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Chapter 21 – Magnetism
21.1 – Magnets and Magnetic Fields
Magnetic Forces
- Magnetic force is the force a magnet exerts on another magnet, on a metal (iron) or on moving charges.
- 1 aspect of electromagnetic force
- Acts over a distance
- Magnetic poles – all magnets have 2 – N and S…these are regions where the magnetic force is strongest
o Direction of magnetic force depends on the direction the poles face
o Like poles repel and opposite attract
Magnetic Fields
- Surround a magnets and can exert magnetic forces
- Strongest near poles and can attract or repel other magnets that enter the field
- Field lines indicate what direction a compass needle would point in space. They start neat the NP and extend
towards the SP. Close together = strong mag. Force, farther apart = weaker mag force
- Magnetic field around Earth
o Magnetosphere – area around earth that is influenced by E’s magnetic field
o Compass points N because it aligns with E’s magnetic NP
o Magnetic poles are not the same as geographic poles magnetic declination: the angle between the
direction to true N and to magnetic N – varies with your location to Earth
Magnetic Materials
- Electron cloud movement and spin of electrons cause e-s to act like magnets
- Unpaired e-s produce magnetic fields –
o each e- can be paired with another with opposite spin weak mag force
o fields don’t align b/c arrangement of atoms isn’t right  weak mag force
o Fe, Ni and Co arrange the unpaired e-‘s to make a strong mag force – fields align to produce mag domain
 Mag domain = region that has very large # of e-s aligned to make a mag field
 Ferromagnetic materials (Fe) can be magnetized b/c they contain magnetic materials
 When material is magnetized, most of its magnetic fields are aligned
- Non magnetized materials – domains on a material are aligned randomly – domains cancel, no magnet
o Ex) iron nail – domains can align, but don’t
Magnetized materials
- You can magnetize a nonmagnetic material by placing it in a magnetic field
- Temporary – if moved out of magnetic field, motion in nail atoms causes the domains to randomly orient again
- Permanent – if the domains stay aligned for a long period of time
- Cut magnets is half will always have N and S poles
21.2 – Electromagnetism
relationship b/t electricity and magnetism
Electricity and Magnetism
- Different aspects of the single force of electromagnetism
- Electric force from charged particles
- Magnetic force from mvmt of electrons in an atom – so both cuased by electric charges
- Mag fields around moving charges
o Moving electric charges create a mag field
o May be the vibrating charges that cause waves
o Forces acting on moving charges
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Effect of magnetic field on moving charge is different than electric field on electric charge
 Remember E-field exerts a force on e-chg in the same (+ charge) or opposite (- chg)
direction
Charge moving in a magnetic field is perpendicular to both the magnetic field and the velocity of
the charge
Draw 636 – 8
Solenoids and Electromagnets
- To use electromagnetic force must be able to control it
- Use current carrying wires
o Coil of a current carrying wire that produces a magnetic field is called a solenoid
 Place a ferromagnetic material (fe rod) inside the coil of a solenoid the strength of magnetic
field INC – causes the Fe rod to become a magnet
 Electromagnet – a solenoid with a ferromagnetic core
 Changing the current in the electromagnetic field controls the strength and the direction
of its magnetic field
 Strength depends on current in solenoid, # of loops in coil and the type of ferromagnetic
core
o INC strength? – INC current flowing – greater the current the stronger mag field
Electromagnetic devices
- Convert electrical NRG into motion and work
- Galvanometers, electric motors and loudspeakers convert electrical NRG into mechanical NRG
o Galvanometers
 Device that uses a solenoid to measure small amts of current
o Electric motors
 Device that uses an electromagnet to turn a motor
21.3 – Electrical NRG Generation and Transmission
Generating Electric Current
- A magnetic field can be used to produce electric current
- Electromagnetic induction is the process of generating a current by moving and electrical conductor relative to a
magnetic field
o Faraday’s law – voltage is induced in a conductor by changing a magnetic field
Generators
- Deice that converts mechanical NRG into electrical NRG by rotating a coil of wire in a magnetic field
- Electric current is generated by relative motion of a conduction coil in a mag field
- Ac and DC generators – READ
Transformers
- Electrical NRG produced by pwr plants is transmitted through pwr lines at high voltages - can’t go into homes
like that
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A transformer is a device that INC or DEC the voltage and current of 2 AC linked circuits – changes the highvoltage current in pwr line to 240-volt current usable by homes
Only works with AC b/c only AC induces a constantly changing mag field
Change voltage and current by inducing a changing mag field in one coil which then induces an AC in a nearby
coil with a different # of turns
READ WHY, CHANGING and TYPES
Electrical NRG for your home - READ