Edexcel A Level Physics

Revision Notes

7.8 Representing Radial & Uniform Electric Fields

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Using Field Lines & Equipotential Diagrams

  • The direction of electric fields is represented by electric field lines
  • Electric field lines are directed from positive to negative
    • Therefore, the field lines must be pointed away from the positive charge and towards the negative charge
    • Hence, field lines show the direction of force on a positive test charge

Representing Radial Fields

  • A radial field spreads out from a spherical charge in all directions
    • e.g. the field around a point charge 

  • Around a point charge, the electric field lines are directly radially inwards or outwards:
    • If the charge is positive (+), the field lines are radially outwards
    • If the charge is negative (-), the field lines are radially inwards

Radial E field lines, downloadable AS & A Level Physics revision notes

Radial electric field lines point away from a positive charge and point towards a negative charge

  • This shares many similarities to radial gravitational field lines around a point mass
    • Since gravity is only attractive, the field lines will look similar to the negative point charge, directed inward
    • However, electric field lines can be in either direction

  • The electric field strength in a radial field follows an inverse square law
    • This means the field strength varies with distance r by 1 / r2

Representing Uniform Electric Fields

  • A uniform electric field has the same electric field strength throughout the field
    • For example, the field between oppositely charged parallel plates

  • This is represented by equally spaced field lines
    • This shares many similarities to uniform gravitational field lines on the surface of a planet

  • A non-uniform electric field has varying electric field strength throughout
  • The strength of an electric field is determined by the spacing of the field lines:
    • A stronger field is represented by the field lines closer together
    • A weaker field is represented by the field lines further apart

Parallel E field lines, downloadable AS & A Level Physics revision notes

The electric field between two parallel plates is directed from the positive to the negative plate. A uniform E field has equally spaced field lines

  • The electric field lines are directed from the positive to the negative plate
  • The electric field strength in a uniform field is given by the equation E = V / d
    • Hence, E proportional to the potential difference V between the plates
    • E is inversely proportional to the distance d between the plates

Equipotential Diagrams

  • Equipotential lines (2D) and surfaces (3D) join together points that have the same electric potential
  • These are always:
    • Perpendicular to the electric field lines in both radial and uniform fields
    • Represented by dotted lines (unlike field lines, which are solid lines with arrows)

  • The potential gradient is defined by the equipotential lines

Electric Equipotential Lines 1, downloadable AS & A Level Physics revision notes

Equipotential lines in a radial field are circles, showing lines of equal potential around a charge. They intersect radial field lines at 90°

Electric Equipotential Lines 2, downloadable AS & A Level Physics revision notes

Equipotential lines in a uniform field are straight lines. They too intersect uniform field lines at 90°

Worked example

Sketch the electric field lines between the two point charges in the diagram below.WE Representing Electric Fields question diagram, downloadable AS & A Level Physics revision notes

  • Electric field lines around point charges are radially outwards for positive charges and radially inwards for negative charges
  • The field lines must be drawn with arrows from the positive charge to the negative charge

  • In a radial field (eg. a point charge), the equipotential lines:
    • Are concentric circles around the charge
    • Become further apart further away from the charge

  • In a uniform field (eg. between charged parallel plates), the equipotential lines are:
    • Horizontal straight lines
    • Parallel
    • Equally spaced

  • No work is done when moving along an equipotential line or surface
  • Work is only done when moving between equipotential lines or surfaces
    • This means that an object travelling along an equipotential doesn't lose or gain energy and ΔV =  0

Representing_Electric_Fields_Worked_example_solution_diagram, downloadable AS & A Level Physics revision notes

Exam Tip

Always label the arrows on the field lines! The lines must also touch the surface of the source charge or plates.

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