Edexcel A Level Physics:复习笔记7.8 Representing Radial & Uniform Electric Fields

• 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 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

   

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

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

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

• 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