Electric Fields and potential

Electric Fields and potential

1. Electric fields

   • Attractive or repulsive
   • Electric field strength
     • The force per unit positive charge
       • The force that a charge of +1C would experience
     • E = F/Q
     • Field lines are drawn to show the direction of the force that would act on a +ve charge
       • For parallel plates the lines point from the most positive plate/voltage to the least positive
     • Radial Fields
       • Any body that behaves as if all its charge is concentrated at the centre
       • E = Q/(4piE0r^2)
         • E is directly proportional to 1/r^2
       • on a graph of E against r the graph starts at the surface/radius of the sphere
     • Uniform Fields
       • E = V/d

2. Coulomb's law

   • F = (1Q1Q2)/(4PiE0r^2)
     • F is -ve when the charges are opposite and the force is attractive
     • F is +ve when the charges are alike and the force is repulsive
   • F is proportional to 1/r^2
     • Inverse square law
   • Gives the force of attraction or repulsion between two point charges

3. Electric Potential

   • The work done per unit positive charge on a positively test charge to move it from infinity to that point in the field
   • V = Q/(4piE0r)
     • V is +ve when Q is +ve and the force is repulsive
     • V is -ve when Q is -ve and the force is attractive
     • Absolute Magnitude of V is greatest on the surface of the charge
   • The gradient of a V against r graph gives the field strength at that point
     • E = ▲V/▲r
   • Electric Potential Difference
     • ▲W = Q x ▲V
     • The energy needed to move a unit charge