• The?electric field strength?describes how strong or weak an electric field is at that point
• A?point charge?produces a?radial?field
  • A charge sphere also acts like a point charge

   

• The electric field strength?E?at a distance?r?due to a point charge?Q?in free space is defined by:

• Where:
  • Q?= the point charge producing the radial electric field (C)
  • r?= distance from the centre of the charge (m)
  • ε0?=?permittivity of free space?(F m-1)

• This equation shows:
  • Electric field strength in a radial field is?not constant
  • As the distance from the charge?r?increases,?E?decreases by a factor of 1/r2

• This is an?inverse square law?relationship with distance
  • This means the field strength?E?decreases by a factor of?four?when the distance?r?is?doubled

   

• Note:?this equation is only for the field strength around a?point charge?since it produces a radial field

Positive and negative point charges and the direction of the electric field lines

• The electric field strength is a?vector?Its direction is the same as the electric field lines
  • If the charge is negative, the E field strength is negative and points?towards?the centre of the charge
  • If the charge is positive, the E field strength is positive and points?away?from the centre of the charge

   

• This equation is analogous to the?gravitational field strength?around a point mass
  • The only difference is, gravitational field lines are?always?towards?the mass, whilst electric field lines can be towards?or?away from the point charge

Step 1:?Write out the equation for electric field strength

Step 2: Substitute quantities for charge, distance and permittivity of free space

• • The charge on an electron?Q?= –1.6 × 10–19?C
  • The distance?r?= 2 m
  • Permittivity of free space?ε0?= 8.85 × 10–12
  • Therefore:

Step 3: State the direction of the field

• • The negative sign indicates the electric field is directed?towards the electron