceptual

  1. Sep 14, 2012 #1
    1. The problem statement, all variables and given/known data
    Carefully study the following plot of electric field E in N/C versus distance r in m and answer the questions that follow. The electric field is directed radially outward, and the variation of E with r is independent of direction. [/b]
    Efield.jpg
    For each statement, select "True" or "False".[/b]
    The work done by you to bring a point negative charge from infinity to A is positive.[/b]
    The Electric field falls off as 1/r for r greater than R.[/b]
    The force on a negative charge placed at A points radially towards the origin.[/b]
    All points at a given distance 'd' from the origin are at the same electrostatic potential.[/b]
    The above electric field could be that due to a negatively charged spherical shell.[/b]
    The electric potential is constant for r less than R. [/b]

    2. Relevant equations
    Va-Vb=∫Edl
    E=Kq/r^2

    3. The attempt at a solution
    First i figured that since the electric field is point outward the the charge must be positive to accelerate positive charges outward.[/b]
    Therefore the work to bring a negative charge from infinity to the origin would be positive the force is "pulling the negative charge in towards the origin.[/b]
    It sure looks like the graph has a 1/r curve after r > R.[/b]
    If the electric field points outward then a negative charge would accelerate inward. therefore the force on the negative charge is inward.[/b]
    I thought about the equi-potential line at 'd' the voltage should be the same at any point at the same distance away from the charge.[/b]
    The electric field could not be due to a negative charge, If it were the E-Field would point in instead of out.[/b]
    Since electric field is the derivative of Electric Potential and the E-Field is zero for the given range the potential must be constant.

    Where am i going astray here? I was given the hint: "Remember that the Electric Field is the negative spatial derivative of the potential. Look up the definition of electric potential and work done against an electric field. "
     
  2. jcsd
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