SPH3U

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SPH3U Unit 5

• Electricity Magnetism

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Unit Overview

• Electric Energy Circuits
• Electric Charge Electrical Structure of Matter
• Electric Potential
• Elementary Charge (The Millikan Experiment)
• Electric Current
• Resistance
• Electric Power Energy
• Series Circuits
• Parallel Circuits
• Complex Circuits
• Magnetism
• Natural Magnetism
• Magnetic Fields
• Electromagnetism (RHR1 2)
• The Motor Principle (RHR3)
• Electromagnetic Induction
• Lenzs Law
• The Generator Effect
• Transformers

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Electric Power Energy

• 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.
• 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.

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Reviewing Terms
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Defining Power
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Example 1

• In North America, the standard electric outlet
  has a potential difference of 120 V. In Europe,
  it is 240 V. What would be the power output of a
  100W 120 V light bulb if it was connected to a
  240 V system? What would happen to the light bulb?

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Practice

• Pg. 65541, 42
• Pg. 65843-45

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Another Relationship for Power
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Example 2

• An electric kettle is rated at 1500 W for a 120 V
  potential difference.
• What is the resistance of the heating element of
  the kettle?
• What will be the power output if the potential
  difference falls to 108V?

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Practice

• Pg. 66246-50

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Energy Consumption

• A seemingly unusual unit for energy is used when
  talking about electrical energy consumption The
  kilowatt-hour (KW-h).
• One kilowatt-hour is the energy transformed by
  1000 W in one hour (3.6x106J).
• A typical charge by an energy company for
  consumed energy is roughly 0.07 per Kw-h. That
  means that for 7 cents you can buy enough energy
  to lift 360 kg a vertical distance of more than 1
  km!

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Example 3

• A family has its television set on for an average
  of 4.0 h a day. If the television set is rated at
  80 W and energy costs 0.07per Kw-h, how much
  would it cost to operate the television for 30
  days?

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Practice

• Pg. 66451-53

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Series 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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Resistance in Series

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

Rtotal R1 R2 R3...
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Resistance in Series

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

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Example 1

• 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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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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Kirchoffs Voltage Law
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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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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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Kirchoffs Current Law
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Kirchoffs Current Law
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Voltage 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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Example 2

• 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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Advantages of Parallel Circuits

• Parallel circuits have two big advantages over
  series circuits
• Each device in the circuit sees the full battery
  voltage.
• 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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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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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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Resistance in Parallel Circuits
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Example 3

• 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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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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The 3 Circuit Laws
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Solving Circuits Problems

• Identify what the problem is asking you to find.
  Assign variables to the unknown quantities.
• 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.
• 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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Solving Circuits 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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Example 4

• 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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Combined Circuits

• Key Question
• How do we analyze network circuits?

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Combined (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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Example 5

• 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?