Magnets and Magnetic Fields

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Magnets and Magnetic Fields

• Physics 1-2 Chapter 21

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How did magnets get their name?

• First discovered about 3000 years ago
• Magnesia, Greece
• First naturally occurring magnetic rock,
  lodestone
• Made up of iron-based material

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Where do magnetic fields come from?

• All magnetic fields arise from electric currents.
• In the case of permanent magnets in ferromagnetic
  materials, the currents are from unpaired
  electrons orbiting the nucleus.

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Magnetic Poles

• Magnets have a north pole and a south pole
• Like charges
• Opposite poles attract
• Like poles repel
• Cant isolate south pole from north pole
• If magnet is cut, each piece will still have two
  poles

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What are magnetic domains? Magnetic substances
like iron, cobalt, and nickel are composed of
small areas where the groups of atoms are aligned
like the poles of a magnet. These regions are
called domains. All of the domains of a magnetic
substance tend to align themselves in the same
direction when placed in a magnetic field. These
domains are typically composed of billions of
atoms.
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Properties of Magnets

• Permanent ALL the time, called permanent magnets
• Example lodestones
• Classified as magnetically hard or soft
• Soft magnets
• Example iron
• Easily magnetized
• Loses its magnetic properties easily

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Properties of Magnets

• Hard magnets
• Example cobalt, nickel
• More difficult to magnetize
• Dont lose magnetism easily
• Soft magnets
• Example Iron in a staple or in a nail.
• Easy to magnetize
• Can be easily demagnetized by physical shock or
  heating

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Magnets exert magnetic forces on each other

• Example
• When magnet is lowered into bucket of nails, it
  can pick up a chain of nails
• Each nail is temporarily magnetized by nail above
  it (exert magnetic force on nail below it)
• Limit to how long chain of nails can be
• The farther from the magnet? smaller magnetic
  force

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Magnets exert magnetic forces on each other

• Eventually, magnetic force not strong enough to
  overcome force of gravity? bottom nails fall

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Magnetic Fields

• Force exerted by magnets acts
• at a distance
• Example
• Move south pole of magnet toward another south
  pole
• Magnet will move away
• Other forces act at a distance
• Gravitational forces, force between electric
  charges

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Magnetic Fields

• ALL magnets produce a magnetic field
• Strength of magnetic field depends on
• Material magnet made from
• How much object is magnetized
• How far from magnet.
• Magnetic field lines used to represent magnetic
  field
• Like electric field lines represent electric
  field

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Magnetic Field Lines

• Direction is defined as the direction that the
  north pole of a compass will point at that
  location. ( go from N to S )
• Form closed loops
• Field lines that are closer together? strong
  magnetic field
• Field line that are farther apart? weak magnetic
  field
• Magnetic field strongest near poles

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Magnetic Field Lines
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How do compasses work?

• Analyze magnetic fields direction
• Compass magnet on top of pivot
• Aligns with Earths magnetic field
• Can be used to determine direction as Earth acts
  like a giant bar magnet

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Earths Magnetic Field

• Earths magnetic poles not same as geographic
  poles
• Geographic north pole (Canada)? magnetic south
  pole
• Geographic south pole (Antarctica)? magnetic
  north pole
• Poles of magnet named for geographic pole they
  point to
• N north-seeking pole
• S south-seeking pole

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Earths Magnetic Field
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Earths Magnetic Field

• Source of magnetism is unknown
• Earths core made mostly of iron but too hot to
  have magnetic properties
• Circulation of ions or electrons in liquid layer
  of Earths core?
• Direction of Earths magnetism has changed
• 20 reversals in last 5 million years
• We are due for a reversal in the next few
  thousand years!
• Aurora Borealis/Australis
• Solar wind (charged particles emitted from sun)
  is deflected by Earths magnetic field

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Aurora Borealis- Northern lights
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Aurora Australis

• Aurora Australis

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Auroras

• Auroras are only visible at night in extreme
  northern or southern latitudes.
• In cases of unusually high solar activity, the
  auroras may be visible further south.

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Ch21.2 Electromagnetism

• There is a magnetic field associated with any
  current (there is no magnetic field without a
  current!)
• The magnetic field lines are co-encentric circles
  around a straight wire. The field line direction
  is given by the right hand rule. Thumb points in
  the direction of the current and fingers gripping
  the straight wire point in the direction of the
  field.

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Right hand rule (P770, hons P662)
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Solenoid

• A long helically wound, insulated electric wire.
  The magnetic field is concentrated within the
  coil. It is further concentrated when a
  ferromagnetic material is placed inside the coil.
  
• Electromagnet A magnet that consists of a
  solenoid and a ferromagnetic core. The magnetic
  field can be switched on and off with the flow of
  electric current.

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Solenoid
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21.3 Relationship between current and magnetic
field

• A charge moving (a current) through a magnetic
  field experiences a force.
• F B q v Sin ?
• F is force in Newtons
• B is magnetic field strength in Tesla, T
• q is charge in coulombs and,
• V is the velocity of the charge
• T is the angle between mag field and motion
  direction.

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Right hand rule shows direction of force (P774,
P650 Hons) on a positevely charged particle USE
LEFT HAND FOR ELECTRON
v
B
F
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Homework

• Hons. P679 Q1,2,3 (use Voltage to calculate v
  first), 4, 5, 6. Draw picture!)
• Reg. P 775 Q1 to 5. Draw a picture!

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Force on a current carrying conductor

• F B I l
• Where I is the current and l is the length of the
  conductor
• There is therefore a force between any 2 current
  carrying conductors (note the demo)
• Do Q 1 5 page 778
• AND P779 1-5

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• The force on a current carrying conductor has
  uses in motors, moving coil meters (any meter
  with a needle) and in any device where electrical
  energy converts to kinetic energy where motion
  is produced.

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An explanation of how a motor works

• http//youtu.be/fWyzPdyCAzU

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Chapter 22 Induced current

• If a conductor is placed in a varying magnetic
  field, a voltage is induced in the conductor.
  (Faradays First Law)
• If the conductor can form a circuit, a ______
  will flow.
• This induced voltage (emf) can happen in one of
  the following ways
• 1) Move the conductor into or out of the field.

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Inducing voltage (contd.)

• 2) Circuit is rotated in the field (angle between
  conductor and field changes)
• 3) Change the intensity of the magnetic field.
• Before we go and do numerical problems based on
  this idea do some concept problems on the
  previous topics
• P769 Q1-4
• P779 Q1-5

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Practical applications of electromagnetic
induction

• Motor This is more of an application of F BIL
  . A current flowing through a loop of wire
  between two magnet poles experiences a force that
  causes rotation (See and understand demo). When
  the motor turns 180, a commutator (switch)
  changes the direction of the current so that the
  force is now changed 180, and rotation continues.

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• A moving coil meter (galvanometer) is like a
  motor without the commutator and it also has a
  spring to return it to zero.
• Generator Identical to motor in construction
  BUT the coil is forced to rotate with the
  magnetic field by an outside force (ex. A
  turbine) and the induced voltage causes a current
  to flow.

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• Speaker works due to F BIL.
• Sound is just a pattern of changing pressure
  (vibration).
• A loudspeaker or headphone has a wire coil placed
  in a permanent magnetic field. Current passed
  through the coil causes the coil to experience a
  ______ .
• If the current changes at the same rate as the
  sound, the speaker coil and the permanent magnet
  interact to vibrate the coil at the same
  frequency as the desired sound.

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• Microphone identical to the speaker
  construction but the coil moves due to sound
  vibrations, causing a current to be induced in
  the coil. This current can than be recorded or
  increased (amplified).
• A microphone is to a speaker as a motorgenerator
  is to a motorgenerator
• A motor is to a generator as a microphonespeaker
  is to a microphonespeaker
• A guitar pickup works the same way as a
  microphone.

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Transformer

• Two coils of wire that have a magnetic material
  between them. When an A.C. current flows in the
  primary coil, a changing magnetic field is
  produced in the magnetic material. This changing
  magnetic field induces a changing current in the
  secondary coil. Voltages may be changed
• V2 N2 / N1 V1
• Where 1 means primary, 2 secondary, N of
  turns of wire
• Do Q1-6 page 818 Hons. P722 Q54-57 and 59

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• Pole transformer
• 38,000V to 240V
• Why are the 38kV
• wires (on top) thinner
• than the 240V wires?
• Pad transformer

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Transformers (contd.)

• Transformers do not work with direct (constant)
  current as a changing magnetic field is necessary
  to induce a voltage in the secondary.
• Alternating current (changes direction 60 times
  per second in US, 50 /sec outside Americas) is
  necessary for a transformer to work.

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• Depending on the ratio of N2 /N1 the voltage may
  be stepped up or down by any amount.
• Efficiency can be as high as the high 90s with
  some energy lost as heat in the coils and due to
  eddy currents producing heat in the magnetic core
  itself. Large transformers have cooling systems
  to remove this heat which can lead to failure.

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• Thomas Edison built a network of power plants in
  major cities producing DC current.
• They had to be very close to the power users and
  the voltage produced was the same as voltage
  consumed.
• Nicola Tesla emigrated to the US and was asked to
  solve the problem of supplying power to gold and
  silver mines in the West. Mines (where power was
  used) were often miles from fast-running rivers
  (where power was produced). Wires had to be very
  thick () if low voltage used (PVI). Equipment
  in mines dangerous if high voltage used in mines.

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• Teslas Trillion dollar idea
• Dont use DC use AC!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
  
• Use transformers at power plant to make voltage
  very high before distribution (thin wires).
• Use transformers at mine to reduce the voltage to
  safe, practical levels.
• This is what we do today.
• Power is distributed from the power plants at V
  of the order of 106 Volts over great distances
  over a grid. Voltage is stepped down successively
  by transformers until your house receives 240V or
  120V.

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• Interesting video of utility linemen
• http//youtu.be/Oy81YP-q8R4

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Answers to P818

• 1) 1.2 X102 V
• 2) 25 Turns
• 3) 1561
• 4) 3.5 X 104 turns
• 5) 2.6 X10 4 V
• 6) 147 V

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Lenzs Law

• The induced voltage in a conductor in a changing
  magnetic field, produces a current that creates
  its own magnetic field which opposes the original
  magnetic field (the induced field opposes the
  change that produced it in the first place)
• Induced currents in transformer cores and any
  conductor in a changing magnetic field flow in
  circles. The larger the area, the higher these
  currents are. They are called eddy currents
• Sometimes these induced currents are desirable
  ex. eddy current braking etc.

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• GBSPhysics163 - How does the Giant drop work?
  (Nick)
• Eddy current brakes are used on
• passenger trains a lot. The braking
• depends on speed (FBqv) so a nice
• smooth stop results.

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Back e.m.f. and motors

• As motor speed increases, a voltage (emf) is
  induced in the coil that opposes the original
  current that made the motor rotate. The net
  effect is to reduce the original current.
• Motors have a high starting current which tapers
  off as speed of rotation increases.

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Faradays first law

• Induced V (emf) -N? (AB (CosT))/?t
• N is the number of turns of wire, A is area, T is
  angle between B and the circuit or wire. The
  current direction found by R.H.R. . Could you
  figure the current direction?

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Find current direction