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Lecture series for Conceptual Physics, 8th Ed.
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Flow of Charge
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Recall that heat flows from the hot end to the cold end.
Current flows from the high potential (V) end to the low V
end of a conductor.
Current flows as long as there is a potential difference, ΔV.
Potential (V) is like
pressure (kPa)
A difference in pressure
causes water to flow.
A pump does the same
job that a battery does.
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Electric Current
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Water current is made up of water molecules.
Electric current is the flow of electric charges.
In a wire, conduction electrons flow while the
protons stay put.
A slight problem: current (I) flows from positive to
negative. The electrons are going the other way.
The electric field also goes from the positive end of
the wire to the negative end of the wire.
+
+
current
-
eCurrent (I) is measured in Amperes (A).
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Voltage Sources
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Batteries (DC) and Generators (AC)
The work done by batt.s
and gen.s to separate
charges creates the
potential difference, ΔV,
between their terminals.
Oh! And…
The + and – terminals have a
difference in potential energy
per charge or voltage.
Become familiar
with these
symbols.
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Electrical Resistance
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This wire has more
resistance than
this wire.
A hose resists
the flow of
water.
Longer, skinnier
hoses have more
resistance than
short, fat hoses.
Most of the time, hot wires have higher resistance than cold
wires. Something to do with the greater jostling of atoms at higher temp.s.
Electrical resistance is measured in ohms,
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Ohm’s Law
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V=IR
Try this:
I=
V
R
What is the current in a wire with a potential difference
of 10 V across its ends and a resistance of 2 ohms?
10 V
I = 2 ohms = 5 A
Now, what if you…
doubled the voltage?
doubled the resistance?
Inside electronic devices, current is regulated by resistors.
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I=
R=
V
R
V
I
12V
=
60 ohms = 0.2 A
120 V
= 10 ohms
=
12 A
Why isn’t the bird shocked?
His feet have the same potential.
There is no potential difference, V.
But, if it were to touch something else…
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Ohm’s Law and Electric Shock
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See Table 22.1 p403: Small currents can be dangerous.
I=
V
R
12 V
= 100,000 ohms
= 0.00012 A
To ground.
Protects you from any wire
touching the inside of the
appliance.
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Direct Current and Alternating Current
Batteries produce dc.
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Electrons go from – to + terminals.
Generators produce ac.
AC travels better.
Electrons just move back and forth in the 1/60 Hz
change in electric field polarization.
Converting ac to dc
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A gadget called a diode takes the ac going one way only and
passes it along in pulses.
A capacitor is used to smooth out the pulses.
A capacitor works in a manner
similar to the water system at left.
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Speed and Source of Electrons in a Circuit
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When you flip the switch on, an electric field is established
immediately in the circuit.
But, the electrons don’t travel very far or very fast.
In fact, they vibrate back and forth in the 1/60 Hz ac current.
Fig 22-13 shows how a wire channels the elec. field.
Fig 22-14 shows how the elec.s
travel through a wire.
Elec.s are jostled by atoms as
they try to accelerate and end
Basically, e’s just vibrate.
up starting over.
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Electrons jostle back and forth in the
filament to produce heat and light
with their “friction”.
Do you get the elec.s from the power
company?
No! They come with the appliance.
In a dc current, the elec.s would drift along at 33cm/hour.
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Electric Power
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Some of the
power is in the
form of light and
a bit more than
half goes as heat.
At 120V this bulb
would produce 100 W.
That’s 100 joules per
second.
P=IV
P = I V = (15 A)(120 V) = 1800 W
Yes. With 600 W to spare.
1 kW
10 cents
1200W x
= 12 cents
x
1
hr
x
1000 W
12
kWh
Electric Circuits
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1.5 V
open
e-’s flow along a circuit.
The current
stops when the
switch is open,
or a light
burns out.
The current is the same everywhere in a series circuit.
Resistors in a series act like one resistor according to
this formula: Rtotal = R1 + R2 + R3 + ……..
If each resistor is 5 ohms, all of them are 15 ohms. AND…
1.5 V
V
= 0.1 A
I= R =
The current in the circuit is
13
15 ohms
What current would be
indicated on the
ammeter?
Rtot = R1 + R2 + R3 = 2 + 3 + 1 = 6 ohms
I = V / R = 6 V / 6 ohms = 1 A
Watt is the power output of the total circuit
P = I V = (1 A)(6 V) = 6 W
What is the voltage drop (IR) of each resistor?
I R2 = (1 A)(2 ohms) = 2 V
I R3 = (1 A)(3 ohms) = 3 V
I R1 = (1 A)(1 ohm) = 1 V
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Again
What is the power output of each resistor?
P2 = I V2 = (1A)(2V) = 2 W
P3 = I V3 = (1A)(3V) = 3 W
6 W total power
P1 = I V1 = (1A)(1V) = 1 W
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Parallel Circuits p410
1.5V
There are 3
pathways for the
electrons to follow.
If a light fails in
one, the others will
continue.
The voltage through each light equals the voltage of
the battery. The resistors in parallel act like one resistor
according to this formula:
1/Rtot = 1/R1 + 1/R2 + 1/R3 + …..
If each resistor is 5 ohms, what
is the total resistance? 1/5 + 1/5 + 1/5 = 3/5
5/3 = 1.7 ohms
What is the total current through the circuit?
I = V / R = 1.5V / 1.7 ohms = 0.88 A
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What is the current through the ammeter?
First, find the total resistance.
1/2 + 1/3 + 1/1 = 3/6 + 2/6 + 6/6 = 11/6
0.545 ohms
I = V/R = 1.50V / 0.545A = 2.75 A
1.5V
What current is going through each resistor?
I2 = V/R2 = 1.50V/2 ohms = 0.750 A
I3 = V/R3 = 1.50/3 = 0.500 A
I1 = V/R = 1.50/1 = 1.50 A
What is the
What power is going through each resistor?
total power?
P2= I2V = (0.750)(1.50) = 1.13 W
P=IV=(2.75)(1.50)
P3 = I3V = (0.500)(1.50) = 0.750 W
P1 = I1V = (1.50)(1.50) = 2.25 W
P = 4.13 W
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Parallel circuits and Overloading
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Adding more appliances decreases the resistance
AND increases the current through the parallel
circuit.
Safety Fuses
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Now, circuit breakers
have replaced this
type of fuse.
The end
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