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Warm Up: Battery and Bulb
What you need: At the front of the room, take a
bulb, a single wire and a single D-cell battery
• Can you make a bulb light using
only a bulb, a D-cell battery and one
piece of wire?
• Draw a picture of all the ways you
try to light the bulb.
• Identify which ones worked and
which did not.
• Why do you think the methods that
worked behaved the way they did?
Electric Circuits
Chapter 18
What Methods Worked?
Why did this circuit work?
What is Electricity?
Electricity is the flow (current) of
electric charges from a high potential
to a low potential.
Two Things are needed to make
current flow.
1)A closed path of conducting
material.
2)A difference in electric
potential.
What We Already Know:
Just like how a mass inside a gravitational field will
move from an area of high gravitational potential
energy to an area of low gravitational potential
energy, Charges will flow from an area of
high electrical potential energy to an area
of low electrical potential energy.
-The amount of Potential energy a
charge has divided by the amount
of charge present is called Voltage
Therefore:
Voltage (V)
PE
V
q
Where,
V = Voltage: measured in Volts (V)
&

q = Charge: measured in Coulombs (C)
Other name for Voltage:
Potential Difference
Electromotive Force (EMF)
What we already know
continued:
Conductor: Allows Electricity to flow
easily (Metals, water with dissolved materials in it)
Insulator: Does NOT allow electricity
to flow easily (Glass, rubber, plastic, air, pure
water)
Current ( I ): The net flow of
electrons through a conductor
Current ( I )
Current: The number of electrons
that pass a specific point in a circuit
in one second
• Current is measured in amperes (A) (amps)
•

q
II = q/t… so the units are 1A = 1C/s
t
Voltage does not go anywhere,
the charges are the only things that move.
Current ( I )
The way we say current flows through a circuit is
actually in the opposite direction (called Conventional
current) that electrons move… I know… its weird
Conventional Current, just like in the
electrostatics chapter, flows from positive to
negative.
Types of Current
Direct Current: Current in a circuit flows in only one
direction.
-Positive and negative ends are fixed and current
flows from + to -.
-Batteries are DC
Alternating Current: Current in a circuit oscillates back and
forth.
-Positive and negative end switch back and forth so current’s
direction switches back and forth.
-Wall Socket has AC
-Many motors are AC motors
Voltage and Current
Voltage is what “pushes” the electricity
through the circuit. That is why it is called
the EMF. It causes the current to flow.
Current
Voltage
Source
Voltage and Current
Flow
Pump
• Think of the Current as water. Water flows from
Higher PE to lower PE
• Eventually the water settles and “loses” its
energy
• The pump acts like a battery, because it brings
the water from Low Energy to High Energy
You built this…
It is known as a circuit.
- A circuit is the path in which current
can flow between a potential
difference
Refresh: What two things are needed
to make a circuit work (for current to
flow)?
What if I wanted to give you a
diagram of how to build a circuit?
Without special symbols we would
be forced to draw complicated
pictures like this:
So, you built this…
BUT…we would draw
it like this…
Symbollogy…How to draw
circuit diagrams
Resistance
All of the components we have just talked
about affect how much current runs
through them
•The amount of current flowing through
a circuit depends on 2 things:
1.How big the voltage is (how hard the
electrons are being pushed)
2.How big the resistance is.
What is resistance?
Resistance
• Resistance- degree of opposition
(sort of like friction) that charges
encounter as they move through a
conductor.
• Measured in Ohms (Ω)
•What factors affect the resistance of a
wire?
Factors that change
Resistance
1) Length:
Longer wires have _________ Resistance.
2) Thickness (gauge):
Thicker wires have _________ Resistance.
3) Temperature:
Hotter wires have _________ Resistance.
4) Material
We will fill these in after the lab.
Ohm’s Law
Ohm’s Law shows the relationship
between the 3 major values we have
discussed (voltage, current, and
resistance)
V = IR
Voltage = Current x Resistance
Power Law
The Power Law shows the
relationship between the voltage,
current, and power)
P = IV
Power = Current x Voltage
Power Law
We have discussed power in this
class already in an earlier unit. Is
this the same power? Can you
prove that electrical power is the
same as mechanical power?
Building Circuits
Building Circuits
Closed circuit Vs Open Circuit
Closed Circuit
Open Circuit
Circuit Rules: Voltage
Voltage Drop: The portion of the total
voltage in a circuit used up by a resisting
component.
-Resistors in a circuit will reduce the voltage as
current flows
Series Circuit: Voltage
V1 = 4V
VT = 9V
V2 = 5V
As current flows through a circuit voltage is lost
across each component.
The total voltage is equal to the sum of the
voltage drop across each resistor.
VT  V1  V2  ... Vn
Series Circuit: Current
I1= 0.2A
IT= 0.2A
I2= 0.2A
The current does not change as it flows
through the circuit.
IT  I1  I2  ...  In
Series Circuit: Resistance
R1 = 10
R2 = 20
The Equivalent (total) resistance is equal to the
sum of the individual resistors.
Req  R1  R2  ... Rn
 Req  10  20  30
Parallel Circuit: Voltage
VT = 120V
V1 = 120V
V2 = 120V
The voltage through each branch is the
same. Each branch needs to use up all of
the available voltage.
VT  V1  V2  ...  Vn
Parallel Circuit: Current
IT = 4A
I1 = 1A
I2 = 3A
As current flows through a circuit, it splits and distributes its
parts through each branch. How much in each branch
depends on the resistance in that branch.
The total current is equal to the sum of the
currents through each branch of the circuit.
IT  I1  I2  ... In
Parallel Circuit: Resistance
R1 = 10
R2 = 20
The inverse of the Equivalent resistance is
equal to the sum of the inverse of the individual
resistors.
1
1
1
1
 
 ...
Req R1 R2
Rn
1
1
1
3



Req 10 20 20
20
Req 
 6.67
3
Circuit Rules
(To sum it all up)



Series Circuits
Parallel Circuits
IT  I1  I2  ...  In
IT  I1  I2  ... In
VT  V1  V2  ... Vn
VT  V1  V2  ...  Vn

Req  R1  R2  ... Rn

1
1
1
1
 
 ...
Req R1 R2
Rn
Examples of Series and
Parallel
Series Circuits
Fuses in your
house
Parallel Circuits
-Houses
-Christmas Lights
What is a Short Circuit?
Generally caused when electricity is allowed to
flow in a path that was unintended. Like when
a resistance free wire closes a path between
hot wires and ground wires.
Can cause circuits or appliances to
break, or even heat wires enough to
start fire.
The danger from a short circuit
comes from a current through a wire
that is way too high.
Example: Battery and steel wool
More on Fuses
A parallel circuit can act like a short.
As branches are added to a
parallel circuit, resistance
decreases and current
increases.
A fuse in your house will break
when you have too many things
on, and the current through your
house becomes too great.
Ohm’s Law and Electric
Shock
• Damaging effects of electric shock are from
current traveling through the body. The initial
cause of electric shock is the voltage, however
the current does the damage.
• Your body typically offers 100,000 Ohms of
resistance. A current of 0.01 Amperes could
cause muscle spasms.
The
faster the current passes through you, the
more damage it does (pg 537 in “Conceptual
Physics”)
Transformers
• increases or decreases AC voltage
• based on induction
• insulated current-carrying wire coiled on one
side of iron ring (primary)
• second wire coiled on other side of iron ring
(secondary)
Math of Transformers
secondary voltage # turns on secondary
------------------------- = ---------------------primary
voltage
# turns on primary
Vs/Vp = Ns/Np
Vp Ip = Vs Is
Is/Ip = Vp/Vs = Np/Ns
(ideal transformer power is conserved)