Electric Fields (Cambridge O Level Physics)

• A charged object creates an electric field around itself
  • This is similar to the way in which magnets create magnetic fields

• This can be shown by electric field lines
  • Fields lines always point away from positive charges and towards negative charges 

Direction of Field Lines for Point Charges   

Electric fields are always directed away from positive charges and towards negative charges

• The direction of the field lines in an electric field is described as:

The direction of the force on a positive charge at that point

Field lines show the direction that a positive charge would experience if it was at that point

• Although the definition of the force direction refers to a positive charge, in demonstrations it is always electrons (negative charges) which are free to move according to that force

• The strength of an electric field depends on the distance from the object creating the field:
  • The field is strongest close to the charged object - this is shown by the field lines being closer together
  • The field becomes weaker further away from the charged object - this is shown by the field lines becoming further apart

• Objects in an electric field will experience an electrostatic force
  • Since force is a vector, the direction of this force depends on whether the charges are the same or opposite
  • If the charges are the same (negative and negative or positive and positive), this force will be repulsive and the second charged object will move away from the charge creating the field
  • If the charges are the opposite (negative and positive), this force will be attractive and the second charged object will move toward the charge creating the field
  • The force is either attractive or repulsive

Force on a Negative Charge Placed in the Electric Field of a Positive Charge

Electric field lines show force direction and force strength

• The size of the force depends on the strength of the field at that point
• This means that the force becomes:
  • Stronger as the distance between the two charged objects decreases
  • Weaker as the distance between the two charged objects increases

• The relationship between the strength of the force and the distance applies to both the force of attraction and force of repulsion
  • Two negative charges brought close together will have a stronger repulsive force than if they were far apart

Field Lines Around a Point Charge

• The electric field is the region in which another charge will experience a force

• Fields lines always go away from positive charges and towards negative charges – they have the same direction as the direction of the force on a positively charged particle at a point in that field

Fields Lines For Point Charges

Electric fields are always directed away from positive charges and towards negative charges

Field Lines Between Two Oppositely Charged Parallel Conducting Plates

• The electric field between two parallel plates is a uniform electric field
• The field lines are:
  • Directed from the positive to the negative plate
  • Parallel
  • Straight lines

Field Lines for Parallel Plates

The electric field between two parallel plates

Field Lines Around a Charged Conducting Sphere

• The field lines around a charge conducting sphere are symmetrical, as with a point charge
  • This is because the charges on the surface of the sphere will be evenly distributed
  • The charges are the same, so they repel
  • The surface is conducting, allowing them to move     

• This field line pattern can be demonstrated using a Van der Graaff Generator
• One method using streamers is shown
• Other methods often demonstrated in schools include
  • Small pieces of paper
  • Polystyrene beads
  • Aluminium foil containers