Transcript Electric Circuits

Simple Circuits
Challenge Questions
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1. Why can a bird be perched on a high voltage wire?
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No potential difference between bird’s feet, therefore no current.
http://www.youtube.com/watch?v=GLW6MEZ9Dcs
2. If a parachutist grabs onto a wire, what happens?
What if it breaks? Why should the parachutist let go as it
falls to the ground?
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No potential difference in the first situation, so no current. If they
hold on and their feet touch the ground, there will be a current
due to potential difference between the wire and the ground.
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http://www.youtube.com/watch?v=jleAxuFGknk
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http://www.youtube.com/watch?v=BtQtRGI0F2Q&feature=related
Battery and Light Bulb
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Consider the diagram of the circuit you
created to light the light bulb.
Light Bulb
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How does a light bulb make a full conducting
path?
Electric Circuits
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Electric Circuit
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A set of electrical components connected so that
they provide one or more complete paths for the
movement of charges
Ex. Light Bulb
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Filament is a resistor. When wire connects battery
to the bulb, charges built up on one terminal of battery
have a path to reach the opposite charges on the
other terminal. Charges move creating a current.
Current causes filament to heat and glow.
Electric Circuits
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Circuit
 The path where electrons flow.
Current
 The rate at which the charge flows past a point.
Voltage
 The amount of “push” behind electrons.
Resistance
 Equal to potential difference divided by current.
EMF
 The energy per unit charge supplied by a source of electric current
Load – any element in a circuit that dissipates energy (ex. Bulb)
Closed circuit – a complete path from one battery terminal to
another.
Open circuit – no complete path, therefore no current
Schematic Diagram
Symbols
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Wire
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Resistor
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Bulb
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Plug
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Battery
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Switch
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Capacitor
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Think of Christmas lights. What happens when one
light burns out?
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The circuit is no longer closed and all the bulbs go
dark.
So why use this?
- It decreases the current needed.
Several lesser resistances can add up to a single
greater resistance.
Important to have no current if something fails (ex.
Burglar alarm)
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Resistors in Parallel
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Parallel
 Describes two or more components in a circuit that are
connected across common points or junctions, providing
separate conducting paths for the current.
 Ex. Christmas Lights
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In series, if a single burns out, they all go dark. In parallel, they have an
alternative path. Current varies, potential difference remains the same.
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Resistors in Series
 Req = R1 + R2 + R3…
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Equivalent resistance equals the total of individual resistances in series.
I = ΔV/Req
ΔV = IR1 and ΔV = IR2
VT = V1 + V2 + V3…
Resistors in Parallel
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1/Req = 1/R1 + 1/R2 + 1/R3…
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Equivalent resistance of resistors in parallel can be calculated using a
reciprocal relationship.
IT = I1 + I2 + I3…
I = ΔV/Req
ΔV =IReq
Series and Parallel Resistors
Series
Parallel
Current
same as total
add to find total
Potential Difference
add to find total
same as total
12V
V = IR
Each Resistor =10 Ω
V = (0.4A)(10 Ω)
Req = 10+10+10
V = 4 volts
Req = 30 Ω
VT = 4V + 4V + 4V = 12V
I = V/R
I = V/R
I = 12V / 30 Ω
I = 4V / 10 Ω
I = 0.4A
I = 0.4A
P = IV
P = (0.4A)(4V)
P = 1.6 W
12V
Each Resistor =10 Ω
1/ Req = (1/10)+(1/10)+(1/10) = 3/10
Req = 3.33 Ω
I = V/R
I = 12V / 3.33 Ω
I = 3.6A
V = 12V
I = V/R
I = 12V / 10 Ω
I = 1.2A
IT = 1.2A + 1.2A + 1.2A = 3.6A
P = IV
P = (1.2A)(12V)
P = 14.4W