Electric Fields (OCR Gateway GCSE Physics)

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

• An electric field can be defined as:

A region where an electric charge experiences a force

• If other charges enter the field then they will experience an electric force, attracting or repelling them from the object
• Since force is a vector, the direction of this force depends on whether the charges are the same or opposite
• The force is either attractive or repulsive
  • 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

When the negative particle enters the electric field, it feels a force of attraction towards the positive charge

• 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

Repulsive forces decrease as like charges move further apart

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

Point Charges

• Electric field lines around point charges are spherical:

Electric fields lines around point charges

• When two opposite charged point charges are brought close together, their attraction is shown by the field lines
• The electric field pattern between two oppositely charged spheres (or point charges) looks like:

Electric field pattern for two opposite charges

Parallel Plates

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

Electric field lines between two parallel plates

• The strength of an electric field is determined by the spacing (the concentration) 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 

Strong and weak electric field lines on a parallel plate