Electromagnets and Induction CPO Science by alicejenny


									Electricity and Magnetism
   Unit 5: Electricity and Magnetism
   Chapter 16: Electromagnets and
 16.1   Electric Current and Magnetism

 16.2   Electric Motors

 16.3   Electric Generators and Transformers
16.1 Investigation: Electromagnetic Forces

Key Question:
How does an electric motor work?

   Build a simple electric motor.

   Describe the components required for an electric motor to work.

   Test the effects of changing different variables on the function of
    an electric motor.
Electric Current and Magnetism
 In 1819, Hans Christian
  Oersted, a Danish physicist
  and chemist, and a
  professor, placed a compass
  needle near a wire through
  which he could make electric
  current flow.
 When the switch was closed,
  the compass needle moved
  just as if the wire were a
Electric current and magnetism
 Electric
         current is made of moving charges
 (electrons), which creates the magnetic field around
 a current-carrying wire

 Magnetism   is created by these moving charges.
The magnetic fields of straight wire
 Themagnetic field lines are concentric circles with
 the wire at the center of the circles.
 The direction of the field depends on the direction of
 the current in the wire.
The magnetic fields of straight wire
        The strength of the magnetic field near the wire
         depends on two factors:
    1.    The strength is directly proportional to the current, so
          doubling the current doubles the strength of the field.
    2.    The field strength is inversely proportional to the distance
          from the wire. (Decreasing the distance to the wire by half
          doubles the strength of the field.)
The magnetic fields of straight wire
                       Near a straight wire, the
                        north pole of a compass
                        needle feels a force in the
                        direction of the field lines.

                       The south pole feels a
                        force in the opposite

                       As a result, the needle
                        twists to align its north-
                        south axis along the
                        circular field lines.
The magnetic fields of loops and coils
     The magnetic field around a single wire is too
      small to be of much use.
     There are two techniques to make strong
      magnetic fields from current flowing in wires:
     1.   Parallel wires can be bundled together. (10 wires,
          each with 1 A of current, create a magnetic field 10X
          as strong as 1 wire carrying 1 A).
     2.   A wire can be looped into a coil so the magnetic field
          is concentrated at the center.
The magnetic fields of loops and coils
 The most common form of
 electromagnetic device is a
 coil with many turns called a
A coil takes advantage of
 these two techniques
 (bundling wires and making
 bundled wires into coils) for
 increasing field strength.
Magnetic forces and electric currents
    Two wires carrying electric current exert force on each
     other, just like two magnets.

    The forces can be attractive or repulsive depending on
     the direction of current in both wires.
   Unit 5: Electricity and Magnetism
   Chapter 16: Electromagnets and
 16.1   Electric Current and Magnetism

 16.2   Electric Motors

 16.3   Electric Generators and Transformers
16.2 Investigation: Electromagnetic Induction
Key Question:
How does an electric generator work?

   Explain how an electric generator works.

   Describe the relationship between the voltage output of a
    generator and the speed of the rotor.

   Modify the design of a generator to test the effects of different
    factors, such as the number of magnets and the orientation of
    the magnets.
Electric motors
   Electric motors convert electrical energy into mechanical

   The disk in the motor is called the rotor because it can

   The disk will keep spinning as long as the external magnet is
    reversed every time the next magnet in the disk passes by.

   One or more stationary magnets reverse their poles to push
    and pull on a rotating assembly of magnets.
Using magnets to spin a disk
 Reversing the magnet in your fingers attracts and
 repels the magnets in the rotor, making it spin.
 The process of reversing the current in the
 electromagnet is called commutation and the
 switch that makes it happen is called a
Electric Motors
            All types of electric motors have three
             key parts:
           1. A rotating element (rotor) with magnets.
           2. A stationary magnet that surrounds the
           3. A commutator that switches the
              electromagnets from north to south at
              the right place to keep the rotor
AC and DC motors
 Motors that run on alternating current (AC) electricity
 are easier to make because the current switches
 direction all by itself—a commutator isn’t needed.
Electric motors
 The  rotating part of the
  motor, including the
  electromagnets, is called
  the armature.

 Ithas 3 electromagnets
  that correspond to the 3
Electric motors
 The permanent magnets
 are on the outside, and
 they stay fixed in place.

 The wires from each of
 the three coils are
 attached to three metal
 plates (commutator) at
 the end of the armature.    commutator
Electric Motors
   As the rotor spins, the three plates come into
    contact with the positive and negative brushes.
   Electric current flows through the brushes into the
   Unit 5: Electricity and Magnetism
   Chapter 16: Electromagnets and
 16.1   Electric Current and Magnetism

 16.2   Electric Motors

 16.3   Electric Generators and Transformers
16.3 Investigation: Generators and
Key Question:
   How do electricity and magnetism work
    together in generators and transformers?

   Apply an understanding of electricity and magnetism to describe
    how generators and transformers function.
Electromagnetic Induction
 If
   you move a magnet near a coil of wire, a current
  will be produced.

 This  process is called electromagnetic induction,
  because a moving magnet induces electric current
  to flow.

 Moving electric charge creates magnetism and
  conversely, changing magnetic fields also can
  cause electric charge to move.
 Current is only produced if
  the magnet is moving
  because a changing
  magnetic field is what
  creates current.

 Ifthe magnetic field does
  not change, such as when
  the magnet is stationary,
  the current is zero.
 Ifthe magnetic field is increasing, the induced
  current is in one direction.

 Ifthe field is decreasing, the induced current is in
  the opposite direction.
Faraday’s law of induction

A moving magnet
 induces current in a
 coil only if the
 magnetic field of
 the magnet passes
 through the coil.
Faraday’s law of induction
            Michael  Faraday (1791–1867), an
            English physicist and chemist, was
            first to explain how moving magnets
            and coils induced voltage.

            Faraday’s found that the induced
            voltage is proportional to the rate of
            change of the magnetic field through
            the coil.
Faraday’s law of induction
                     Faraday’s   law says
                      the current in a coil is
                      proportional to the rate
                      at which the magnetic
                      field changes.
Faraday's Law
A generator is a device that uses induction to
 convert mechanical energy into electrical energy.
Electrical generators
                 The electrical energy created
                  by a generator is not created
                  from nothing.
                 Energy   must continually be
                  supplied to keep the rotating
                  coil or magnetic disk turning.
                 In hydroelectric generators,
                  falling water turns a turbine
                  which spins a generator to
                  produce electricity.
Producing and transporting energy
                   Hoover  Dam is called a
                   hydroelectric plant
                   because it converts the
                   energy of falling water into

                   Usingthe potential energy
                   of water is one way to
                   produce electricity.
Energy flow
 Witheach transformation (green arrows), some
 energy is lost to the system in the form of heat (red
Electricity from different resources
A nonrenewable resource is not replaced as it is

 Anyfossil fuel is an good example of
 nonrenewable resource.

 Besides their growing scarcity, burning fossil fuels
 produces sulfur oxide emissions that reduce air
 quality and may be accelerating climate change.
Electricity from different resources
                 A  renewable resource can
                  be replaced naturally in a
                  relatively short period of

                  Falling
                         water, energy from
                  the Sun, wind energy, and
                  geothermal energy are
                  examples of renewable
Geothermal, biomass and hydroelectric
 Geothermal  power plants use Earth’s internal heat in
 the form of water or steam, to produce electricity.

 Biomass,such as organic material from plants or
 animals or municipal waste, can be burned to
 produce steam for a turbine.

 Impoundment  and pumped storage hydroelectric
 power plants use falling water differently to generate
Electrical Power
 Recall that electrical power (in
 watts) is the rate at which
 electrical energy is changed into
 other forms of energy such as
 heat, sound, or light.

 Anything   that “uses” electricity is actually converting
  electrical energy into some other type of energy.

 Utilitycompanies charge customers for the number
  of kilowatt-hours (kWh) used each month.
 Transformers    are
 extremely useful because
 they efficiently change
 voltage and current, while
 providing the same total
 Thetransformer uses
 induction, similar to a
    Consider the transformer between the outside
     power lines and your house:
    1. The primary coil is connected to outside power
       lines. Current in the primary coil creates a
       magnetic field through the secondary coil. The
       primary coil’s field is shown by the magnetic field
       lines (green arrows)
    2. The current in the primary coil changes constantly
       because it is alternating current.
3.   As the current changes, so does the strength and direction
     of the magnetic field through the secondary coil.

4.   The changing magnetic field through the secondary coil
     induces current in the secondary coil. The secondary coil
     connects to your home’s wiring.
                                    The relationship between
                                     voltages and turns for a
                                     transformer is the result of
                                     two coils having a different
                                     number of turns.
 Inthe same changing
 magnetic field, a coil with
 100 turns produces 10
 times the voltage of the
 induced current as a coil
 with 10 turns.
                       Changing voltage in a transformer
A cell-phone AC adapter reduces the 120 V AC to the 6 V DC
needed by the phone’s battery. If the primary coil has 240 turns,
how many turns must the secondary coil have?
1.   Looking for: …no. of turns of the secondary coil.
2.   Given: …voltage of each coil (120VAC and 6 VDC) and the
     no. of turns of the primary coil (240)
3.   Relationships: …V1 = N1 Solve for N2 = V2 x N1
                     V2   N2                    V1

4.   Solution: … 6 V x 240 = 12 turns
                     120 V
                                       Michael Faraday

   Despite little formal schooling,
    Michael Faraday rose to become
    one of England’s top research
    scientists of the nineteenth

   He is best known for his
    discovery of electromagnetic
    induction, which made possible
    the large-scale production of
    electricity in power plants.

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