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Field Lines

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Title: ELECTRICITY and MAGNETISM Author: Jenn Last modified by: Jenn Created Date: 5/18/2009 12:59:11 PM Document presentation format: On-screen Show (4:3) – PowerPoint PPT presentation

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Title: Field Lines


1
Field Lines
2
Rules for drawing electric field lines
  • At every point the direction of the field is
    tangent to the line
  • Strength is represented by density
  • Lines go from positive and to negative

3
(No Transcript)
4
Rules for Gravitational Field Lines
  • The force of gravity is always attractive
  • The field lines will always point towards the
    center of a spherical mass and arrive
    perpendicular to the surface

5
Rules for Magnetic Field Lines
  • Field lines leave the N-pole, enter the S-pole
    and continue to form a closed loop inside the
    magnet
  • Magnetic Flux the number of magnetic field
    lines passing through a particular unit area.
    Proportional to the magnetic field intensity

6
Questions from Provincial Exam Draw appropriate
lines to describe field near the objects shown
  • A)
  • B)

7
Questions from Provincial
  • Three points are indicated as A, B, C on the
    diagram. List the letters in order of increasing
    field strength (weakest first)

8
Questions from Provincial
  • The diagram shows 3 magnetic poles mapped with
    iron filings. Identify which two poles are
    alike. Explain your reasoning.

9
ELECTRICITY and MAGNETISM
10
Rules for Magnetic Interactions
  • 1. Like poles repel each other
  • 2. Unlike poles attract each other
  • 3. The force of attraction varies inversely as
    the square of the distance between the poles
  • Magnetic Dipole
  • Magnets always seem to come with a N-pole paired
    with an S-pole

11
ELECTROMAGNETISM
  • Electrons produce a magnetic field and a changing
    magnetic field will cause electrons to move.
  • Discovered by accident in 1819 by Hans Christian
    Oersted

12
USES
  • Discovery marked the beginning of modern science
    and technology
  • Radio, television, computers, tape recorders,
    VCRs, CD players, lasers, electric motors and
    generators, etc

13
Magnetic Fields
  • Each point of a current carrying conductor
    creates a magnetic field around itself
  • The field lines are a set of concentric closed
    circles perpendicular to the direction of the
    current

14
Right-hand rule 1 finding direction of magnetic
field lines around a conductor
  • Grasp a current carrying conductor with a your
    right hand, thumb lies in the direction of the
    conventional current (positive flow)
  • The fingers encircle the conductor in the
    direction of the magnetic field lines caused by
    the current

15
Magnetic Fields on a Coil
  • Field lines are closed loops
  • Inside the coil, uniformly spaced to represent
    the uniform nature of the field
  • Outside, spread out to indicate the weakened
    field
  • Xs indicate field lines that go into the page
  • Dots indicate field lines out of the page

16
Magnetic Fields around a Solenoid
  • Solenoid a closely wound helix. The field from
    a solenoid is stronger and more uniform.
  • The field lines leave one end of the solenoid,
    circle around and enter the other end.

17
Right-hand rule 2 finding the N-pole of a coil
of wire
  • Place fingers of right hand along the wire of the
    coil so that your fingers point in the direction
    of the current in the coil
  • Extend your thumb
  • This will indicate the direction of the field
    lines as they pass through the coil, and thus the
    face of the coil that acts as the N-pole

18
INTERACTIONS OF MAGNETIC FIELDS
  • AMPERE

19
Andre Ampere
  • 1820
  • Developed mathematical law describing the
    relationship between the current in a conductor
    and its resulting magnetic field

20
Current in the same direction
  • Magnetic fields between the two conductors are in
    opposite directions
  • The magnetic fields will apply a force drawing
    the conductors together

21
Current in opposite directions
  • Magnetic fields between the two conductors are in
    the same direction
  • The magnetic fines of force will repel each other

22
Question from previous provincial
  • Two long straight current-carrying wires are
    placed so that they are parallel to one another.
    The picture shows a cross-section of the 2 wires.
    Draw a representation of the field lines and
    draw force vectors showing the direction of the
    magnetic force on each wire.

23
Questions Section Review
  • pg 767 1 a, b, c

24
The AMPERE
  • The unit of measure of electric current intensity
  • The amount of current in each of 2 long straight
    parallel conductors, one meter apart, that will
    cause a force of 2 x 10-7N to act on each meter
    of wire.

25
The Motor Force
  • F kILB
  • The force exerted by a magnetic on the magnetic
    field of a current carrying conductor
  • magnetic field that acts perpendicular to the
    conductor (B )
  • current (I)
  • length of the conductor inside the field (L)
  • k is the proportionality constant

26
The Electric Motor
  • Activity from resource book

27
Electromagnetic Induction
28
Electromagnetics
  • Core of ferromagnetic material placed inside a
    solenoid increases the strength of the magnetic
    field inside the solenoid
  • 3 things affect the strength of the
    electromagnet size of current, number of turns
    of the coil, permeability of the core

29
Strong Electromagnets
  • Must be super-cooled to the point where the coils
    become superconductors, and lose their
    resistance.
  • MRI, High speed trains, Particle accelerators

30
Ampere and Faraday
  • 1820 Ampere shows that an electric current
    produces a steady magnetic field
  • 1831 Faraday predicts that a steady magnetic
    field should produce an electric current

31
Lenzs Law
  • An induced electric current flows in a direction
    that opposes the change that produced it.
  • Heinrich Friedrich Emil Lenz (1804-1865)
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