# PHYS 142 Lecture Notes - Lecture 14: Valence Electron, Number Density

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Course PHYS 142: Electromagnetism and Optics - Lecture 14: The Electric Current Part 1
Electric Current:
As the capacitor is discharging, there is a current in the wire.
When a current is flowing, the conductors are not in electrostatic equilibrium.
Though you cannot see current directly, there are certain indicators that current is present in a
wire
Charge Carriers:
The outer electrons of metal atoms are only weakly bound to the nuclei.
In a metal, the outer electrons become detached from their parent nuclei to form a fluid-like
sea of electrons that can move through the solid.
Electrons are the charge carriers in metals.
Electron Current:
We define the electron current ie to be the number of electrons per second that pass through a
cross section of the conductor.
The number Ne of electrons that pass through the cross section during the time interval Δt is
If the number density of conduction electrons is ne, then the total number of electrons in the
Ne = neV
= neAΔx
= neAvdΔt
So the electron current is:
In most metals, each atom contributes one valence electron to the sea of electrons.
Thus the number of conduction electrons ne is the same as the number of atoms per cubic
meter.
ne is called the electron density, which is specific to every metal
The electric field strength E in a wire of cross-section A causes an electron current.
The electron density ne and the mean time between collisions τ are properties of the metal.
The electron current is directly proportional to the electric field strength.
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## Document Summary

Phys 142: electromagnetism and optics - lecture 14: the electric current part 1. As the capacitor is discharging, there is a current in the wire. When a current is flowing, the conductors are not in electrostatic equilibrium. Though you cannot see current directly, there are certain indicators that current is present in a wire. The outer electrons of metal atoms are only weakly bound to the nuclei. In a metal, the outer electrons become detached from their parent nuclei to form a fluid-like sea of electrons that can move through the solid. Electrons are the charge carriers in metals. We define the electron current ie to be the number of electrons per second that pass through a cross section of the conductor. The number ne of electrons that pass through the cross section during the time interval t is. If the number density of conduction electrons is ne, then the total number of electrons in the shaded cylinder is.

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