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

Electric fields exist around all charges in 3-dimensions. Usually we draw them in 2-dimensions (i.e. on a sheet of paper) and use lines to show their shape.

An electric field line is the path in which a light positive charge would move if free to move.

Characteristics of electric field lines:

1.	Field lines are directed from positive to negative charges.
2.	Field lines never intersect each other.
3.	Field lines enter or leave the surfaces of conductors at right angles.
4.	Field lines do not exist inside a charged conductor.
5.	The spacing of the field lines is an indication of relative strength of the electric field.
6.	The two common types of electric fields are radial field and uniform field.
7.	A uniform field is indicated by parallel and equally-spaced straight lines.

Guidelines for drawing electric field patterns

1. Identify sign of charged objects
2. Draw electric field lines
3. Insert arrows on lines (out from positive charge, into negative charge)
4. Check for any symmetry expected in the electric field pattern
5. Ensure field lines are drawn according to their characteristics
6. Draw sufficient number of field lines to show a clear representation of the electric field pattern. If more lines are added to this, the pattern should remain similar.

Some Common Electric Fields:

electric field around a small positive charge (radial field)

 

electric field around a small negative charge (radial field)

electric field around two opposite charges

 

electric field around two similar charges

 

single charge next to an earthed metal plate
(being near the +ve charge the plate is induced to become negative)

 

 

parallel field in the region between two parallel plates

Forces on a Charge

The electrostatic force experienced by a charge placed in an electric field will depend on:

• nature of the charges (to determine the direction)
• field strength to determine the magnitude

 

Example 1
Two small, identical charges, q1 and q2, are placed near a positively charged sphere as shown: Which charge will experience the larger force? q2 will experience the larger force as the field strength at that point is greater. This is indicated by the density of field lines drawn. Show Answer

Example 2

Two small, identical positive charges, q3 and q4, are placed near a positively charged sphere as shown: Draw force arrows to indicate the direction and magnitude of the forces acting on q3 and q4. q3 is at a greater distance from the positive charge (and is thus in an area of lower field strength) so will experience a smaller electrostatic force. Show Answer   Note Field Lines are used to show the direction of the field, there is nothing special about q4  being “on” the field line, we could easily draw a field line through q3 if we want to. Note If charges q3 and q4 had been negative charges then they would have been pointing in the other direction, i.e. against the direction of the field lines. Remember, the direction of a field lines is defined as being the direction I which a positive charge would move.

Example 3
A small, negative test charge, q5, is placed between parallel charged plates as shown: Indicate the direction of the force on particle q5. Show Answer

Example 4
Two small, negative charges, q6 and q7, are placed between parallel charged plates as shown: What can you say about the relative magnitude of the forces acting on the q6 and q7? The two forces will be equal as the field strengths are equal at both points. Show Answer

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