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An introduction to electric fields, the behavior of point charges, and the concept of electric potential. It also covers the relationship between electric potential and electric potential energy, the role of conductors in storing electrical energy, and the concept of electric current. Examples and explanations of various phenomena related to electric fields and current, such as the difference between electric and gravitational forces, the role of resistance in conduction, and the impact of collisions on the movement of electrons.
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Field concept: just a reformulation
q
“test charge”
Note analogy to gravity: Field g=GM/r^2 , Force mg= GMm/r^2
A) obey the inverse-square law.
B) act over shorter distances.
C) are weaker.
D) attract.
E) repel or attract.
Energy is stored in an electric field!
It’s a form of potential energy and defined in terms of electric potential. If we bring 2 charges together, they exert a force on each other for t he distance over which you bring them together—you must do work, and you store potential energy!
electric potential = electric potential energy / charge
The units of potential are volts: 1 volt = 1 Joule / 1 Coulomb
We can store electrical energy in many ways, e.g. batteries. Energy is stored whenever + and – charges are kept separated.
Simplest device: capacitor (2 charged metal plates)
Many devices use this electrical potential energy: camera flashes, TV sets...
What’s current exactly? Suppose electric charges were balls rolling down a tube: it would be easy to count how many balls passed a point A in a given time.
If we looked at any given interval, there might be 3 balls passi ng, or 0. But on the average there would be a certain number passing A per time. If each ball carried a charge q , we could also in principle count the average number of Coulombs which passed per time. Define
Ions
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Ions electrons
Q: Why don’t they keep accelerating due to electric force?
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