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Bell Ringer

  • Give an example of something that you observed over break which involved physics.
  • Be sure to thoroughly explain your answer.

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Magnetism

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Magnetism

  • Force of attraction or repulsion cause by the movement of charged participles
    • It is not charge
    • It is caused by the movement of charge

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Magnetic Poles

  • Magnetic Poles: A region on a magnet which produces magnetic forces
  • The poles of a suspended magnet will align themselves to the poles of the Earth
  • Fundamental Rule: Like poles repel; opposite poles attract

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Magnetic Poles

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    • Electric charges can be isolated
    • Magnetic poles cannot
  • Magnetic poles behave similarly to electric charges EXCEPT:

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Magnetic Fields

  • Magnetic Field: The space around a magnet in which a magnetic force is exerted
    • The shape of a magnetic field is revealed by magnetic field lines
      • Directed away from north poles and toward south poles
  • The strength of magnetic fields are measured in units of Tesla (T)

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Magnetic Fields

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Magnetic Fields

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Magnetic Fields

  • Magnetic fields are caused by the motion of electric charges
    • Since motion is relative, so are magnetic fields
  • Magnets at rest consist of charges in motion
    • Every spinning electron is a tiny magnet
    • Electrons spinning in the same direction produce a stronger magnet

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Magnetic Domains

  • Magnetic Domain: Clusters of aligned atoms

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Magnetic Domains

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Bell Ringer

  • 1.) Draw the magnetic field lines for a bar magnet with its North Pole on the bottom and its south pole on top.
  • 2.) How are the domains arranged in a permanent magnet?

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Magnetic Domains

  • If the magnetic domains of a material can align, this material is called ferromagnetic.

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Magnetic Domains

  • Permanent magnets are made by placing materials in strong magnetic fields
    • Aligns the domains
  • It is possible to weaken a magnet by dropping or heating it
    • Pushes the domains out of alignment

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Earth’s Magnetic Field

  • Currents in the molten part of Earth beneath the crust create the Earth’s magnetic field
    • Moving charges loop within the Earth
  • Earth’s magnetic field is not stable
    • Magnetic pole and geographic pole are offset

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Earth’s Magnetic Field

  • Magnetic poles are located:
    • North: Hudson Bay region of northern Canada (1800 km from geographic pole)
    • South: Just south of Australia
  • Magnetic Declination: Difference between geographic and magnetic poles

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Bell Ringer 5/13

  • From what was discussed in yesterday’s class…what do you believe the true name of our school should be?

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Earth’s Magnetic Field

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Earth’s Magnetic Field

  • The magnetic field of Earth reverses on an irregular cycle
  • 1 million years ago, the field completely “shut off” for 10,000-20,000 years
  • Its strength has decreased by 5% over the last 100 years
    • Indicates a reversal possibly in the next 200 years

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Earth’s Magnetic Field

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Earth’s Magnetic Field

Courtesy of NASA, JPL and GSFC

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Electric Current

  • Recall: Electric current is the movement of electric charges
    • Current produces a magnetic field
  • When current is carried through a conductor, the magnetic field produces a pattern of concentric circles

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Electric Current

  • This concept is known as Oersted’s Principle
  • If the current switches direction, the compass needles will turn 180o

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Electromagnets

  • If a current carrying wire is bent into a loop:
    • Field lines become bunched inside the loop
  • Two overlapping loops:
    • Twice the concentration of magnetic field lines

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Electromagnets

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Electromagnets

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Electromagnets

  • Electromagnet: A current-carrying coil of wire with many loops
    • The magnetic domains of a piece of iron placed in the coil will be aligned
    • The will intensify the magnetic field
    • The strongest electromagnets use superconductors as a core

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Electromagnet Applications

  • An alarm bell uses an electromagnet with a changing magnetic field to move a plunger

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Junkyardpickerupper

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Bell Ringer

How does a speaker work?

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Moving Particles

  • A moving charged particle could be deflected by a magnetic field
  • For particles moving parallel to the magnetic field, no deflection occurs
  • For particles moving perpendicular to the magnetic field, maximum deflection occurs

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Moving Particles

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Moving Particles

  • A current carrying wire will also be deflected when it encounters a magnetic field
    • The particles trapped inside the wire are deflected, therefore pushing the wire itself
  • If the direction of the current is reversed, the deflecting force acts in the opposite direction

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Moving Particles

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Applications

  • The follow items use the idea
    • TVs
    • MRIs
    • Aurora Borealis

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Bell Ringer

  • When will a charged particle experience the maximum amount of force when entering a magnetic field?

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Motors

  • A simple DC motor uses a permanent magnet to establish a field where an electromagnet is made to rotate
    • When connected to a battery, the electro-magnet will rotate one half turn
    • Its south pole will align with the north pole of the permanent magnet

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Motors

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Motors

  • These two devices change the direction of the current at exactly the correct moment so that the poles of the electromagnet reverse
  • This causes the electromagnet to keep spinning

Brushes

Commutator

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Motors

  • As the armature passes through the horizontal position, the poles of the electromagnet reverse due to the commutator

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Bell Ringer

  • Where the motors that you made last week operating on alternating or direct current?
  • What would happen with your motor from yesterday if you took off all the insulation off of each end?

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Meters

  • Galvanometer: A sensitive instrument used to detect electric current
    • Consists of a magnetic needle centered under loops of insulated wire
    • Electric current will cause the needle to pivot, detecting even very small currents
    • May be calibrated to measure current (ammeter) or voltage (voltmeter)

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Meters