• The electric field strength of a uniform field between two charged parallel plates is defined as:

• Where:
  • E =?electric field strength?(V m^-1)
  • ΔV = potential difference between the plates (V)
  • Δd = separation between the plates (m)
• Note:?the electric field strength is now also defined by the units?V m^-1
• The equation shows:
  • The greater the voltage between the plates, the stronger the field
  • The greater the separation between the plates, the weaker the field
• Remember this equation cannot be used to find the electric field strength around a?point charge?(since this would be a radial field)
• The direction of the electric field is from the plate connected to the?positive?terminal of the cell to the plate connected to the?negative?terminal

The E field strength between two charged parallel plates is the ratio of the potential difference and separation of the plates

• Note:?if one of the parallel plates is?earthed, it has a voltage of 0 V

Step 1:?Write down the known values

Potential difference, ΔV = 7.9 kV = 7.9 × 10³?V

Distance between plates, Δd = 3.5 cm = 3.5 × 10^-2?m

Charge, Q = 2.6 × 10^-15?C

Step 2:?Calculate the electric field strength between the parallel plates

Step 3:?Write out the equation for electric force on a charged particle

F = QE

Step 4:?Substitute electric field strength and charge into electric force equation

F = QE = (2.6 × 10^-15) × (2.257 × 10⁵) = 5.87 × 10^-10?N = 5.9 × 10^-10?N (2 s.f.)

• The electric field strength at a point describes how strong or weak an electric field is at that point
• The electric field strength?E?at a distance?r?due to a point charge?Q?in free space is defined by:

• Where:
  • Q = the charge producing the 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 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 decreases by a factor of?four?when the distance 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)