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Phy 103: Fundamentals of Physics. Chapter 24: Magnetism Lecture Notes. Magnetic Poles. N S. Since magnetism is created by moving charge, there is no single source of magnetism (such as a single electron) All magnets have both a north pole a south pole Magnetic poles are never

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Phy 103 fundamentals of physics

Phy 103: Fundamentals of Physics

Chapter 24: Magnetism

Lecture Notes


Magnetic poles
Magnetic Poles

N S

  • Since magnetism is created by moving charge, there is no single source of magnetism (such as a single electron)

  • All magnets have both

    • a north pole

    • a south pole

  • Magnetic poles are never

    occur themselves

  • Aside from the obvious geographic reference, the south and north poles are magnet equivalent to positive and negative charge


Magnetism magnetic force
Magnetism & Magnetic Force

N

N

S

S

S

N

N

S

  • The magnetic poles interact similarly to electric charge

  • “like” poles repel

  • “unlike” poles attract


Magnetic fields
Magnetic Fields

S

B

N

  • The movement of electric charge produces a magnetic field (a B field)

  • Magnets have magnetic fields associated with them

  • Magnetic fields are essentially the magnetic influence a magnet has on the space around it

  • Magnetic field lines always point from North to South

    • The field lines originate at the North pole

    • The field lines terminate at the South pole

  • The units of magnetic field are called Tesla (T)

    1 Tesla (T) = 1 N.s/C.m


Electric currents magnetic fields
Electric Currents & Magnetic Fields

  • Wires carrying electric current produce magnetic fields

  • The magnitude of the magnetic field is related to:

    • Current in wire (I)

    • Distance from wire (r)

      B ~ I/r

  • To calculate the B field:

  • The magnetic field “curls” around the wire!!


Andre marie ampere 1775 1836
Andre Marie Ampere (1775-1836)

  • First person to describe current as the flow of electricity along a wire

  • Published his derivation of the electrodynamic (electric) force law in 1826

  • Considered to be the “Newton of electricity”

  • Not formally educated (well did not attend school)

  • Reputed to have mastered all known mathematics by the age of 12 (probably untrue)

  • Worked on an electrical & magnetic wave theory of light


Electromagnets
Electromagnets

  • A coil of wire with current running through is an electromagnet

  • The magnetic field inside the coil is constant and points long the length of the coil

  • The strength of the electromagnet depends on:

    • The current (I)

    • The number of coils (N)

    • The length of the coil (L)

      Bcoil ~ I.N/L



Magnetic force on charged particles
Magnetic Force (on charged particles)

  • When an electric charge (such as an electron) moves through an magnetic field, it experiences a magnetic force

  • The magnitude of the magnetic force depends on:

    • The charge (q)

    • The velocity of the charge (v)

    • The magnetic field (B)

      Fmagnetic = q.v.B

  • The direction of the magnetic force is perpendicular to both:

    • The direction of the field

    • The direction of motion (v)


Magnetic force on current carrying wires
Magnetic Force(on current carrying wires)

  • When two current carrying wires are aligned parallel to each other they exert a magnetic force on each other

  • Each wire is in the magnetic field of the other

  • The direction of the force depends on the relative direction of the electric currents in te wires:

I1

I1

F

F

Repulsive:

Attractive:

F

F

I2

I2


Earth s magnetic field
Earth’s Magnetic Field

  • The Earth has a magnetic field and acts like a giant big magnet

  • We define the magnetic “north” direction as the direction the North end of a compass points

    • The geographic “North Pole” is really the South pole of the magnetic field

    • The geographic “South Pole” is really the North pole of the magnetic field

  • Although its value varies depending on location, the magnitude of the Earth’s magnetic field (Bearth) is

    Bearth ~ 60 x 10-6 T