Physics 201- Introduction to Electricity & Magnetism
September 4- Intro to Electric Fields

A few concepts behind "fields"

• In order to completely understand EM interactions we need to go beyond the simple picture given to us by Coulombís law.
• Coulombís law gives us an action at a distance model.  It implies that the instant we move one charge another charge would feel the effects.  Each charge acts directly on the other charges.  Why is this bad?  Well, it would give us infinitely quick information propagation and this does not happen.
• So, Coulombís law is fine for charges at rest (which is what we're doing here), but when we start to deal with moving charges, we'll need to modify it.
• There must be something which actually exerts the force on charges.  This is where the idea of a field comes into play.
• Let's go back to gravity for a moment... What is actually pulling you toward the surface of the earth?  Is the earth reaching up with a hand and pulling you down?  Of course not.  There must be something which exerts the force, and it is the gravitational field that the earth creates.

• The field model gives us the picture of having the space which surrounds a charged particle being filled with an electric field. Unlike a point charge, it occupies all of space simultaneously.
• A field (whether it is gravitational, electric or magnetic) is very real, it has momentum and energy just like particles.

• The previous (incorrect) picture was

• Our new picture for how the particles interact is

• The information of a wiggling charged particle is now propagated through the field at the speed of light.  You can (sort of) think of this interaction like ripples on a pons surface spreading out.
• Fields can be difficult to comprehend, but they are the cornerstone of this semester.

Now we can move on to more quantitative description of electric fields.

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 Jeff Phillips phillips@lmu.edu Loyola Marymount University Fall 2002