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Electric Fields

Physics (Year 12) - Electro-magnetism

Dev Lohar

Electric Field

An electric field is defined as the electric force per unit of positive charge. An electric field surrounds positive and negative charges and it exerts a force on any other charge within the field. Its direction is the same as the direction of force it would exert on a positive test charge.

The strength of electric fields can be calculated using the following relationship:

Electric Field Lines

Electric fields are represented by electric field lines (i.e. arrows) and these field lines always point away from positive charges and towards negative charges. The density (the amount of lines) of the field lines indicate the relative strength of the electric field. For example, consider two electric fields A and B, where electric field A is twice as strong as electric field B. Therefore, there will be twice as many electric field lines that represent electric field A, compared to the electric field lines that represent electric field B.

These are the following rules you need to follow when drawing electric field lines (failure to follow these rules may result in loosing marks):

  • Electric field lines go from positive to negative

  • They start at 90 degrees to the surface with no gap between the line and the surface

  • Electric field lines can never cross/intersect each other

  • Around small charged spheres (called point charges), draw a minimum of 8 electric field lines

  • Between 2 point charges, the direction of the electric field is the resultant vector of adding the field of each point charge

  • Between 2 oppositely charged parallel plates, the electric field is evenly spread; meaning it is the same at any point between the plates

Electric Field Examples

Force in Electric Fields

As mentioned earlier, electric fields exert a force on any free charge within the field. The following is the mathematical relationship between electric field strength and the force exerted on charges:

A positive charge in a field would experience a force in the direction of the electric field (positive to negative). Whilst a negative charge in an electric field would experience a force in the opposite direction to the field.

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