Electric Fields

The diagram above shows the Electric field around two isolated point charges, one positive charge, one negative charge.

Note - By convention, field lines are always drawn moving from Positive towards Negative. The direction of an Electric field is defined as the direction that a Positive charge would move in that region.

When Two (or more) point charges are brought near to each other, their fields interact in the following ways :-

 In the above diagram :-

When unlike charges are brought together, their fields attract and combine, with the field direction Positive to Negative.

When like charges are brought together, their fields repel.

Coulombs Law

The above diagram shows two point charges separated by a distance r .

In 1784, French Scientist Charles Augustin de Coulomb discovered that the Electrostatic Force between two point charges was :-

1. Directly proportional to the magnitude of each charge.

2. Inversely proportional to the square of the distance separating the charges.

Coulomb went on to experimentally derive the following formulae :-

Where Q1 and Q2 are separated by air or a vacuum.

The complicated constant that Coulomb included within his calculation is called the constant of proportionality and has a value of 9x109 Nm2C-2.  

The term ε0 is the Permittivity of Free Space and has a value of 8.85x10-12 Fm-1 .

Note - In some textbooks, the constant of proportionality  will be denoted simply as k. Also, Electrostatic Force is a vector, and as such has a magnitude and direction.

Example 1 - 

Three identical charge are positioned as shown :-

Each has a charge of +8 nC 

Calculate :-

1. The Forces exerted on charge A by charges B and C

2. The resultant Force on charge A

The resultant Force on A = 1.8x10-6 N at an angle 29o anti-clockwise from the line BA 

The video below gives a summary of Coulomb's Law