Do Now (10/14/13):

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Do Now (10/14/13)

• What is Ohms Law?
• What do you know about electric circuits?
• In your own words, what is electric current?

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ELECTRICAL CIRCUITS

• All you need to be an inventor is a good
  imagination and a pile of junk.
• -Thomas Edison

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ReviewOhms Law
I V / R
I Current (Amperes) (amps) V Voltage
(Volts) R Resistance (ohms)
Georg Simon Ohm (1787-1854)
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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)
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How you should be thinking about electric
circuits
Resistance friction that impedes flow of current
through the circuit (rocks in the river)
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How you should be thinking about electric
circuits

Current the actual substance that is flowing
through the wires of the circuit (electrons!)
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Would This Work?
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Would This Work?
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Would This Work?
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The Central Concept Closed Circuit
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Series and Parallel Circuits
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Circuits

• Can either be series or parallel.

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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.

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Series

• Current only takes one path for electrons
• Current flows through every part of the circuit

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Lights in a Series
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Series

• If you add a resistor (like another light)
• Total resistance goes UP since all the current
  has must go through each resistor.

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Adding Resistors to Series

• Current in the circuit will go DOWN (lights will
  dim)
• If you remove a light bulb or one burns outall
  go out!

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Current in Series

• Current is the same at all points
• Use Ohms Law to find current using resistance
  and voltage

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Calculating Current Example

• A 6 V battery is connected to

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Voltage in Series

• Voltage is reduced by each resistance voltage
  drop

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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 2O resistor?

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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.

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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
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Calculating Resistance Example

• A series circuit contains a 12O, 10 O and 8O
  resistor. What is the equivalent resistance?

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Circuit Symbols
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Exit Question

• Draw a series circuit with the following
• A 12 V battery
• Two 3O resistors
• One 5O resistor
• What is the one variable that will not change
  throughout the circuit?

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Do Now (10/15/13)

• Draw a series circuit diagram containing the
  following
• A 2O resistor
• A 1O resistor
• A 6V battery
• Which variable does not change throughout a
  series circuit?

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20.1 Total resistance in a series circuit

• Light bulbs, resistors, motors, and heaters
  usually have much greater resistance than wires
  and batteries.

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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?

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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
• Ohms law is used to calculate the voltage drop
  across each resistor.

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Sample Problem 1

• 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 circuit.

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Do Now (10/24/13)

• Draw an example of what you a parallel circuit
  would look like.

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Extra Credit Opportunity

• Open House Sunday, October 28, 12 3 PM
• 5 service learning hours!
• Free pizza!!
• Extra credit!!!

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Parallel Circuits

• Has at least one point where current divides
• More than one path for current to flow
• Paths are also known as branches

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Lights in Parallel
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Parallel

• If you add a resistor
• Total resistance goes down
• Total current goes up when you add another path

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Removing a Light Bulb

• If you remove a light bulb or one burns out, the
  others stay on because the circuit is still
  closed.

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Current in Parallel

• Current flows into a branching point, the same
  total current must flow out again
• Current depends on resistance in each branch

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Calculating Current Example
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Voltage in Parallel

• Voltage is the same across each branch because
  each branch is on the same wire

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Calculating Voltage Example
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Resistance in Parallel

• Calculate current in each branch based on
  resistance in each branch by using Ohms Law

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Calculating Resistance Example
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Practice problem 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?

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Toll RoadCircuit Analogy
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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.

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Batteries in Series and Parallel
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• In seriesThe voltage is increased.
• In parallelNo change in voltage these batteries
  will last longer!

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One More FINAL Thing

• Two Types of Current
• DCDirect Currentproduced by solar cells and
  chemical cells (batteries)
• Current only flows in one direction.

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2nd type of current

• ACAlternating Current
• Current flows back and forth (alternates)
• Found in homes
• Generators produce AC current

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Practice

• Work on the paper Parallel Circuits.
• Be prepared for an exit question!

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Exit Question

• Draw a parallel circuit containing 3 branches
• Each branch should have one resister (10O, 2O,
  5O)
• The battery is 6 V
• What is the voltage in each branch?