Ch. 21: Magnetism

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# Ch. 21: Magnetism - PowerPoint PPT Presentation

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
• 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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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
• 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
• Hans Christian Oersted (1820) first noticed that
• an electric current will deflect a compass needle
• firstright hand rule
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
• 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
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
• 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
• 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: an instrument that measures the mass of charged particles
• used to identify elements present in a sample
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?

### Chapter 22: Induction and Alternating Current

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: the magnetic field of an induced current opposes the change in the applied magnetic field
• energy is conserved due to this “magnetic friction”
• N = number of loops
• A = area
• B = magnetic field
• t = time
• Use this law to calculate the voltage generated by a spinning coil.
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
• 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
• motor: a device that converts electric energy (AC or DC) to KE
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