# Drawing Field Lines

Show/Hide Sub-topics (Static Electricity | O Level)
Show/Hide Sub-topics (Electric fields | A Level)

The electric field lines are parallel to the direction of force experienced by a positive test charge placed at that point.

## Basic conventions when drawing field lines

• Field lines extend out from positive charges
• Field lines go into negative charges
• All field lines are continuous curves or lines without breaks
• Field lines never cross each other’s path
• Field lines in the same direction tend to stay as far apart as possible
• Field lines in the opposite direction tend to cancel each other
• When electric field strength E increases (i.e. charge is stronger), the number of field lines increase
• Field lines are always perpendicular from its source
• Stronger field lines retain their shape better therefore are less distorted by other charges around

When 2 or more charges are placed in the vicinity of each other, the resultant E-field shape will be a combination of the individual E-field.

• When both point charges are of the same magnitude, the shape will be symmetrical
• When both point charges are of different magnitude, the shape will be asymmetrical

## What does it mean by uniform electric field?

An uniform electric field is defined as an electric field whose influence over a charge is constant regardless of the point of the space taken into consideration.

An electric field generated by a point charge is not uniform because the electric field lines are spaced further apart as the distance from the charge increases. (The electric field gets weaker.)

Almost uniform E-field can be obtained with oppositely charged parallel plates when the length of the plates is much longer than the distance between them.

## Electric field strength due to double and multiple point charges in space

$E_{y} = E_{1y} + E_{2y}$

$E_{x} = E_{1x} + E_{2x}$

$E = \sqrt{E_{x}^{2} + E_{y}^{2}}$

### Procedure:

• Resolve the electric field vectors into x and y components
• The x and y components of the resultant electric field can be found using the first two equations above
• The resultant electric field is then computed using the third equation

Note: If you are familiar with vectors, the resultant electric field at a point is just the vector sum of the two or more electric fields due to the charges.