Unit 7, Chapter 20

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Unit 7, Chapter 20
CPO Science Foundations of Physics
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Unit 7 Electricity and Magnetism
Chapter 20 Electric Circuits and Power

• 20.1 Series and Parallel Circuits
• 20.2 Analysis of Circuits
• 20.3 Electric Power, AC, and DC Electricity

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Chapter 20 Objectives

1. Recognize and sketch examples of series and
   parallel circuits.
2. Describe a short circuit and why a short circuit
   may be a dangerous hazard.
3. Calculate the current in a series or parallel
   circuit containing up to three resistances.
4. Calculate the total resistance of a circuit by
   combining series or parallel resistances.
5. Describe the differences between AC and DC
   electricity.
6. Calculate the power used in an AC or DC circuit
   from the current and voltage.

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Chapter 20 Vocabulary Terms

• series circuit
• parallel circuit
• short circuit
• network circuit
• circuit analysis
• power
• Kirchhoffs voltage law
• voltage drop direct
• current (DC)

• alternating current (AC)
• kilowatt
• Kirchhoffs current law
• horsepower
• power factor
• circuit breaker
• watt
• kilowatt-hour

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20.1 Series and Parallel Circuits

• Key Question
• How do series and parallel circuits work?

Students read Section 20.1 AFTER Investigation
20.1
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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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20.1 Adding resistances in series

• Each resistance in a series circuit adds to the
  total resistance of the circuit.

Rtotal R1 R2 R3...
Total resistance (ohms)
Individual resistances (W)
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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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20.1 Series and Parallel Circuits

• In parallel circuits the current can take more
  than one path.
• Because there are multiple branches, the current
  is not the same at all points in a parallel
  circuit.

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20.1 Series and Parallel Circuits

• Sometimes these paths are called branches.
• The current through a branch is also called the
  branch current.
• When analyzing a parallel circuit, remember that
  the current always has to go somewhere.
• The total current in the circuit is the sum of
  the currents in all the branches.
• At every branch point the current flowing out
  must equal the current flowing in.
• This rule is known as Kirchhoffs current law.

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20.1 Voltage and current in a parallel circuit

• In a parallel circuit the voltage is the same
  across each branch because each branch has a low
  resistance path back to the battery.
• The amount of current in each branch in a
  parallel circuit is not necessarily the same.
• The resistance in each branch determines the
  current in that branch.

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20.1 Advantages of parallel circuits

• Parallel circuits have two big advantages over
  series circuits
• 1. Each device in the circuit sees the full
  battery voltage.
• 2. Each device in the circuit may be turned off
  independently without stopping the current
  flowing to other devices in the circuit.

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20.1 Short circuit

• A short circuit is a parallel path in a circuit
  with zero or very low resistance.
• Short circuits can be made accidentally by
  connecting a wire between two other wires at
  different voltages.
• Short circuits are dangerous because they can
  draw huge amounts of current.

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20.1 Calculate current

• Two bulbs with different resistances are
  connected in parallel to batteries with a total
  voltage of 3 volts.
• Calculate the total current supplied by the
  battery.

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20.1 Resistance in parallel circuits

• Adding resistance in parallel provides another
  path for current, and more current flows.
• When more current flows for the same voltage, the
  total resistance of the circuit decreases.
• This happens because every new path in a parallel
  circuit allows more current to flow for the same
  voltage.

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20.1 Adding resistance in parallel circuits

• A circuit contains a 2 ohm resistor and a 4 ohm
  resistor in parallel.
• Calculate the total resistance of the circuit.

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20.2 Analysis of Circuits

• Key Question
• How do we analyze network circuits?

Students read Section 20.2 AFTER Investigation
20.2
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20.2 Analysis of Circuits

• All circuits work by manipulating currents and
  voltages.
• The process of circuit analysis means figuring
  out what the currents and voltages in a circuit
  are, and also how they are affected by each
  other.
• Three basic laws are the foundation of circuit
  analysis.

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20.2 Three circuit laws
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20.2 Voltage divider circuit
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20.2 Voltage divider

• A circuit divides any supplied voltage by a ratio
  of the resistors.

Input voltage (volts)
Output voltage (volts)
resistor ratio (W)
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20.2 Solving circuit problems

1. Identify what the problem is asking you to find.
   Assign variables to the unknown quantities.
2. Make a large clear diagram of the circuit. Label
   all of the known resistances, currents, and
   voltages. Use the variables you defined to label
   the unknowns.
3. You may need to combine resistances to find the
   total circuit resistance. Use multiple steps to
   combine series and parallel resistors.

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20.2 Solving circuit problems

1. If you know the total resistance and current, use
   Ohms law as V IR to calculate voltages or
   voltage drops. If you know the resistance and
   voltage, use Ohms law as I V R to calculate
   the current.
2. An unknown resistance can be found using Ohms
   law as R V I, if you know the current and the
   voltage drop through the resistor.
3. Use Kirchhoffs current and voltage laws as
   necessary.

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20.2 Solving circuit problems

• A bulb with a resistance of 1O is to be used in a
  circuit with a 6-volt battery.
• The bulb requires 1 amp of current.
• If the bulb were connected directly to the
  battery, it would draw 6 amps and burn out
  instantly.
• To limit the current, a resistor is added in
  series with the bulb.
• What size resistor is needed to make the current
  1 amp?

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20.2 Network circuits

• In many circuits, resistors are connected both in
  series and in parallel.
• Such a circuit is called a network circuit.
• There is no single formula for adding resistors
  in a network circuit.
• For very complex circuits, electrical engineers
  use computer programs that can rapidly solve
  equations for the circuit using Kirchhoffs laws.

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20.2 Calculate using network circuits

• Three bulbs, each with a resistance of 3O, are
  combined in the circuit in the diagram
• Three volts are applied to the circuit.
• Calculate the current in each of the bulbs.
• From your calculations, do you think all three
  bulbs will be equally bright?

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20.3 Electric Power, AC, and DC Electricity

• Key Question
• How much does electricity cost and what do you
  pay for?

Students read Section 20.3 AFTER Investigation
20.3
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20.3 Electric Power, AC, and DC Electricity

• The watt (W) is a unit of power.
• Power is the rate at which energy moves or is
  used.
• Since energy is measured in joules, power is
  measured in joules per second.
• One joule per second is equal to one watt.

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20.3 Reviewing terms
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20.3 Power in electric circuits

• One watt is a pretty small amount of power.
• In everyday use, larger units are more convenient
  to use.
• A kilowatt (kW) is equal to 1,000 watts.
• The other common unit of power often seen on
  electric motors is the horsepower.
• One horsepower is 746 watts.

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20.3 Power
Voltage (volts)
P VI
Current (amps)
Power (watts)
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20.3 Calculate power

• A light bulb with a resistance of 1.5O is
  connected to a 1.5-volt battery in the circuit
  shown at right.
• Calculate the power used by the light bulb.

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20.3 Paying for electricity

• Electric companies charge for the number of
  kilowatt-hours used during a set period of time,
  often a month.
• One kilowatt-hour (kWh) means that a kilowatt of
  power has been used for one hour.
• Since power multiplied by time is energy, a
  kilowatt-hour is a unit of energy.
• One kilowatt-hour is 3.6 x 106 joules.

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20.3 Calculate power

• Your electric company charges 14 cents per
  kilowatt-hour. Your coffee maker has a power
  rating of 1,050 watts.
• How much does it cost to use the coffee maker one
  hour per day for a month?

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20.3 Alternating and direct current

• The current from a battery is always in the same
  direction.
• One end of the battery is positive and the other
  end is negative.
• The direction of current flows from positive to
  negative.
• This is called direct current, or DC.

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20.3 Alternating and direct current

• If voltage alternates, so does current.
• When the voltage is positive, the current in the
  circuit is clockwise.
• When the voltage is negative the current is the
  opposite direction.
• This type of current is called alternating
  current, or AC.

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20.3 Alternating and direct current

• AC current is used for almost all high-power
  applications because it is easier to generate and
  to transmit over long distances.
• The 120 volt AC (VAC) electricity used in homes
  and businesses alternates between peak values of
  170 V and -170 V at a frequency of 60 Hz.
• AC electricity is usually identified by the
  average voltage, (120 VAC) not the peak voltage.

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20.3 Power in AC circuits

• For a circuit containing a motor, the power
  calculation is a little different from that for a
  simple resistance like a light bulb.
• Because motors store energy and act like
  generators, the current and voltage are not in
  phase with each other.
• The current is always a little behind the voltage.

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20.3 Power for AC circuits

• Electrical engineers use a power factor (pf) to
  calculate power for AC circuits with motors

Avg. voltage (volts)
Avg. current (amps)
P VI x pf
Power (watts)
power factor 0-100
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Application Wiring in Homes and Buildings
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Application Wiring in Homes and Buildings