AP Physics Chapter 19 Magnetism

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AP Physics Chapter 19Magnetism
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Chapter 19 Magnetism

• 19.1 Magnets, Magnetic Poles, and Magnetic Field
  Direction
• 19.2 Magnetic Field Strength and Magnetic Force
• 19.3 Electromagnetism The Source of Magnetic
  Fields
• 19.4 Omitted
• 19.5 Magnetic Forces on Current-Carrying Wires
• 19.6 Applications of Electromagnetism
• 19.7 Omitted

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Homework for Chapter 19

• Read Chapter 19
• HW 19.A p.626-627 6,9-13,15,24,25,28,29.
• HW 19.B p.629-63152-56,58,61,62,75,76,78,79.

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19.1 Magnets, Magnetic Poles, and Magnetic Field
Direction
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static cling
bar magnet
electromagnet
electric generator
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• poleforce law or law of poles
  - like magnetic poles repel each other, and
  unlike magnetic poles attract each other.
• The north pole of a compass is the north-seeking
  end .
• dipole - poles always occur in
  pairs, never singly (monopole). If you break a
  magnet in half, you end up with two smaller
  dipole magnets.
• magnetic field (B) - The
  direction of a magnetic field (or B field) at any
  location is the direction that the north pole of
  a compass at that location would point.
• Magnetic field lines always point
• from north to south .
• Magnetism and electricity are two
• aspects of a fundamental force, the
• electromagnetic force .

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19.2 Magnetic Field Strength and Magnetic Force
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• A magnetic field can exert forces only on moving
  charges.
• When a charged particle enters a magnetic field,
  the particle experiences a force that is evident
  because the charge is deflected from its original
  path.
• The electron beam in a cathode ray tube (made
  visible by fluorescent paper) is normally
  horizontal between the end electrodes but is
  deflected here because of the magnet.
• We will use the convention
• X X X X
• X X X X
• is into the page and
• ? ? ? ?
• ? ? ? ?
• is out of the page.

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On Gold Sheet
Right Hand Force Rule For a positively charged
particle the force is in the direction your palm
is facing for a negatively charged particle, the
force is in the direction of the back of your
hand.
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Example 19.1 An electron moves with a speed of
4.0 x 106 m/s along the x-axis. It enters a
region where there is a uniform magnetic field
of 2.5 T, directed at an angle of 60 to the x
axis and lying in the xy plane. Calculate the
initial force and acceleration of the electron.
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A larger mass will have a larger radius.
Fc mac qvB mv2 r
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Example 19.2 A proton has a speed of 4.5 x 106
m/s in a direction perpendicular to a uniform
magnetic field, and the proton moves in a circle
of radius 0.20 m. What is the magnitude of the
magnetic field?
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• Check for Understanding
• When the ends of two bar magnets are near each
  other, they attract one another. The ends must be
• one north, the other south
• one south, the other north
• both north
• both south
• either a or b
• Answer e
• 2. If you look directly down on the S pole of a
  bar magnet, the magnetic field points
• a) to the right
• b) to the left
• c) away from you
• d) toward you
• Answer c

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• Check for Understanding
• A proton moves vertically upward in a uniform
  magnetic field and deflects to the right as you
  watch it. What is the magnetic field direction?
• directly away from you
• directly toward you
• to the right
• to the left
• Answer b, according to the right hand rule

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Check for Understanding
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Check for Understanding
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19.3 Electromagnetism The Source of Magnetic
Fields
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Although a bit unlikely, the idea is that by
jarring the domains in the presence of the
earths magnetic field, they will align with it.
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On Gold Sheet
Right-Hand Source Rule If a current-carrying
wire is grasped with the right hand with the
extended thumb pointing in the direction of the
current (I), the curled fingers indicate the
circular sense of the magnetic field direction.
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? Since magnetic field is a vector, you must use
vector addition to find the net
field if there are contributions from
two or more sources.
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Example 19.3 What current is required for a long
straight wire to produce a magnetic field of
magnitude equal to the strength of the Earths
magnetic field of about 5.0 x 10-5 T at a
location 2.5 cm from the wire?
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• Example 19.5 Two long parallel wires carry
  currents of 20 A and 5.0 A in opposite
  directions. The wires are separated by 0.20 m.
• What is the magnetic field midway between the two
  wires?
• At what point between the wires are the magnetic
  fields from the two wires the same?

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• Check for Understanding
• A long, straight wire is parallel to the ground
  and carries a steady current to the east. At a
  point directly below the wire, what is the
  direction of the magnetic field the wire
  produces?
• north
• east
• south
• west
• Answer a, according to the right hand source
  rule.
• 2. A long, straight current-carrying wire is
  oriented vertically. On its east side, the field
  it creates points south. What is the current
  direction?
• a) up
• b) down
• Answer b, according to the right hand source
  rule.

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Homework for Chapter 19

• HW 19.A p.626-627 6,9-13,15,24,25,28,29.

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19.5 Magnetic Forces on Current-Carrying Wires
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no current in the wire
X
X
X
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On Gold Sheet
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a) Use the right hand force rule to determine the
direction of force. Point the thumb in the
direction of the conventional current I and the
fingers in the direction of the B-field. The
force F is the direction of the palm.
b) Here the current is flowing in the opposite
direction. Point the thumb in the direction of
the current I and the fingers in the direction of
B. F is the direction of the palm.
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• Forces exist between two parallel
  current-carrying wires. This is because the
  magnetic field produced by the current in one
  wire exerts a force on the other wire.
• If the currents are in the same direction, the
  forces attract. If the currents are in opposite
  directions, the forces repel. Use the right hand
  rule for

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Example 19.6 A wire carries a current of 6.0 A
in a direction 60 with respect to the direction
of a magnetic field of 0.75 T. Find the magnitude
of the magnetic force on a 0.50 length of the
wire.
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• Torque on a Current-Carrying Loop
• A magnetic field can exert torque on a
  current-carrying loop.
• Suppose that the loop in figure a is free to
  rotate about an axis passing through opposite
  sides. There are no net forces or torques on the
  pivot sides of the loop. When these sides are
  parallel to the B field, the force on them is
  zero. At any other angle to the field, the forces
  on them are equal and opposite in the plane of
  the loop and so produce no net force or net
  torque.
• The other two sides of the loop do produce a net
  torque, which tends to rotate the loop.

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Torque on a Current Carrying Coil ? NIAB sin
? (not on gold sheet) Where ? is the
torque N is the number of loops in the
coil I is the current A is the area of
the loop. It can be any shape. B is the
magnetic field ? is the angle between the
normal to the plane of the loop and the
B-field.
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• Example 19.7 A circular loop of wire radius 0.50
  m is in a uniform magnetic field of 0.30 T. The
  current in the loop is 2.0 A. Find the magnitude
  of the torque when
• the plane of the loop is parallel to the magnetic
  field,
• the plane of the loop is perpendicular to the
  magnetic field,
• the plane of the loop is at 30 to the magnetic
  field.

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Example 19.8 Two long, straight wires separated
by a distance of 0.30 m carry currents in the
same direction. If the current in one wire is 10
A and the current in the other 8.0 A, find the
magnitude and direction of the forces per unit
length (per meter) between the wires. What if the
currents are in opposite directions?
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• Check for Understanding
• A long, straight, horizontal wire is located on
  the equator and carries a current directed toward
  the east. What is the direction of the force on
  it due to the Earths magnetic field?
• east
• west
• south
• upward
• Answer d, according to the right hand force
  rule

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Check for Understanding
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Check for Understanding
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19.6 Applications of Electromagnetism
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electron is not moving
no current in the wire
X
X
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• The dc Motor
• An electrical motor is a device that converts
  electrical energy into mechanical energy.
• A pivoted, current carrying coil with N loops in
  a magnetic field will rotate freely, but for only
  a half-cycle, or through a maximum angle of 180.

• Recall that ? NAIB sin ?, and when the
  magnetic field is perpendicular to the plane of
  the coil (sin ? 0), the torque is zero and the
  coil is in equilibrium.
• To provide for continuous rotation, the current
  is reversed every half turn so that the
  torque-producing forces are reversed. This is
  done by means of a split-ring commutator.
• Contact brushes provide a path for current.

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• When current is supplied, one half-ring is
  electrically positive and the other negative. The
  coil and ring rotates.
• When the coil and ring have gone through half a
  rotation, the half rings come in contact with the
  opposite brushes. Their polarity is reversed, and
  the current in the coil is in the opposite
  direction. This changes the directions of the
  torque. The process repeats and the spinning
  continues.

For a real motor, the rotating shaft is called
the armature.
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• Cathode Ray Tube (CRT)
• The cathode ray tube is a vacuum tube that is
  used in an oscilloscope, such as those in some
  laboratories.
• Electrons, negatively charged, are boiled off
  a hot filament in an electron gun and accelerated
  by a voltage applied between the cathode (-) and
  anode ().
• The picture tube in older television sets and
  computer monitors is also a cathode ray tube.
• Magnetic coils are usually used there to deflect
  the electron beam.

Open this Cathode Ray Tube Simulation in
Explorer http//highered.mcgraw-hill.com/olcweb/
cgi/pluginpop.cgi?itswf100100/sites/dl/fr
ee/0072512644/117354/01_Cathode_Ray_Tube.swfCath
ode Ray Tube
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• The Mass Spectrometer
• A mass spectrometer is a device used to measure
  the mass of atoms or molecules. It is often used
  to separate isotopes, or atoms of different
  masses.
• Actually, the masses of ions are measured since
  electric and magnetic fields have motional
  effects only on charged particles. (An ion is an
  atom or molecule with a net electric charge.)
• Ions with a known charge (q) are produced by
  heating the substance to be analyzed.

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• The resulting beam of ions introduced into the
  mass spectrometer has a distribution of speeds.
  Ions with a particular velocity are selected by
  means of a velocity selector, made up of charged
  plates and a magnetic field that allow particles
  traveling at only that velocity to go
  undeflected.
• The values of the E and B fields between the
  plates of the velocity selector determine the
  velocity.
• For a positively charged ion, the E-field
  produces a downward force F qE. The B-field
  produces an upward force F qvB1.
• If the beam is not deflected, the resultant
  force must be zero, so
• qE qvB1
• or v E
• B1
• If the plates are parallel, E V/d. Since the
  voltage
• and plate separation are controllable
  quantities, a
• more practical version of the equation is
• v V ion speed in a velocity selector
• B1d

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• Beyond the velocity selector, the beam passes
  through a slit into another magnetic field (B2),
  which is perpendicular to the direction of the
  beam.
• The force due to this magnetic field (F qvB2)
  is always perpendicular to the velocity of the
  ions, which are therefore deflected along a
  circular path.
• The magnetic force supplies the centripetal
  force for this motion, and
• mv2 qvB2 so, m qdB1B2
  r particle mass via
• r V mass spectrometer

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• Example 19.9 In a mass spectrometer, a
  single-charged particle has a speed of 1.0 x 106
  m/s and enters a uniform magnetic field of 0.20
  T. The radius of the circular orbit is 0.020 m.
• What is the mass of the particle?
• What is the kinetic energy of the particle?

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• Check for Understanding
• A mass spectrometer
• can be used to determine the masses of atoms and
  molecules
• requires charged particles
• can be use to determine relative abundances of
  isotopes
• all of these
• Answer d
• 2. Why can a nearby magnet distort the display of
  a computer monitor or television picture tube?
• Answer The magnetic force on the electron beam,
  which prints pictures, causes the deflection of
  the electrons.

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Homework for Chapter 19

• HW 19.B p.629-63152-56,58,61,62,75,76,78,79.

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Chapter 19 Formulas
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v E V ion speed in a
velocity selector B1 B1d m
qdB1B2 r qB2r particle mass via V
v mass spectrometer