Electric Field (E-Field)

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Electromagnetic waves are made up of Electric Fields (often called the E-field) and magnetic fields. What is an E-field?

Technically, the E-field at a point in space is a measure of how strong the force would be on a unit point charge (a small sphere with an electric charge of 1 Coulomb on it). Hence, the units of the E-field are Newtons/Coulomb [N/C]. These units are equivalent to Volts/meter [V/m], which is what the E-field is commonly quoted in (for instance, 10 V/m).

The E-field is a vector quantity - this means at every point in space it has a magnitude and a direction. For instance, lets say an E-field exists in space given by:

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This is the E-field of a plane wave travelling in the +z-direction, and the E-field is linearly polarized and 'points' in the y-direction (k is the wavenumber). The amplitude of the wave is A Volts/meter.

At time t=0 and z=0, the E-field is A Volts/meter in the +y-direction. This means that a unit point charge (1 Coulomb) at this location would experience a force of A Newtons in the +y-direction.

The electric field also relates to voltage - a stronger E-field incident upon an antenna will induce a larger voltage difference across the antenna's terminals. However, except for low frequencies, the relationship between E-fields and Voltage is not simple (the voltage is a potential which is subject to different definitions). At d.c., when the fields are static (no variation with time), the E-field and voltage V are related to each other by:


The electric field associated with a point charge with a positive charge point away from it at every location; the fields associated with a pont charge with a negative charge point towards it.

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