ELECTRIC FIELDS

1. The electric field

An electric field is a region in space in which an electric charge experiences a force.

The STRENGTH of such a field at a point in space is defined as the force exerted on a positive charge at that point, divided by the magnitude of the charge, that is,

E, the ELECTRIC FIELD STRENGTH, is a vector quantity and its direction is the same as that of the force. It has units NEWTONS PER COULOMB, N·C-1.

A charged spherical particle will create an electric field. The strength of the field, E, is proportional to the magnitude of the charge Q, and inversely proportional to the square of the distance, r, from the CENTRE of the particle.

Referring to the diagram on the right, at the point P, the field strength due to the charge Q is E = kQ/r2 where k is Coulomb's constant.


2. Field at a point due to a group of charges

The diagram on the left shows a group of four charges, Q1, Q2, Q3 and Q4 some distance from a unit positive point charge P. Each charge will generate an electric field at P. The total field, E at P will be the VECTOR SUM of each field E1, E2, E3 and E4. Thus, E = E1 + E2 + E3 + E4

See also: Definition of an electric field and the vector nature of force and the electric field.


3. Electric field lines

Electric field lines are IMAGINARY LINES along which a small POSITIVE test charge would move.

The force experienced by the positive test charge is always in the direction of the tangent to the field line.

Electric field lines have the following properties:

  1. They start on a positive charge and end on a negative charge.
  2. They never cross.
  3. They are closer together in regions where the field is stronger and further apart where the field is weak.
  4. They begin and end perpendicularly to the charged surface.

Of course, field lines as represented on this screen or on paper are inaccurate illustrations of the electric field because:

  1. the field is continuous, while one draws the lines at discrete intervals
  2. the field is three-dimensional, while the field as we draw it is a representation of a two-dimensional section through the field.


4. Additional questions


Definition of electric field

Measuring the field strength at a point using a test charge presents a problem, as the charge has a field and this field will add to the field being measured. Thus we need to use a test charge which is very small or approaching zero.







The vector nature of force and electric field

  1. Charge is a scalar quantity.
  2. Force is a vector quantity.
  3. The presence of a charge creates an electric field in space, the strength of which is a vector quantity.
  4. The force between two positive charges is directed along the line joining these charges.
  5. If the sign of the charges is included in calculations of the force between two charges using Coulomb's Law, then the forces which have a negative sign are attractive and those which have a positive sign are repulsive.