Electric Field - A Field of Force
An electric field is defined as being present in any region where a charged object experiences an electric force. This is a fancy way of saying that the only way we can tell if a field exists is to place a test charge at that spot and see if it feels a force. (In other words, it takes one to know one.) The animation below shows a stationary charge (Although we have no way of knowing it is charged until we bring in a test charge.) When a test charge is brought in, a force is present on that charge and so it shows evidence of a field being present. The closer the test charge is brought to the stationary charge the greater the force. The greater the force on the test charge, the stronger the field is.
As long as we are using the same test charge it is possible to vaguely
describe the strength of the field as stronger or weaker. However, the force that is
felt is also dependent on the amount of charge on the test charge. It is for this
reason that the 
Field Intensity or Field Strength is
described as the ratio of Force to the amount of test charge. The field intensity for an
electric field is measured in Newtons per Coulomb [N/C]. This describes the amount
of force present for every coulomb of charge used as a test charge. Field Intensity
is a vector quantity. The size of the vector is given by the equation to the left
but the direction of the vector is based on the direction of the force felt by a positive
test charge. The field strength equation has no way of specifying the direction of
the field, therefore you should ignore any negative signs that get created in your
answer. You must simply look at the region you are calculating the field strength
for and then determine the direction that a positive test charge would be forced.
Another very useful hybrid equation can be created by combining the formula for Coulombs law and the formula for field intensity.
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combine  common terms |
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This lesson continues with electric field mapping.
