Electric Fields And Field Lines (Copy)

• An electric field is a region of space where a positive test charge experiences a force.
• Electric fields are a type of field of force, similar to gravitational fields, but they act on charges rather than masses.
• Definition:
  The electric field strength (E) at a point is defined as the force per unit positive charge:
  • E = F / q
  • Where:
    F = force experienced by the charge (N)
    q = magnitude of the charge (C)
    E = electric field strength (N/C or V/m)
• SI unit of electric field strength:
  • Newton per coulomb (N/C)
  • Also equivalent to volt per metre (V/m)

• Direction of Electric Field Lines:
  • Field lines always show the direction of force on a positive test charge.
  • Field lines point:
    • Away from positive charges
    • Towards negative charges
  • The closer the field lines, the stronger the electric field.
  • Uniform field: Field lines are equally spaced and parallel.
  • Radial field: Field lines spread radially from a point charge.

• In a uniform electric field, the electric field strength is the same at all points:
  • Can be created between two parallel plates connected to a voltage source.
  • The formula for electric field strength in a uniform field:
    • E = V / d
    • Where:
      V = potential difference between plates (V)
      d = separation between the plates (m)
      E = electric field strength (V/m)

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change A2 Level Physics Full Scale Course

• Force on a charge in an electric field is given by:
  • F = qE
  • Where:
    q = charge (C)
    E = electric field strength (N/C)
    F = force (N)
• The direction of this force:
  • For positive charges, same as direction of field lines
  • For negative charges, opposite to field lines
• This implies that a charge placed in an electric field will experience a linear force (vector), which can cause it to accelerate if unopposed.

• Example:
  • If a 3.0 C charge experiences a 6.0 N force in an electric field:
    E = F / q = 6.0 / 3.0 = 2.0 N/C
• Example:
  • If two plates are 0.05 m apart and the potential difference across them is 500 V, then:
    E = V / d = 500 / 0.05 = 10,000 V/m

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change A2 Level Physics Full Scale Course

• Properties of electric field lines:
  • They never cross.
  • They start on positive charges and end on negative charges.
  • The density of lines indicates field strength.
  • Field lines perpendicular to surfaces of conductors in electrostatic equilibrium.
• Comparison with gravitational fields:
  • Both are inverse-square laws.
  • Gravitational fields act on mass, electric fields act on charge.
  • Gravitational fields are always attractive; electric fields can be attractive or repulsive depending on charge sign.

• Radial Electric Field (from a point charge):
  • E = kQ / r² or E = (1 / 4πε₀) × Q / r²
  • Where:
    Q = source charge (C)
    r = distance from point charge (m)
    ε₀ = permittivity of free space = 8.85 × 10⁻¹² F/m
    E = field strength (N/C)
• Electric field strength in a radial field decreases with distance following the inverse square law.

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change A2 Level Physics Full Scale Course

• Direction of force and motion in electric fields:
  • Positive charges move in the same direction as the electric field.
  • Negative charges move in the opposite direction.
• Work done by an electric field:
  • Work = q × V = qEd
  • Units: Joules (J)
• In uniform fields, a charged particle will accelerate with a constant acceleration, similar to an object in uniform gravitational field:
  • a = F / m = qE / m

Important vector rule:

• Electric field is a vector quantity.
• In two-dimensional fields (like those involving two charges), field vectors must be added vectorially.

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change A2 Level Physics Full Scale Course