Chapter 30
Current and Conductivity
Phys 133 – Chapter 30
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Current
When there is current, the bulb glows and the compass
needle deflects.
Phys 133 – Chapter 30
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Electron Current (i)
i = electrons/sec
n = electrons/m3
i = nAv d
vd  10-4 m/s
Phys 133 – Chapter 30
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Question
The light switch is located approximately 2 m from the
light. How long will it take an electron to travel this
distance? (vD=10-4 m/s)
= 333 hours
Do Workbook 28.7
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Conservation of electron current
Electrons cannot be created or destroyed
(conservation of charge)
The electron current is the same at all points in a
current-carrying wire.
The electron current into a junction is the same as the
electron current leaving a junction.
i0 = i1 + i2
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How to create current
need an electric field
static
dynamic
E ¹0
(not static equilibrium)
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What creates Electric Field?
Surface charges make E field - creates current
Phys 133 – Chapter 30
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Do Workbook 28.8 & 9
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Battery
Battery: charge escalator
“Pump”, no charge created
Move charge against
electric field
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Current (I)
(as opposed to electron current)
Current - amount of charge passing per unit time
dQ
I =
, direction of E
dt
= ie
Current Density -charge passing per unit time per unit area
I
J = = nevd
A
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Kirchoff’s junction rule
(conservation of charge/current)
å I =å I
in
out
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Phys 133 – Chapter 30
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Collisions/energy transfer
No electric field
With electric field
Electrons move randomly
Electrons tend to move
against electric field
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Conduction/resitivity
-Fields cause current (add energy)
-Collisions (take away energy)
-Current density (J) linear in electric field
-Depends on conductivity (resistivity)
J = sE =
1
r
E
-material dependent
-environment (temperature, magnetic field, …)
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Simulation
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Problem 28.33
The electron beam inside a TV tube is 0.4 mm in
diameter with current 50 A. Electrons strike screen.
a) How many electrons strike screen each second?
b) What is the current density in the beam?
c) The electrons move at 4.0x107m/s. What electric field
is necessary to accelerate the electrons to this speed
over a distance of 5.0 mm?
d) Assume each electron gives its kinetic energy to the
screen. What power is delivered to the screen?
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Problem 28.33 ans
a)
How many electrons strike screen each second?
I
50 ´ 10 -6 A
14 -1
n= =
=
3.12
´
10
s
e 1.60 ´ 10 -19 C
b)
What is the current density in the beam?
c)
The electrons move at 4.0x107m/s. What electric field is necessary to
accelerate the electrons to this speed over a distance of 5.0 mm?
ae
d)
I
50 ´ 10 -6 A
A
J= =
=
398
A p (0.00020 m) 2
m2
F eE
å
=
=
me
me
Assume each electron gives its kinetic energy to the screen. What
power is delivered to the screen?
Phys 133 – Chapter 30
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Potential in a circuit/wire
Apply to wire
Ewire is a constant
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Potential and current in a wire
I = JA
= (sE wire )A = (E wire )
A
r
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“Ohmic” vs “non-ohmic” materials
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