Representing 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
- A radial field spreads uniformly to or from the charge in all directions
- e.g. the field around a point charge or sphere
Electric Fields Around Point Charges
- 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
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 an attractive force, the field lines will look similar to the negative point charge, whilst electric field lines can be in either direction
- 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 and 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 represented by the spacing of the field lines:
- A stronger field is represented by the field lines which are closer together
- A weaker field is represented by the field lines which are further apart
Electric Field Lines Between Two Parallel Plates
- The electric field lines are directed from the positive to the negative plate
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
- A radial field is considered a non-uniform field
- Electric field strength E varies with distance from a charged particle
Worked example
Sketch the electric field lines between the two point charges in the diagram below.
Answer:
- Electric field lines around point charges have arrows which point radially outwards for positive charges and radially inwards for negative charges
- Arrows (representing force on a positive test charge) point from the positive charge to the negative charge
Exam Tip
Always label the arrows on the field lines! The lines must also touch the surface of the source charge or plates.