2. Coulomb’s law
For charges in a vacuum
F α Q1 Q2 F = k Q1 Q2
d2 d2
k = 1/4πε0 = 9 x 109 Nm2 C-2
ε0 = 8.854 x 10-12 Fm-1
F = Q1 Q2
4πε0d2
In a medium such as water F = Q1 Q2
4πεr2
ε = ε0 εr εr = relative permitivity
3. Intensity of an electrostatic field
An electric field exerts a force on a charged
body. The intensity E of an electrostatic field
at any point is the force per unit charge
which it exerts at that point.
E = F/Q
If F = 1 Q1 Q2
4πε0 d2
E = Q
4πε0d2 NC-1
4. Calculating electric field flux
Ψ = E x area
= Q x 4πr2 = Q
4πεr2 ε
Eg calculate the flux of a charge 3nC in a
vacuum
3 x 10-9
ε0
5. Potential of a charge
What is the potential of a charge 3nC
positioned at 0.05m from a certain point in
vacuum?
6. solution
Potential of a charge is the work done in
bringing a unit positive charge from infinity
to that point against action of the field
V = Q
4πε0r2
= 3x 10-9 x 9 x109 = 54,000 V
0.05