PHYA22 Chapter 31.doc

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17 Apr 2012
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Chapter 31: Current and Resistance
The Electron Current
Current: the motion of charges.
A current is present in a wire when it makes the wire warm and deflects a
compass needle.
Charge Carriers: the charges that move in a conductor.
When a metal bar accelerates to the right, inertia causes the charge carriers to
be displaced to the rear surface and the front surface becomes oppositely
charged.
The charge carriers in metals are electrons as found when negatively charged
particles move to the rear surface as it accelerates.
Conduction electrons in metals undergo random thermal motions but there is
no net motion.
Drift Speed (vd): the net motion causing the entire sea of electrons to move in
one direction.
Electron Current (ie): the number of electrons per second that pass through a
cross section of a wire or other conduction and unit are in s-1.
Number of Electrons that pass through the cross section during the time
interval t: Ne = iet = neAx = neAvdt
Increasing the drift speed will increase the number of electrons passing through
a wire each second.
Electric current in the wire: ie = neAvd
You can increase the electric current by making them move faster, by having
more of them per cubic meter or by increasing the size of the pipe they’re
flowing through.
Discharging a wire occurs instantaneously because the wire is already full of
electrons and as soon as the excess electrons move from the negative
capacitor plate into the wire, they immediately push an equal number of
electrons out the other end of the wire and onto the positive plate, thus
neutralizing it.
Creating a Current
The sea of electrons will quickly slow down and stop unless you continue
pushing it with an electric field.
An electron current is a nonequilibrium motion of charges sustained by an
internal electric field so (E 0).
An electric field creates a current.
The nonuniform surface charge density creates an electric field inside the wire;
it is positive at the positive capacitor plate, zero at the midpoint and negative
at the negative plate.
The on-axis field of a ring of charge:
oPoints away from a positive ring and toward a negative ring
oIs proportional to the amount of charge on the ring
oDecreases with distance away from the ring.
A nonuniform distribution of surface charges along a wire creates a net electric
field inside the wire that points from the more positive end of the wire toward
the more negative end of the wire. This is the internal electric field (E) that
pushes electron current through the wire.
Without an electric field, an electron bounces back and forth between collisions
(with ions in the lattice of the metal) but its average velocity is zero and it
undergoes no net displacement.
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