3.7.3 Electric fields

3.7.3 Electric fields

1. Coulomb's law

   • F = (1/4πε0)(Q1Q2/rr)
     • ε0 is the permittivity of free space
     • air can be treated as a vacuum
     • inverse-square
   • consider charge to be at the centre of a charged sphere

2. Electric field strength

   • for a radial field
     • E = F / Q
       • E = (1/4πε0)(Q/rr)
   • for a uniform field
     • E = V / d
   • force per unit charge on a positive test charge placed at that point
     • equal to the negative of the potential gradient
   • work done derivation

3. Electric Potential

   • work done per unit positive charge on a positive test charge when it is moved from infinity to that position
   • V = (1/4πε0)(Q/r)
   • electric potential is 0 at infinity
     • reason for electric potential being negative
   • work done, ΔW = QΔV
   • equipotential surfaces
     • surfaces of constant potential
     • test charge moving along an equipotential has constant potential energy
       • no work is done because the force due to the field is at right angles to the equipotential
         • no work done to move from Y to Z
   • potential gradient in an electric field is the change of potential per unit change of distance in a given direction
     • E = V /d
       • ΔV is the area under a graph of E against r

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