IB Coulomb's Law with Dielectrics

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A tutorial sheet of problems with dielectrics separating the charges is given below

  1. Two point charges, +2.0 nC and +2.0 nC are placed 5.0 cm apart in a vacuum. Draw the electric field around the charges and determine the electric force between the charges.
  2. The point charges in question 1 are now placed in a material of relative permittivity 4. Draw the electric field lines. Give a reason why the electric force between the charges increases/decreases/stays the same.
  3. A point charge -4.0 nC is at a height of 6.0 cm above a deep, still, freshwater lake. Draw the electric field lines in air and water. If the dielectric constant of water is 81 determine the electric force on the charge.[See p184 Electromagnetism by Lorrain and Corson, attraction, k(K-1)/(K+1)q2/(4d2), 9.75x10-6N]
  4. Two point charges +6.0 μC and -6.0 μC are placed 12.0 cm apart in a vacuum. An infinite rectangular slab of dielectric material of thickness 6.0 cm and relative permittivity 4 is placed with its surface 3.0 cm from each charge perpendicular to the line joining the charges. Draw the electric field lines and determine the electric force acting on each charge.