Transcript 1700_current_2012Aug06

Electric Current
Dr. Bill Pezzaglia
Updated 2012Aug08
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V. Current & Conduction
A. Current
B. Resistance & Ohms Law
C. Electric Power
D. Circuits
A. Current
1) Current as Flow of Charge
2) Conduction Model
3) Conservation of Charge
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1. Current is the Flow of Charge
a. Definition: the flux of (positive) charge
Q
I
t
b. SI Units:
Amp=Coulomb/sec
often use mA or A.
•
•
•
•
•
2 mA
10 mA
20 mA
100 mA
100,000 A
threshold of feeling
pain
can’t let go
DEAD
Lightning
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2. Conduction Model
In metals, it’s the electrons that
really move (smaller, lighter than
protons).
NOTE: Electrons flow in
opposite direction of
“conventional current”
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3. Conservation of Charge
Conservation laws in the universe:
•
•
•
Conservation of momentum
Conservation of Energy
Conservation of Angular momentum
• Conservation of Charge
•
Current flowing out of a Leyden Jar (capacitor) must match the
loss of charge in jar:
Q
I 
t
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B. Resistance
1) Ohm’s Law
2) Resistivity
3) Non-Ohmic devices
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1. Ohm’s Laws (1872) [1826?]
• Current through a device is
proportional to voltage
• Resistance “R” is in units of
“ohms”
• Ohm=Volt/Amp=kgm2/(sC2)
V  IR
V
R
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2. Resistance of a device
•
Resistance of a device is proportional to length,
inversely proportional to cross section area
•
Resistivity of materials divided into classes:
•
•
•
•
Insulator:
Semiconductor
Conductor
Superconductor
High resistance (Rubber)
Medium resistance (e.g. Carbon)
Low resistance (metals)
Zero resistance!
The historical definition of the “ohm was a 1 meter column of mercury with
cross section area of 1 mm2. The unit system has been changed slightly
since that time, such that the column of mercury would only be 0.96 ohms.
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3. Non-Ohmic Behavior
a)Resistance changes with temperature (and the
temperature changes with current).
• Conductors: resistance increases with temperature
• Semiconductors: resistance decreases with
temperature
b)Light Bulb: resistance approximately increases
linearly with current (more current, higher
temperature)
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c) Vacuum Tube: Non linear
I  bV
3
30
2
25
20
15
10
5
0
0
2
4
6
8
10
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3d Diodes
Diode (e.g. LED)
• Has low resistance in one direction
• High resistance in other direction
• Behaves like a “one way street”. Current
can only flow in direction of arrow.
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C. Electric Power
1) Source of Power (Batteries)
2) Electric Work
3) Joule Heating
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1. The Electric Battery
(a) First Battery? 400 AD? The
Baghdad Battery is the common name
for a number of artifacts probably
discovered in the village of Khuyut
Rabbou'a (near Baghdad, Iraq) in 1936.
These artifacts came to wider attention
in 1938, when Wilhelm König, the
German director of the National Museum
of Iraq, found the objects in the
museum's collections, and in 1940
(having returned to Berlin due to illness)
published a paper speculating that they
may have been galvanic cells, perhaps
used for electroplating gold onto silver
objects.
-wikipedia
1b. Luigi Galvani (1737-1798)
•1786 first battery cell (two
different metals in contact)
•1791 Animalistic nature of
electricity (frog legs jump
from electric charge)
•(Mary Shelly used this idea
in her “Frankenstein” book)
•Did he also do work on
corrosion? (galvanized
nails?)
http://www.corrosion-doctors.org/Biographies/VoltaBio.htm
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1c. Voltic Pile
•
1786 Galvani creates first cell
•
1793 Volta shows cell creates an
electric current
•
1800 makes 30 volt “Voltic Pile”
from a column of cells
•
1826 Ohm determines voltage is
the driver of current
•
1830 Faraday figures out the
electrochemical reactions of a
battery.
http://www.corrosion-doctors.org/Biographies/VoltaBio.htm
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Alessandro Volta
(1745-1827)
2. Electric Work
(a) Battery is like a “pump” that increases energy of
charge (current) passing through it
•
The “voltage gain” is called EMF (electromotive force),
measured in units of “volts”
•
Change in potential energy of charge q passing through
battery is: U=V q
•
Often “EMF” is given the symbol “” or E
•
“Cathode” is plus end, “anode” the negative
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2b. Electrical Power
•
Power is units of Watts=Joule/Second
•
1841 Joule shows electrical work is equivalent to
mechanical work
•
Electric Power:
•
Hence: Watt=VoltAmp
Often we use Kilowatts or Megawatts
U Vq

 VI
t
t
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2c. Energy Usage
•
Energy or work done is Power x Time
•
Hence: Joule=Wattsecond
•
However, the PGE uses the weird unit of:
Kilowatt-Hour
•
1 kWh=3.6106 Joules
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3. Joule Heating
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In a resistor, the electrical energy is converted into heat.
(a) Power lost in Resistor:
V2
2
P  VI 
 RI
R
(b) You can show that less energy is lost in
transmission wires if you use high voltage with low
current, rather than low voltage with high current.
D. Circuits
1) Networks of Resistors
2) House wiring
3) Rectifiers
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1. Networks of Resistors
(a) Resistors in series add
(same current)
R  R1  R2
(b) Resistors in parallel have
same voltage, total
resistance:
1 1 1
 
R R1 R2
R1R2
R
R1  R2
I
R1
V
R2
I
I
V
I
I1
I2
R1
R2
2. Which circuit will give most light?
•
Batteries in series give most voltage
•
Bulbs in parallel will each get same
voltage (in series they would share it)
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3. Rectifiers
(a) DC or “Direct” Current is
constant
(b) AC or “Alternating” Current
oscillates polarity
(c) Rectifiers change AC into DC
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