Magnetism

1
Magnetism

• Chapter 19

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Chapter 19 Homework

• Conceptual Questions P.611
• 1,3,4,5,7,8,10
• Homework P.611-618
• 1-5,11,14,18,27,29,30,34,37,44,46

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Magnets

• North and South Poles
• Magnetic Field lines leave the North pole and
  terminate on the South pole
• Like poles repel, unlike poles attract
• Magnetic poles always occur in pairs and cannot
  be isolated
• Bar Magnets

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Magnets
5
Magnets

• Magnetic Materials
• Soft magnetic materials
• Easily magnetized
• Tend to lose their magnetism easily
• Hard magnetic materials
• Hard to magnetize
• Tend to retain their magnetism longer

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Magnets Electricity

• Similarities
• Attractive/Repulsive Forces
• Methods for making magnets and charging objects
• E-Field B-Field

• Differences
• Magnetic poles come in pairs electric charges
  can be separated

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Magnetic Field of the Earth

• Geographic North ? Magnetic South
• Geographic South ? Magnetic North
• Dip Angle

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Magnetic Field of the Earth
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Magnetic Fields, B

• A charged particle moving through a magnetic
  field experiences a magnetic force
• This force is at a maximum when the charge moves
  perpendicular to the field.
• F q n B sinq

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Magnetic Fields, B

• B F
• q n B sinq
• q C, F N, v m/s, B Tesla Wb/m2
• Maximum Force
• F qvB _at_ q 90

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Magnetic Fields, B

• Right Hand Rule
• Fingers in direction of B-Field, Thumb in
  direction of velocity, then the palm will face
  the direction of the Magnetic Force on a positive
  charge.
• If charge is negative, then the Magnetic Force is
  in the opposite direction.

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Right Hand Rule
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A charged particle moves in a straight line
through a certain region of space. The magnetic
field in that region (a) has a magnitude of zero,
(b) has a zero component perpendicular to the
particle's velocity, or (c) has a zero component
parallel to the particle's velocity.
QUICK QUIZ 19.1
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(b). The force that a magnetic field exerts on a
charged particle moving through it is given by F
qvB sin ? qvB , where B is the component of
the field perpendicular to the particles
velocity. Since the particle moves in a straight
line, the magnetic force (and hence B , since qv
? 0) must be zero.
QUICK QUIZ 19.1 ANSWER
15
The north-pole end of a bar magnet is held near a
stationary positively charged piece of plastic.
Is the plastic (a) attracted, (b) repelled, or
(c) unaffected by the magnet?
QUICK QUIZ 19.2
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(c). The magnetic force exerted by a magnetic
field on a charge is proportional to the charges
velocity relative to the field. If the charge is
stationary, as in this situation, there is no
magnetic force.
QUICK QUIZ 19.2 ANSWER
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Magnetic Force on a Current-Carrying Wire

• Right Hand Rule Applies
• Notation is always blue for B-Fields
• ? means into the board
• ? means out of the board
• ? means in the plane of the board

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Magnetic Force on a Current-Carrying Wire

• Finding the total force on a current carrying
  conductor
• F qvB
• I nqvdA
• Total Force (F) B I l sin q

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Torque on a Current Loop

• Torque Fd (d lever arm)
• F B I l sin q
• Lever arm half of the loop
• Only works when B is parallel to the plane of the
  loop

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Torque on a Current Loop

• Torque ( t ) Area B I sin q
• Works for all types of loops
• Torque ( t ) NBIA sin q
• For coil of wire with N turns

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Galvanometer its Applications

• A device used in the construction of Ammeters and
  volt meters, which has the following components
• Magnetic Field
• Coil of wire free to rotate
• Torque is proportional to current
• Works for small currents

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Motion of a Charged Particle in a Magnetic Field

• The magnetic force acting on a charged particle
  is always toward the center of a circle
• Causes centripetal acceleration
• F qvB mv/r2
• r mv2/qvB
• What happens if the particles velocity is not
  perpendicular to the B-field?

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Motion of a Charged Particle in a Magnetic Field
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Amperes Law

• B-field of a long straight wire
• B moI
• 2pr
• Right hand rule
• If the wire is grasped in the right hand with the
  thumb in the direction of the current, the
  fingers will curl in the direction of the B-Field

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Amperes Law
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Magnetic Force Between Two Parallel Conductors

• Force per unit length
• F1 moI1I2
• l 2pd
• The Force is attractive between the wires if the
  current in each wire is in the same direction,
  and repulsive if the current is not in the same
  direction.

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Magnetic Force Between Two Parallel Conductors
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If I1 2 A and I2 6 A in the figure below,
which of the following is true(a) F1 3F2, (b)
F1 F2, or (c) F1 F2/3?
QUICK QUIZ 19.5
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(b). The two forces are an action-reaction pair.
They act on different wires, and have equal
magnitudes but opposite directions.
QUICK QUIZ 19.5 ANSWER
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Magnetic Field of Current Loop

• All segments of the current loop produce a
  magnetic field at the center of the loop directed
  out of the page