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Electricity and Magnetism

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Electricity and Magnetism Electricity and Magnetism Properties of Magnets Magnetic Properties of Materials The Magnetic Field of the Earth – PowerPoint PPT presentation

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Title: Electricity and Magnetism


1
Electricity and Magnetism
Electricity and Magnetism
  • Properties of Magnets
  • Magnetic Properties of Materials
  • The Magnetic Field of the Earth

2
Chapter Objectives
  1. Predict the direction of the force by using the
    right-hand rule.
  2. Explain the relationship between electric current
    and magnetism.
  3. Describe and construct a simple electromagnet.
  4. Explain the concept of commutation as it relates
    to an electric motor.
  5. Explain how the concept of magnetic flux applies
    to generating electric current using Faradays
    law of induction.
  6. Describe three ways to increase the current from
    an electric generator.

3
Chapter Vocabulary Terms
  • gauss
  • right-hand rule
  • coil
  • solenoid
  • magnetic field
  • tesla
  • Faradays law
  • induction
  • induced current
  • magnetic flux
  • commutator
  • generator
  • electromagnet
  • polarity

4
Electric Current and Magnetism
  • Key Question
  • Can electric current create a magnet?

5
23.1 Electric Current and Magnetism
  • In 1819, Hans Christian Oersted, a Danish
    physicist and chemist, placed a compass needle
    near a wire
  • When the switch was closed, the compass needle
    moved just as if the wire were a magnet.

6
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7
Electric Current and Magnetism
  • 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.

8
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9
Electric Current and Magnetism
  • A single wire is too small to be of much use.
  • There are two ways to make strong magnetic fields
    from current in wires
  • Bundling together, this allows the same current
    to create many times the magnetic field of one
    wire.
  • Coiling the wires concentrate the magnetic field
    in their center.

10
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11
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12
Electric Current and Magnetism
  • A wire coil with many turns called solenoid.
  • A coil takes advantage of these two techniques
    (bundling wires and making coils) for increasing
    field strength.

13
The true nature of magnetism
  • The magnetic field of a coil is identical to the
    field of a disk-shaped permanent magnet.

14
Electric Current and Magnetism
  • The orbiting electrons carry electric charge.
  • Moving charge makes current the orbiting
    electrons make currents
  • These currents create the magnetic fields.
  • Substances that have all their orbits going the
    same way have permeant magnetic fields.

15
Magnetic force on a moving charge
  • The magnetic force on a wire is really due to
    force acting on moving charges in the wire.
  • A charge moving in a magnetic field feels a force
    perpendicular to both the magnetic field and to
    the direction of motion of the charge.

16
Magnetic force on a moving charge
  • A magnetic field that has a strength of 1 tesla
    (1 T) creates a force of 1 newton (1 N) on a
    charge of 1 coulomb (1 C) moving at 1 meter per
    second.
  • This relationship is how the unit of magnetic
    field is defined.

17
Magnetic force on a moving charge
  • A charge moving perpendicular to a magnetic field
    moves in a circular orbit.
  • A charge moving at an angle to a magnetic field
    moves in a spiral.

18
23.1 Magnetic field near a wire
  • The field of a straight wire is proportional to
    the current in the wire and inversely
    proportional to the radius from the wire.

Current (amps)
Magnetic field (T)
Radius (m)
19
Magnetic fields in a coil
  • The magnetic field at the center of a coil comes
    from the whole circumference of the coil.

No. of turns of wire
Magnetic field (T)
Current (amps)
Radius of coil (m)
20
Calculate magnetic field
  • A current of 2 amps flows in a coil made from 400
    turns of very thin wire.
  • The radius of the coil is 1 cm.
  • Calculate the strength of magnetic field (in
    tesla) at the center of the coil.

21
Electromagnets and the Electric Motor
  • Key Question
  • How does a motor work?

Students read Section 23.2 AFTER Investigation
23.2
22
Electromagnets and the Electric Motor
  • Electromagnets are magnets that are created when
    electric current flows in a coil of wire.
  • A simple electromagnet is a coil of wire wrapped
    around a rod of iron or steel.
  • Because iron is magnetic, it concentrates and
    amplifies the magnetic field created by the
    current in the coil.

23
Electromagnets and the Electric Motor
  • The right-hand rule
  • When your fingers curl in the direction of
    current, your thumb points toward the magnets
    north pole.

24
The principle of the electric motor
  • An electric motor uses electromagnets to convert
    electrical energy into mechanical energy.
  • The disk is called the rotor because it can
    rotate.
  • 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.

25
The principle of the electric motor
26
Commutation
  • The process of reversing the current in the
    electromagnet is called commutation and the
    switch that makes it happen is called a
    commutator.

27
Electric Motors
  • Electric motors are very common.
  • All types of electric motors have three key
    components
  • A rotating element (rotor) with magnets.
  • A stationary magnet that surrounds the rotor.
  • A commutator that switches the electromagnets
    from north to south at the right place to keep
    the rotor spinning.

28
Electric Motors
  • If you take apart an electric motor that runs on
    batteries, the same three mechanisms are there
    the difference is in the arrangement of the
    electromagnets and permanent magnets.

29
Electric motors
  • The rotating part of the motor, including the
    electromagnets, is called the armature.

30
23.2 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
31
23.2 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 coils.

32
23.3 Induction and the Electric Generator
  • Key Question
  • How does a generator produce electricity?

Students read Section 23.3 AFTER Investigation
23.3
33
23.3 Induction and the Electric Generator
  • 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.

34
23.3 Induction
  • Current is only produced if the magnet is moving
    because a changing magnetic field is what creates
    current.
  • If the magnetic field does not change, such as
    when the magnet is stationary, the current is
    zero.

35
23.3 Induction
  • If the magnetic field is increasing, the induced
    current is in one direction.
  • If the field is decreasing, the induced current
    is in the opposite direction.

36
23.3 Magnetic flux
  • A moving magnet induces current in a coil only if
    the magnetic field of the magnet passes through
    the coil.

37
23.3 Faraday's Law
  • Faradays law says the current in a coil is
    proportional to the rate at which the magnetic
    field passing through the coil (the flux) changes.

38
23.3 Faraday's Law
39
23.3 Generators
  • A generator is a device that uses induction to
    convert mechanical energy into electrical energy.

40
23.3 Transformers
  • Transformers are extremely useful because they
    efficiently change voltage and current, while
    providing the same total power.
  • The transformer uses electromagnetic induction,
    similar to a generator.

41
23.3 Transformers
  • A relationship between voltages and turns for a
    transformer results because the two coils have a
    different number of turns.

42
Application Trains that Float by Magnetic
Levitation
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