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### Course: AP®︎/College Physics 2>Unit 2

Lesson 2: Electric fields

# Electric fields

Electric fields are present around objects with charge, which allows the electric force to act on charged objects which are not in contact. An electric field causes an electric force on other charges that enter the field. The electric field is defined as the amount of electric force per charge at a point in space. Created by David SantoPietro.

## Want to join the conversation?

• why cant a charge experience a force on itself by its own electric field(its silly but it always bugs me)
• Maybe it can, but since it's pushing itself with the same force from all directions, the result is the same as if it wasn't doing anything.

It would be weird if it was pushing more in one direction than others, right? Where would that asymmetry come from?
• is the direction of electric field and electric force same?
• Not always. The vector equation for Electrostatic force can be written as:
`F` = q • `E`
where the highlighted characters `F` and `E` denote that they are vectors.
If the test charge q is positive then `F` and `E` will have the same sign. But if q is negative, `F` will be in a direction opposite to that of `E`.

Hope this helps.
• so will a charged particle in an electric field always move along electric field lines?
• not necessarily. The force will always be along the field lines but if the charge already has motion perpendicular to the field, then its dircetion / speed will change.

ok?
• what bout the distance at which the charge is placed
• If the positive charge is in a vacuum, with no contradicting charges(other charges), would the electric field expand into infinity? And if it does, then why??
• yes it does.

because thats what radial, inverse square fields do.... they fade out to zero at infinity
• So electric fields are NOT forces that apply a certain force on anything that comes in its way. Right?
• theyre not forces themselves, but they apply a force on objects that approximate depending on their magnitude, sign, and distance
(1 vote)
• Is it necessary that the source charge, generating the field is positive?
• Is there a speed with which an electric field spreads? If there is what happens when an item is newly charged but is already in the range of effect of an older electric field? Does it begin to move without actually having an effect on the other charge? I am guessing that it would not move since it would violate Newton's 3rd law, but can someone elaborate on the mechanics please. Thank you in advance!
• First of all there is a electromagnetic field throughout all space. Electrically charged particles change the energy in this field. In a vacuum changes in the electromagnetic field propagate at the speed of light. When you have a material the velocity of propagation depends on the permittivity of that material, if you have a conductor like a wire the speed of propagation is proportional to the impedance of the conductor.

Since there is a low of conservation of charge any time you create a - charge you produce a + charge as well moving away from the same point. The momentum of the particles is what ever is leftover after you create the mass of the particles. This momentum along with the external electromagnetic dictates how the particles move. There is no violation of any of Newtons laws.