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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 CIRCUITSAll 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)
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!)
How you should be thinking about electric circuits:
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.
SeriesIf 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 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 SeriesAdd up all resistors to get totalTotal 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?
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 1.5-volt 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 Draw a series circuit with two 1.5 V
batteries, 3 resistors, and a current of 0.5 A.
1. What is the total voltage of the circuit?
2. What is the resistance of each resistor?
3. What is the voltage drop across each resistor? Label on your circuit.
Extra Credit Opportunity
Open House – Sunday, October 28, 12 – 3 PM
5 service learning hours!Free pizza!!Extra credit!!!
Parallel CircuitsHas at least one point where current divides
More than one path for current to flow
Paths are also known as branches
Parallel:If you add a resistor:
Total resistance goes down
Total current goes up when you add another path
Removing a Light BulbIf 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
Voltage in Parallel
Voltage is the same across each branch – because each branch is on the same wire
Resistance in ParallelCalculate current in each branch based on resistance in each branch by using Ohm’s Law
Practice problem #2
Draw a parallel circuit with two resistors (one on each branch) and a 12 V battery.
1. What is the voltage through each resistor?
2. What is the current flowing through each branch?
Toll Booth ExplanationAdding toll booths in series
increases resistance and slows the current flow.
Adding toll booths in parallel lowers resistance and increases the current flow.
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.
2nd type of current:AC—Alternating CurrentCurrent flows back and forth (alternates)
Found in homesGenerators produce AC current