Source of magnetism Magnetic field Magnetic force ... of magnetism Magnetic field Electromagnetic Induction Applications ... orbital and spins create magnetic fields

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  • Magnetism

    Lecture 15

    Source of magnetism

    Magnetic field

    Electromagnetic Induction

    Applications

    Magnetic force

    Electric shock

    Bioelectricity

    Physiological effects

    Resting potential

    Cell potentials

    Action Potential

  • Magnetism

    First magnets were pieces of iron-bearing

    rock called loadstone (magnetite, Fe3O4)

    found originally in Asia (magnesia)

    Iron is one of a few materials

    (also Nickel, Cobalt) that can be permanently

    magnetised.

    These are called

    ferromagnetic materials

    Historical

    Materials

  • N

    S S

    N

    All magnets have two poles, North and South.

    named with reference to alignment in Earths

    magnetic field.

    Magnets and Magnetic Force

    S

    Magnetic pole that points to Earths geographic

    north pole is called the north pole of the magnet

  • A magnetic field

    surrounds the

    magnet.

    When two magnets

    are near to each

    other or in contact,

    they exert a force

    on one another, in

    a manner similar to

    electrical charges.

    This force is the

    magnetic force.

    Magnets and magnetic force

    Lines indicate direction

    and magnitude of

    magnetic field

    N

    S

    N

    S

    N

    S

    F F S

    N

    N

    S

    F F

    Like poles repel,

    unlike poles attract.

  • Earths Magnetic Field

    Rotation

    Axis North

    Geographic

    Pole

    Earths magnetic field pattern similar to

    that of a bar magnet.

    11

    South

    magnetic

    pole

    North Geographic Pole is approximately

    2,000km from the south magnetic pole

  • The motion of electrical charges (current)

    is the only source of magnetism.

    When a current passes through a simple wire,

    a magnetic field is created around the wire,

    this is due to the flow of the electrons.

    Example:

    Source of magnetism

    Permanent magnets are not the only items

    with magnetic properties

    What about permanent magnets?

    I

    Magnetic field lines

  • The microscopic origin of the magnetism

    in magnets;

    Electrons are moving around the nucleus,

    electrons orbiting constitute a circular

    current loop, (moving charge) so each

    electron generates a tiny magnetic field

    Source of magnetism

    Atom

    Electrons also act as though they are spinning

    about an axis through their centres.

    Spinning electron also act like a current loop

    and so creates a tiny magnetic field

    Both these electron motions in atoms,

    orbital and spins create magnetic fields.

    Orbiting Electrons

    Spinning Electrons

  • Origin of magnetism

    Externally applied magnetic field can align the

    domains so that the material becomes

    magnetised.

    In ferromagnetic materials the magnetic effects

    of the electrons do not fully cancel each other

    out, atoms then are like tiny magnets.

    In most atoms the magnetic effect of the

    electrons cancel each other out

    Ferromagnetic materials consists of small

    regions (called domains) in which all the

    magnetic effects of atoms are aligned.

    magnetised Disorder non-magnetic

    domains

    N

    S

  • Some metals (iron for example), can be

    magnetised under the influence of an

    external magnetic field

    When the magnetic field is switched off,

    the magnetization disappears.

    Magnetization

    Uses

    Electromagnet

  • Magnetic force

    A wire carrying a current in an external

    magnetic field.

    Current and magnetic field are

    perpendicular to each other as shown.

    Force on the wire:

    Perpendicular to the plane containing the wire

    Perpendicular to the line of the magnetic field.

    i.e. into the page

    N N S S

    I

    F

    F

  • B = F/IL (B perpendicular to I)

    Force (F) on the wire is proportional to the current

    and the length (L) of the wire in the magnetic field

    B is the magnetic field strength

    units of B are Newtons per metre per ampere

    (SI unit) of magnetic field strength Tesla

    Nikola Tesla (18561943) Croatia

    1T = 1N.m-1.A-1

    N N S S

    I

    F

    F

    F =B*I*L F IL

    Magnetic force

  • This effect is used in generators to produce

    AC current (power plants, bikes, cars)

    Magnetic Induction

    Moving charges (current) cause a magnetic field

    A changing magnetic field in the vicinity of a wire or coil will induce a voltage in the wire or coil

    N S

    I

  • Electromagnetic Induction

    A changing magnetic field in the vicinity of a wire or coil will induce a current in the wire or coil

    Faradays law of magnetic induction

    Induced voltage is proportional to the rate of

    change of magnetic flux through the coil.

    emf Nt

    - + ammeter

    0.0 98

    N =number

    of turns in coil

    Secondary

    circuit Primary

    circuit

    magnetic flux f = Magnetic field strength x area

    f = B.A

  • Electromagnetic Induction

    Transformer

    Iron

    ac Voltage in ac Voltage out

    Charger:

    mobile phone

    I pod

    etc

    ac voltage

    X-ray machinehigh voltage

    Application

    Voltage out is proportional to the number of

    turns Ns in the secondary coil p p

    s s

    V N

    V N

    Secondary

    circuit Primary

    circuit

    Electric toothbrush

    Charges sitting on base unit

    No metal to metal contact

    Only plastic to plastic.

    How does it charge??

  • Medical uses of Magnetic fields

    Dentistry: Magnets not routinely used

    Orthodontics

    Dental prostheses retentionjaw implant

    Recently developed magnets

    small

    exceptional strength

    Orthodontics

    Magnets

    alternative to elastics & wires

    Magnetic Force

    constant

    Directional

    Biologically safe

  • Magnetic fields can penetrate tissue

    with little or no adverse effects

    ---can be used to probe the body

    Medical uses of Magnetic fields

    Main medical application

    Nuclear Magnetic Resonance NMR

    Magnetic Resonance Imaging MRI

    Like electrons, protons (positively charged)

    in the nucleus also act as though they are

    spinning about an axis through their centres.

    Spinning protons act like a current loop

    and so creates a tiny magnetic field

    Nuclei of some atoms (particularly Hydrogen)

    have small magnetic fields

    In strong magnetic field they align with the field.

  • Medical uses of Magnetic fields MRI

    Non-invasive

    imaging

    technique that

    discriminates

    between body

    tissues.

    Patient in strong magnetic field. Radio frequency

    signal applied (reoriented the proton spin).

    Absorption and re-emission measured and

    computer image generated showing the types

    of tissue present.

    diagnostic tool for soft tissue organs, ligaments,

    the circulatory system, spinal column, brain

    MRI uses superconducting magnet;

    Earths magnetic field 0.5 x10-4tesla.

    fridge magnet 10-3 tesla.

    MRI scanner magnet 3 tesla

    6 x104 times the Earths magnetic field.

  • A wire placed in magnetic field such that

    its length is perpendicular to the field which is

    of strength 10milliTesla. If the current in the

    wire is 15 milliamps and the magnitude of the

    force on the wire is 30x10-6 N, what is

    the length (in cm) of the wire in the magnetic field?

    B = F/IL

    Therefore L =F/BI = 30x10-6 N

    10x10-3 T. 15x10-3 A

    L = 2x10-1m = 0.2m = 20cm.

    Example

  • An electrical shock happens when ones body

    provides an accidental circuit completion

    to earth.

    Electrical Shock

    ~ V

    Appliance

    metal

    case Ground

    live

    neutral

    A current then flows through the body, resulting

    in tissue burn or muscle affect

    Nerves controlling muscles use electric currents

    Muscles involuntarily contract in response

    to external electric currents

    Cant let go effect

    Essentially because contractor muscles which

    close the hand are stronger than the extensor

    muscles which are used to open it

    Current disrupts central nervous system

  • The effects of the electrical shock

    depends on 4 main factors:

    (1) magnitude of current

    (2) DC or AC (frequency)

    (3) duration

    (4) path in body

    Electrical Shock

    Main factors and Physiological effects

    earth

    Voltage

    Resistance of body

    Ohms law

  • onset of sustained

    ventricular contraction

    and respiratory paralysis

    6000 (6A)

    onset of burns, depends on

    concentration of current

    300

    ventricular fibrillation

    possible

    100-300

    onset of pain; heart still

    unaffected

    50

    onset of sustained muscle

    contraction; hand cant let

    go, breathing

    10-20

    maximum harmless current 5

    threshold of sensation* 1

    Effect Current (mA)

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