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Ch. 21: Magnetism

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Ch. 21: Magnetism. Magnetism. magnetic poles produce magnetic forces poles always exist in pairs (N and S) opposite poles attract, like poles repel there are no magnetic “monopoles”. Magnetic Fields. magnetic field lines ( B-field ) always point from N to S. • • • • • • • • • •

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Presentation Transcript
magnetism
Magnetism
  • magnetic poles produce magnetic forces
  • poles always exist in pairs (N and S)
  • opposite poles attract, like poles repel
  • there are no magnetic “monopoles”
magnetic fields
Magnetic Fields

magnetic field lines (B-field) always point from N to S

• • • • •

• • • • •

• • • • •

+ + + + +

+ + + + +

+ + + + +

big and little magnets
Big and Little Magnets
  • currents within the mantle
  • produce the earth’s field
  • in atoms, orbiting and
  • spinning electrons produce
  • tiny magnetic fields
  • Fe, Ni, and Co are the most
  • magnetic elements
objectives
Objectives
  • Understand and apply the first magnetic “right hand” rule.
  • Understand and apply the second magnetic “right hand” rule.
  • Understand practical applications of electromagnets.
  • Understand and explain the concept of magnetic domains.
electric current and b fields
Electric Current and B-Fields
  • Hans Christian Oersted (1820) first noticed that
  • an electric current will deflect a compass needle
  • firstright hand rule
electric current and b fields1
Electric Current and B-Fields
  • a current in a coil (or solenoid)
  • produces an electromagnet
  • second right hand rule

B

I

  • How a Speaker Works
magnetic domains
Magnetic Domains
  • domains are clusters of billions of
  • iron atoms with aligned fields
  • domains will align in a B-field
  • permanent magnets have been
  • exposed to very strong fields
  • heat destroys magnets because
  • domains become random
objectives1
Objectives
  • Understand how magnetic force is applied to moving charges.
  • Apply the third “right hand” rule.
  • Understand some common applications of magnetic force.
  • Solve magnetic force problems.
magnetic force
Magnetic Force
  • a charged particle moving perpendicular to a B-field feels a force
  • 1 Tesla (T) = 1 N/(C · m/s) = N/(A·m)
  • third right hand rule:
magnetic force problem
Magnetic Force Problem
  • A proton moving at 1200 km/s (in the solar wind) runs perpendicular into the earth’s magnetic field (B = 55 mT). How much force is applied to the proton? What is the acceleration of the proton (m = 1.67 x 10-27 kg)?
mass spectrometer
Mass Spectrometer
  • mass spectrometer: an instrument that measures the mass of charged particles
  • used to identify elements present in a sample
magnetic force on a wire
Magnetic Force on a Wire
  • a current-carrying wire in a B-field will feel a force perpendicular to the wire
  • How much force is applied to a 5-cm long wire carrying 12 A of current when it is placed in a 3 mT magnetic field?
magnetic fields and emfs
Magnetic Fields and EMFs
  • Michael Faraday (1831) and Joseph Henry:
  • electromagnetic induction: the production of a current caused when a conductor is moved through a magnetic field (or the magnetic field is changed)
  • emf: electromotive force; an increase in PE per charge (voltage) that pushes charges through a conductor; emf produces a current
  • Use the 3rd right hand rule to determine direction of current.
lenz s law
Lenz’s Law
  • Lenz’s law: the magnetic field of an induced current opposes the change in the applied magnetic field
  • energy is conserved due to this “magnetic friction”
faraday s law
Faraday’s Law
  • N = number of loops
  • A = area
  • B = magnetic field
  • t = time
  • Use this law to calculate the voltage generated by a spinning coil.
objectives2
Objectives
  • Be able to explain how/why a generator works.
  • Be able to explain how/why an electric motor works.
  • Understand how different commutators are used to produce/use AC versus DC.
generators and motors
Generators and Motors
  • generator: converts KE to electrical energy (current)
  • spinning a coil in a B-field causes an AC to form
  • commutator: determines if AC or DC
  • armature: multiple-loop coil
electric motors
Electric Motors
  • motor: a device that converts electric energy (AC or DC) to KE
transformers
Transformers
  • transformer: converts AC to higher or lower voltage (step up or step down)
  • DV2 = DV1N2 / N1
  • Electricity is transmitted at high V, low I (due to “I2R loss”) then stepped down
  • 230kV to 20kV to 120V
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