Electric field – Energy Education

The electric field is one of the fundamental results of electromagnetism, created by a static (stationary) charge, or by a dynamic (changing in time) magnetic field. An electric field is defined as the force per unit charge, given by the equation:

[math]E=\frac{F}{q}[/math]

,

where [math]q[/math] is the charge in coulombs (C). Thus, when a charge is in the presence of an electric field it experiences a force causing the charged particle to move, which is given by rearranging the equation above so that:

The electric field can be looked at as analogous to a gravitational field in many ways, as massive objects are pulled towards each other in the presence of one another, just as charges are. The forces associated with each field also both fall off by an inverse square law, [math]1/r^2[/math] in simple cases. For complex charge distributions the electric field can have quite a few different relationships, see the figure gallery below (for a more complete discussion please see hyperphysics.

However, there is one significant difference: gravitational fields are limited to attraction (pulling objects together) whereas electric fields can be either attractive or repulsive, depending on the charge of the object in the field. By definition, a positive charge in the force equation given above would experience a repulsive force (it would move away from the source of the field) and a negative charge would feel the attractive force.

Due to this attractive/repulsive nature of electric fields, there is a notation for drawing them. A positive stationary charge has its electric field lines pointing radially away from it (perpendicular lines away from its surface, in all directions), and a negative stationary charge has its lines pointing radially towards it.

• Electric field lines
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  Figure 1. A negative charge with field lines pointing towards it.[2]

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  Figure 2. An interaction of positive and negative electric field lines, shown to be attractive.[3]

• 

  Figure 3. An interaction of two positive electric field lines, shown to be repulsive.[4]

For Further Reading

References

1. ↑

   R. Chabay and B. Sherwood, “The Electric Field of a Dipole,” in Matter & Interactions, 3rd ed., Hoboken, NJ: Wiley, 2011, ch.14, sec.6, pp. 556-560

2. ↑

   [GFDL ( http://www.gnu.org/copyleft/fdl.html ) or CC BY-SA 3.0 ( http://creativecommons.org/licenses/by-sa/3.0 )], via Wikimedia Commons

3. ↑

   “VFPt dipole electric manylines” Licensed under CC BY-SA 3.0 via Wikimedia Commons – http://commons.wikimedia.org/wiki/File:VFPt_dipole_electric_manylines.svg#mediaviewer/File:VFPt_dipole_electric_manylines.svg

4. ↑

   “VFPt charges plus plus” Licensed under CC BY-SA 3.0 via Wikimedia Commons – http://commons.wikimedia.org/wiki/File:VFPt_charges_plus_plus.svg#mediaviewer/File:VFPt_charges_plus_plus.svg