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Magnets and Magnetic Fields


Magnets and Magnetic Fields Physics 1-2 Chapter 21 – PowerPoint PPT presentation

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Title: Magnets and Magnetic Fields

Magnets and Magnetic Fields
  • Physics 1-2 Chapter 21

How did magnets get their name?
  • First discovered about 3000 years ago
  • Magnesia, Greece
  • First naturally occurring magnetic rock,
  • Made up of iron-based material

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.

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

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
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

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

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

Magnets exert magnetic forces on each other
  • Eventually, magnetic force not strong enough to
    overcome force of gravity? bottom nails fall

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

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
  • Like electric field lines represent electric

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
  • Magnetic field strongest near poles

Magnetic Field Lines
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

Earths Magnetic Field
  • Earths magnetic poles not same as geographic
  • Geographic north pole (Canada)? magnetic south
  • 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

Earths Magnetic Field
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

Aurora Borealis- Northern lights
Aurora Australis
  • Aurora Australis

  • 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.

Ch21.2 Electromagnetism
  • There is a magnetic field associated with any
    current (there is no magnetic field without a
  • 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

Right hand rule (P770, hons P662)
  • 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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21.3 Relationship between current and magnetic
  • 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

Right hand rule shows direction of force (P774,
P650 Hons) on a positevely charged particle USE
  • 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!

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

  • 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.

An explanation of how a motor works
  • http//

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.

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

Practical applications of electromagnetic
  • 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.

  • 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.

  • Speaker works due to F BIL.
  • Sound is just a pattern of changing pressure
  • 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

  • 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.

  • 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.

  • 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
  • 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.

  • 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

  • Interesting video of utility linemen
  • http//

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

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.

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

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.

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?

Find current direction