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## PowerPoint Slideshow about ' Ch. 21: Magnetism' - aram

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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”

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?

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”

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.

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

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