• 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
• 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

Step 1: Write down the known values

Electric field strength,?E?= 1.5 × 105?V m-1

Radius of sphere,?r?= 15 / 2 = 7.5 cm = 7.5 × 10-2?m

Step 2:?Write out the equation for electric field strength

Step 3:?Rearrange for charge Q

Q?= 4πε0Er2

Step 4: Substitute in values

Q?= (4π × 8.85 × 10-12) × (1.5 × 105) × (7.5 × 10-2)2?= 9.38 × 10-8?C = 94 nC (2 s.f)