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Electric Current
Chapter 34
Flow of Charge
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When the ends of an electric conductor are at
different electric potentials, charge flows from
one end to the other
Potential Difference – the difference in voltage
between the ends of a conductor
The flow of charge will continue until both ends
reach a common potential
To attain a sustained flow of charge in a
conductor, some arrangement must be provided
to maintain a difference in potential while
charge flows from one end to the other
Flow of Charge
Electric Current
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Electric Current – the flow of electric charge
In solid conductors the electrons carry the charge
through the circuit because they are free to move
throughout the atomic network (conduction electrons)
Protons are bound inside atomic nuclei that are more
or less locked in fixed positions
Ampere – the unit of electric current (A); 1 coulomb
of charge per second
A current-carrying wire does not have a net electric
charge, because the number of electrons still equals the
number of protons
Electric Current
Voltage Sources
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Charges will not flow unless there is a potential
difference
Voltage Source – something that provides a
potential difference
Dry cells, wet cells, and generators are capable
of maintaining a steady flow (batteries are two
or more cells connected together), by supplying
energy that allows charges to move
The voltage provides the “electric pressure” to
move electrons between the terminals in a
circuit
Voltage Sources
Electric Resistance
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Electric Resistance – the resistance that the
conductor offers to the flow of charge
The resistance of a wire depends on the
conductivity of the material used in the wire and
also on the thickness of the wire
Thick wires have less resistance than thin wires;
longer wires have more resistance than short
wires
Electric resistance is also dependent on
temperature; the more temperature, the more
resistance
Ohms – unit of electric resistance (Ω)
Electric Resistance
Material
Resistivity
(ohm•meter)
Silver
1.59 x 10-8
Copper
1.7 x 10-8
Gold
2.4 x 10-8
Aluminum
2.8 x 10-8
Tungsten
5.6 x 10-8
Iron
10 x 10-8
Platinum
11 x 10-8
Lead
22 x 10-8
Nichrome
150 x 10-8
Carbon
3.5 x 105
Polystyrene
107 - 1011
Polyethylene
108 - 109
Glass
Hard Rubber
1010 - 1014
1013
Ohm’s Law
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Ohm’s Law – the current in a circuit is directly
proportional to the voltage impressed across the
circuit and inversely proportional to the
resistance of the circuit
Current = voltage/resistance
1 ampere = 1 volt/ohm
Inside electrical devices, such as radio and
television receivers, the current is regulated by
circuit elements called resistors, whose resistance
may range from a few ohms to millions of ohms
Ohm’s Law
Circuit
Diagram
Battery
Voltage
( V)
Total
Resistance
( )
Current
(Amps)
1.
1.5 V
3
0.50 Amp
2.
3.0 V
3
1 Amp
3.
4.5 V
3
1.5 Amp
4.
1.5 V
6
0.25 Amp
5.
3.0 V
6
0.5 Amp
6.
4.5 V
6
0.75 Amp
7.
4.5 V
9
0.50 Amp
Ohm’s Law and Electric Shock
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The damaging effects of shock are the result of
current passing through the body
This current depends on the voltage supplied,
and also on the electric resistance of the human
body
The resistance of your body depends on its
condition and ranges from about 100 ohms if
you’re soaked with salt water to about 500,000
ohms if your skin is very dry
One effect of electric shock is to overheat
tissues in the body or to disrupt normal nerve
functions
Ohm’s Law and Electric Shock
Effect of Various Electric Currents on the Body
Current in
Amperes
Effect
0.001
Can be felt
0.005
Painful
0.010
Involuntary muscle contractions (spasms)
0.015
Loss of muscle control
0.070
If through the heart, serious disruption;
probably fatal if current lasts for more than 1
second
Direct Current and Alternating
Current
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Direct Current – a flow of charge which always flows in
one direction
A battery produces direct current in a circuit because the
terminals always have the same sign of charge (electrons
always move through the circuit in the same direction)
Alternating Current – electric current that repeatedly
switches direction
Nearly all commercial AC circuits in North America
involve voltages and currents that alternate back and forth
at a frequency of 60 cycles per second
Voltage of AC in North America is normally 120 volts,
whereas in Europe the voltage is standardized at 220 volts
(that’s why you need a voltage adapter in other countries)
Direct Current and Alternating
Current
Direct Current
Alternating Current
Converting AC to DC
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Diode – a tiny electronic device that acts as a
one-way valve to allow electron flow in only one
direction
Since alternating current vibrates in two
directions, only half of each cycle will pass
through a diode
The output is rough DC, off half the time
To maintain continuous current while
smoothing the bumps, a capacitor is used
Converting AC to DC
The Speed of Electrons in a Circuit
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At room temperature, the electrons inside a metal wire
have an average speed of a few million kilometers per
hour due to their thermal motion
There is no net flow in any one direction; but when a
battery is connected, an electric field is established
inside the wire
The electrons continue their random motions in all
directions while simultaneously being nudged along the
wire by the electric field
The conducting wire acts as a guide or “pipe” for the
electric field lines
Because the electrons are constantly bumping into each
other as they move, the wire becomes hot
The Speed of Electrons in a Circuit
The Source of Electrons in a Circuit
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The source of electrons in a circuit is the
conducting circuit material itself
When you plug a lamp into an AC outlet, energy
flows from the outlet to the lamp, NOT
electrons
Energy is carried by the electric field and causes
vibratory motion of the electrons that already
exist in the lamp filament
The Source of Electrons in a Circuit
Electric Power
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Electric Power – the rate at which electrical
energy is converted into another form such as
mechanical energy, heat, or light
Electric power = current x voltage
1 watt = (1 ampere) x (1 volt)
A kilowatt is 1000 watts, and a kilowatt-hour
represents the amount of energy consumed in 1
hour at the rate of 1 kilowatt
This is how the electric company charges for the
power you receive
Assignment (Due 4/15/08)
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Read Chapter 34 (pg. 531-544)
Do Chapter 34 #26-52 (pg. 546-547)
Appendix F, Chapter 34 #1-12 (pg. 689)