Do Now (10/14/13):



What is Ohm’s Law?
What do you know about electric
circuits?
In your own words, what is electric
current?
ELECTRICAL
CIRCUITS
All you need to be an inventor is a
good imagination and a pile of junk.
-Thomas Edison
Review:Ohm’s Law
I=V/R
Georg Simon Ohm (1787-1854)
I
= Current (Amperes) (amps)
V
= Voltage (Volts)
R
= Resistance (ohms)
How you should be thinking
about electric circuits:
Voltage: what
pushes the current
through the circuit
(in this picture it
would be
equivalent to
gravity)
How you should be thinking
about electric circuits:
Resistance: friction that
impedes flow of current
through the circuit
(rocks in the river)
How you should be thinking
about electric circuits:
Current: the actual
“substance” that is
flowing through the
wires of the circuit
(electrons!)
Would This Work?
Would This Work?
Would This Work?
The Central Concept:
Closed Circuit
Series and
Parallel Circuits
Circuits
Can
either be series
or parallel.
20.1 Series and Parallel Circuits


In series circuits, current can only take
one path.
The amount of current is the same at
all points in a series circuit.
Series
Current
only takes
one path for
electrons
Current flows
through every part of
the circuit
Lights in a Series
Series
If
you add a resistor (like
another light):
Total
resistance goes UP
since all the current has
must go through each
resistor.
Adding Resistors to
Series:
Current in the circuit
will go DOWN (lights
will dim)
If you remove a light
bulb or one burns
out—all go out!
Current in Series
Current
is the same at
all points
Use Ohm’s Law to find
current using
resistance and voltage
Calculating Current
Example:

A 6 V battery is connected to
Voltage in Series
Voltage
is
reduced by each
resistance –
voltage drop
Calculating Voltage
Example

A 6 V battery is connected to a
series circuit. If the current is 2 A,
what is the voltage across a 2Ω
resistor?
Resistance in Series
Add
up all resistors to
get total
Total
resistance will go
up because all of the
current must go through
each resistor.
Adding resistances in series

Each resistance in a
series circuit adds to
the total (or
equivalent) resistance
of the circuit.
Rtotal = R1 + R2 + R3...
Total resistance
(ohms)
Individual resistances
Calculating
Resistance Example:

A series circuit contains a 12Ω, 10
Ω and 8Ω resistor. What is the
equivalent resistance?
Circuit Symbols
Exit Question:





Draw a series circuit with the following:
A 12 V battery
Two 3Ω resistors
One 5Ω resistor
What is the one variable that will not
change throughout the circuit?
Do Now (10/15/13):

Draw a series circuit diagram
containing the following:




A 2Ω resistor
A 1Ω resistor
A 6V battery
Which variable does not change
throughout a series circuit?
20.1 Total resistance in a series
circuit

Light bulbs, resistors, motors, and heaters
usually have much greater resistance than wires
and batteries.
20.1 Calculate current

How much current flows in a circuit with a 1.5volt battery and three 1 ohm resistances (bulbs)
in series?
20.1 Voltage in a series circuit



Each separate resistance
creates a voltage drop as the
current passes through.
As current flows along a
series circuit, each type of
resistor transforms some of
the electrical energy into
another form of energy
Ohm’s law is used to calculate
the voltage drop across each
resistor.
Sample Problem #1

1.
2.
3.
Draw a series circuit with two 1.5
V batteries, 3 resistors, and a
current of 0.5 A.
What is the total voltage of the
circuit?
What is the resistance of each
resistor?
What is the voltage drop across
each resistor? Label on your
Do Now (10/24/13):

Draw an example of what you a
parallel circuit would look like.
Extra Credit Opportunity
 Open
House – Sunday,
October 28, 12 – 3 PM
 5 service learning hours!
 Free pizza!!
 Extra credit!!!
Parallel Circuits
Has
at least one point
where current divides
More than one path for
current to flow
Paths are also known as
branches
Lights in Parallel
Parallel:
If
you add a resistor:
Total resistance goes
down
Total current goes up
when you add
another path
Removing a Light
Bulb
If
you remove a light
bulb or one burns out,
the others stay on
because the circuit is
still closed.
Current in Parallel
 Current
flows into a
branching point, the same
total current must flow out
again
 Current depends on
resistance in each branch
Calculating Current
Example:
Voltage in Parallel
Voltage
is the same
across each branch –
because each branch is
on the same wire
Calculating Voltage
Example:
Resistance in
Parallel
Calculate
current in each
branch based on
resistance in each branch
by using Ohm’s Law
Calculating
Resistance Example:
Practice problem #2

1.
2.
Draw a parallel circuit with
two resistors (one on each
branch) and a 12 V battery.
What is the voltage through
each resistor?
What is the current flowing
through each branch?
Toll Road—Circuit
Analogy
Toll Booth
Explanation
 Adding
toll booths in series
increases resistance and
slows the current flow.
 Adding toll booths in
parallel lowers resistance
and increases the current
flow.
Batteries in Series
and Parallel:
In
series—The voltage
is increased.
In parallel—No
change in voltage;
these batteries will
last longer!
One More FINAL
Thing:
Two
Types of Current:
DC—Direct Current—
produced by solar cells
and chemical cells
(batteries)
Current only flows in
one direction.
nd
2 type of current:
AC—Alternating
Current
Current flows back and
forth (alternates)
Found in homes
Generators produce AC
current
Practice:


Work on the paper “Parallel
Circuits.”
Be prepared for an exit question!
Exit Question:

Draw a parallel circuit containing 3
branches:



Each branch should have one resister
(10Ω, 2Ω, 5Ω)
The battery is 6 V
What is the voltage in each branch?