prepwalk

1. PHYSICS
ELECTROSTATICS
Short revision series

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

7. end