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Mechanics & Stress Analysis*
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  • A-Level Physics

Electric Fields

Electric fields are regions where non-contact electrical forces can be felt by charged objects. They are generated by electrical charges.

Field lines show the path a positive test charge would take from positive to negative. The closer they are, the stronger the field. Lines never stop in empty space and must never cross. Field lines go from positive to negative, so the point charge below is negative:

Uniformly charged spheres, point and spherical fields, point fields, spherical fields. electric fields., EngineeringNotes, Engineering Notes

Uniformly charged spheres can be modelled as a point charge at its centre, with field lines leaving/entering at right angles all around, to infinity. Closer to the point, you can see that the field lines are closer together. Therefore, the field strength must be greater.

Field strength - distance graph, inverse square law., EngineeringNotes, Engineering Notes

In all radial fields, the field strength is proportional to the distance from the point charge via an inverse square law.

Electric field strength is force per unit charge:

E = F/Q Electric field strength = Force / Charge

The units of electric field strength and newtons per coulomb, N/C

Coulomb's Law

The attractive/repulsive force between two point charges can be calculated using Coulomb's law:

F = Qq / 4πεr² Force = Product of two charges / 4π x ε x separation²

ε is the permittivity of free space, and can be taken as 8.85 E-12.

Since electric field strength is calculated as E = F/Q, coulomb's law can be used to calculate field strength of a point charge by dividing by one charge:

F = Q / 4πεr² Force = Charge / 4π x ε x separation²

Electric vs Gravitational Fields

Newton's law of gravitation is very similar to coulomb's law, and the fields have many other similarities, too. However, there are some differences.