IB Coulomb's Law with Dielectrics

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.[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.
Comment