Ch. 21: Magnetism
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Transcript of Ch. 21: Magnetism
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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”
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Magnetic Fieldsmagnetic field lines (B-field) always point from N to S
+ + + + ++ + + + ++ + + + +
• • • • •• • • • •• • • • •
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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
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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.
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Electric Current and B-Fields
• Hans Christian Oersted (1820) first noticed that an electric current will deflect a compass needle• first right hand rule
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Electric Current and B-Fields
• a current in a coil (or solenoid) produces an electromagnet• second right hand rule
I
B
How a Speaker Works
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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
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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.
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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:
F q v Bmagnetic
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Auroras
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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)?
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Particle Accelerators
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Mass Spectrometer• mass spectrometer: an instrument that measures
the mass of charged particles• used to identify elements present in a sample
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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?
F q v B
F B I Lmagnetic
m agnetic
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Chapter 22: Induction and Alternating Current
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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.
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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”
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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.
em f N ABt
[ co s ]
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Applying Faraday’s Law
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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.
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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
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Electric Motors• motor: a device that converts electric energy (AC or DC)
to KE
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Transformers
• transformer: converts AC to higher or lower voltage (step up or step down)
• V2 = V1N2 / N1
• Electricity is transmitted at high V, low I (due to “I2R loss”) then stepped down
• 230kV to 20kV to 120V