Physics 22.4 Coulomb's Law

Question 1

What are the equations for Coulomb’s Law?

Answer 1

F=k((Q1Q2)/r^2)

and

F=(1/(4πε0))x((Q1Q2)/r^2)

• F is the force on each charge in Newtons [N]
• Q1 and Q2 are the charges of the interacting charge in coulombs [C]
• r is the separation of the charges in metres [m]
• k is a constant of proportionality
• ε0 is the permitivity of free space in farads per metre [F m^-1]

k = 9.0 * 10^9 N m^2 C^-2
ε0=8.85 * 10^-12 F m^-1

Question 2

What are the conditions for Coulomb’s Law?

Answer 2

• The force on Q1 is always equal and opposite to that on Q2
• The force can be attractive or repulsive
• The equation also works for spherically-symmetric charge distributions
• The magnitude of the electric force between two objects is grater than the gravitational force between them
• Air can be treated as a vacuum when calculating force between charges