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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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