Transcript chapter-20-1-with

CH 20-1
Surface Charge Gradient
When a wire is connected
to a battery, a surface
charge gradient (i.e.
variation) is “immediately”
established on the outside
of the wire.
The surface charge
(gradient) creates a
uniform electric field on
the inside of the wire that
points from high potential
to low potential along the
wire.
+ −
High
Potential
Low
Potential
Poll
For an electron at the
point shown, what will be
the direction of the force
(due to the electric field
created by surface
charge) on the electron.
+ −
High
Potential
Low
Potential
Flow of electrons
• Moving charges collide with
atoms and other electrons
in a conductor.
• The electrons travel with a
constant speed -- called
drift speed.
• They lose potential energy
as they travel through the
wire (U = qV).
• Lost potential energy results
in an increase in thermal
energy and light.
Current
Electron Current is the number
of electrons that passes a
cross section of the wire per
second.
+ −
Current is the charge that
passes a cross section of the
wire per second.
High
Potential
Low
Potential
Example
If a current of 1 A flows in a wire, how many
electrons pass a cross-section of the wire in 1
second?
Drift speed
Current depends on charge density, area, and the
drift speed.
Example
What is the number density of conduction electrons in
copper? Copper has a density of 8900 kg/m3, an atomic
mass of 63.5 g/mol, and 1 free electron per atom.
Example
What is the drift speed of electrons in a copper wire of
diameter 1 mm if the current is 1 A.
Resistance
More collisions will mean that
fewer electrons pass a cross
section of wire per second.
This property of the wire is
called resistance.
For a higher resistance of the
wire, less current flows
through the wire when it is
connected to a battery.
Units of R is the ohm 
Ohm’s Law
For a wire (or an “ideal
resistor”)
+ −
For a given potential
difference, current and
resistance are inversely
proportional.
High
Potential
Example
You connect a 1.5 V battery to a 6  light bulb.
What will be the current through the filament of the
bulb?
Resistance of a wire
Resistance of a wire depends on the collisions
that an electron makes as it travels through the
wire. Three factors thus affect resistance:
resistivity (which depends on the particular
metal), the length of the wire, and the area of the
wire.
Resistivity
Analyzing simple circuits
Treat resistors and light bulbs as “ideal resistors” even
though light bulbs certainly have a resistance that
certainly changes with temperature. Assume that all
other wires are “ideal wires.”
Poll
A 1.5 V battery is connected to Bulb A and you observe
its brightness. You then replace the bulb with Bulb B and
observe that it is brighter. Through which bulb is the
current the greatest?
1. Bulb A
2. Bulb B
3. Neither, because the current is the same
through each bulb.
Poll
A 1.5 V battery is connected to Bulb A and you observe
its brightness. You then replace the bulb with Bulb B and
observe that it is brighter. Across which bulb is the
potential difference the greatest?
1. Bulb A
2. Bulb B
3. Neither, because the current is the same
through each bulb.
Poll
A 1.5 V battery is connected to Bulb A and you observe
its brightness. You then replace the bulb with Bulb B and
observe that it is brighter. Which bulb has a greater
resistance?
1. Bulb A
2. Bulb B
3. Neither, because the current is the same
through each bulb.
Poll
Suppose that Filament #1 has the same length as
Filament #2 and they are made of the same material.
But Filament #2 has a greater cross-sectional area.
Which filament has a greater resistance?
1. Filament #1
2. Filament #2
3. Neither, because the filaments have the
same resistance.
Poll
Suppose that Filament #1 has the same length as
Filament #2 and they are made of the same material.
But Filament #2 has a greater cross-sectional area.
Which filament will be brighter in a light bulb that is
connected to a battery?
1. Filament #1
2. Filament #2
3. Neither, because the filaments have the
same resistance.
Poll
Suppose that Filament #1 has the same cross-sectional
area as Filament #2 and they are made of the same
material. But Filament #2 has a greater length. Which
filament will have a greater resistance?
1. Filament #1
2. Filament #2
3. Neither, because the filaments have the
same resistance.
Poll
Suppose that Filament #1 has the same area as
Filament #2 and they are made of the same material.
But Filament #2 has a greater length. If in a bulb and
connected to a battery, which filament will be brighter?
1. Filament #1
2. Filament #2
3. Neither, because the filaments have the
same resistance.