Electric field

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  1. How to draw electric field line diagrams
    Draw the electric field line diagrams for the following:
    1. Positive point charge
      PHYS 8 2 1a
    2. Negative point charge
      PHYS 8 2 1b
    3. Parallel plates with equal magnitude charges
      PHYS 8 2 1c
    4. Two point charges with opposite equal magnitude charges
      PHYS 8 2 1d
    5. Two point charges with positive equal magnitude point charges
      PHYS 8 2 1e
    6. Two point charges with negative equal magnitude point charges
      PHYS 8 2 1f
  2. Calculations with electric field
    A pair of positive and negative point charges are fixed in the following positions:
    PHYS 8 2 2
    1. Find the electric field (magnitude and direction) at point A.
    2. A charge placed at A experiences a force of 5.9×103N5.9 \times 10^{-3} N [right]. Find the magnitude and polarity of the charge.
Topic Notes
In this lesson, we will learn:
  • Meaning of electric field and its relationship to electric force
  • How to draw electric field line diagrams
  • Solving problems with electric field

  • Electric field (E) is the electric force exerted by a charge Q on another charge q, per unit charge of q. It is a vector quantity.
  • By convention, the direction of electric field vectors is defined as the direction that a positive test charge would move if placed in the field. Fe points in the same direction as E for positive charges, and in the opposite direction of E for negative charges.
    • A test charge is a point charge with a very small magnitude. Test charges have a small magnitude charge so that the electric field of the test charge is negligible and does not affect the electric field that is being investigated.

Electric Field

E=FeQE= \frac{F_e}{Q}
or equivalently, by substituting Coulomb's law:
E=kQr2|E| = k \frac{|Q|}{r^2}
E:E: electric field, in newtons per coulomb (N/C)
Fe:F_e: electric force, in newtons (N)
q:q: charge that experiences the field, in coulombs (C)
k:k: Coulomb's law constant, 9.00×109Nm2/C29.00 \times 10^9 N\centerdot m^2 / C^2
Q:|Q|: magnitude of charge that creates the field, in coulombs (C)
r:r: distance from charge, in meters (m)

Coulomb's Law (Electric Force)

Fe=kQ1Q2r2|F_e| = k \frac{|Q_1 Q_2|}{r^2}
Fe:|F_e|: magnitude of electric force, in newtons (N)
k:k: Coulomb's law constant, 9.00×109Nm2/C29.00 \times 10^9 N\centerdot m^2 / C^2
Q1,Q2:|Q_1|, |Q_2|: magnitude of each charge, in coulombs (C)
r:r: distance between charges, in meters (m)