DEFINITION:

Units: (SI: either N/C or V/m = Volts/meter)

• E, the force per unit of charge, is determine at each point by the ratio of the force on an inconsequentially small, positive charge q_o divided by size of the charge.
  
  
• The direction of E in space is the same direction that a small positive charge would move (from rest) if placed at that location, i.e. the direction of the force.
  

Some properties of the E-field:

• Surrounding a charge we assert that there is an Electric Field E that has an independent existence.
  
  
• The Electric Field at some point in space (the field point ) can be thought of as being detached from the charge(s) (the source point(s) ) from which the Electric Field originates. Later you will see that an electromagnetic wave - a light beam for example - is a detached electric and magnetic fields moving through space together at the speed of light.
  
  
• The Electric Field is a Vector Function ; meaning that - in general - the symbol stands for not just one thing but an uncountable infinity of vectors, each with its own magnitude and direction. A reference to is a reference to an infinite number of vectors at all points in space.
  

Electric Field on a Test Charge near two Point Charges IP Simulation
Two charges at fixed locations create and Electric Field around them. A positive test charge can be clicked and dragged around to see the electric field that each charge creates at the location of the test charge as well as the total E-field at the location of the test charge. I started to hide the test charge because the E-field has am existence that independent of test charge. You do not need a test charge to create the E-field only to measure the E-fields value at some location. (This simulation will not work with IP versions 3.0 and above because IP 3.0 or higher will not display the zeroth frame correctly. You have to click the run button to see the total E-field vector.)

Force on a Charged Particle in an Electric Field:

• Turing the definition of the electric field around it is easy to see that if a charge q is in an electric field E then the force acting on the charge is:

• The sign of q is important. If q is negative then the force will be in the opposite direction of the E-field.
  
  
  
  
• The advantage of using the concept of an E-field is that the force on a charge can be easily found if you know the vector value of the E-field at the location of the charge even though the E-field may arise from a complicate arrangement of a large number of charges.

Field Lines:
To help visualize the electric field one often constructs field lines.

• Field lines begin on positive charges and end on negative charges. Field lines point out of +charges and into -charges. (Field lines can start at or go off to infinity when there is only a single charge.

• The electric field at a given point is tangent to the field line passing through that point.
• There are an infinite number of field lines surrounding a charge. Conventionally, the field lines are displayed in such a way that the number of field lines is proportional to the magnitude of the charge from which they originate or terminate.

• In regions where two field lines get closer together, the magnitude of the electric field is becoming larger. In regions where the E-field is becoming weaker, the field lines are spread out.
• A diagram of the field lines surrounding charges is called a field map. A two-dimensional field map is a cross-section of the actual three-dimensional structure.

Superposition Principle:

• The Electric Field at a particular point in space (due to presence of a large number of charges) is equal to the vector sum of the E-Fields due to each source charge.

• The direction of the electric force on a negative charge is opposite to the direction of the Electric field at the location of the charge.