Electric Fields

Electric Fields

1. Electric field strength

   • A positive charge,q, is used as a test charge
     • if q experiences a force proportional to charge then it is in an electric field
       • defined as: E=F/q
   • Electric field strength a vector quantity

2. Coulombs Law

   • two point charges, separated by a distance in a vacuum exert forces on each other
   • F=1/(4πε)*QaQb/(r^2)
   • With Coulombs law: E=1/(4πε)*Q/(r^2)
     • The direction of field for positive Q is radially outwards, radially inwards for negative Q

3. Electric potential

   • V=(work done by field on a charge q as q goes from P to infinity) /q
     • -V is zero at infinity - Top line is electric potential energy
       • EPE =qV
     • ΔV=-EΔx
       • ΔV is potential difference
   • Due to a point Charge
     • V=Q/(4πεr)
     • potential is equal to area under E-r graph
     • If ΔV=-EΔx then ΔV/Δx is the potential gradient, the gradient on an E-r graph

4. Electric field with number of charges

   • use vector addition to find result electric field strength. Find E of individual charges and then resolve in same plane using cos(angle)
   • Potential: A scalar quantity. Just add up remembering sign of charges

5. Electric field lines

   • Their direction gives the direction of the electric field
   • lines start on positive charges and end on negative ones.
     • Lines indicate strength of electric field with distance between each lines indicating so
       • The closer together they are the stronger the field
   • Field lines never intersect but join together to make a resultant field
   • Equipotentials
     • all points have the same potential
     • Equipotentials and field lines cross at right angles
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