R Ch 36 Magnetism pg 1

Text Qs pg 575RQ 1, 2, 4, 5, 7-9,12,13,19

R Ch 36 Magnetism pg 2

• Magnets are essential for modern life, they are used in generators, motors, lights etc.

• Originally called loadstones they were found naturally in Magnesia, Greece about 2,000 yrs ago

• They contain a form of iron called Magnetite• Magnetism & electricity were linked in 1820 by

Hans Oersted while playing with a compass and a current carrying wire

R Ch 36.1 Magnetic Poles pg 3

• Magnets have these properties;• 1) are a no touch force• 2) attract or repel• 3) their effect follows the inverse square law

for distance• 4) Magnetic force = mag. Pole strength x mag. Pole strength / d2

• 5) Magnets always have two poles ( N & S )

R Ch 36.2 Magnetic Fields pg 4

• The space around a magnet is filled with a magnetic force field

• 1) The direction of the field is N -> S• 2) The field is strongest at the poles• 3) Lines can be drawn to represent the field

& the closer the lines the stronger the field

R Ch 36.3 The nature of the Magnetic Field pg 5

• Magnetism is produced by the motion of an electric charge, magnets get their moving charges from the electrons moving around inside

R Ch 36.3 The nature of the Magnetic Field pg 6

• Remember that electrons • 1) orbit the nucleus• 2) spin on their axis, stronger than orbit

• When the spin & orbit match-up, the magnetic fields add

• When the spin & orbit are opposite they cancel

R Ch 36.3 The nature of the Magnetic Field pg 7

R Ch 36.3 The nature of the Magnetic Field pg 8

• Electron usually exist in pairs • When electrons are paired up, their spins

are opposite they cancel• The canceling effects of the various spins &

orbits are the reason most materials are not magnetic

R Ch 36.3 The nature of the Magnetic Field pg 9

• Iron, cobalt & nickel all are magnetic because they have single electrons with spin that is not canceled by a partner electron

• Iron is the strongest with four unpaired electrons,

• cobalt next with three unpaired electrons, • nickel weakest with two unpaired electrons

R Ch 36.3 The nature of the Magnetic Field pg 10

• Iron, cobalt & nickel are mixed with aluminum (for weight) to make common magnets

R Ch 36.4Magnetic Domains pg 11

• Magnetic domains are clusters of atoms aligned with their N & S poles all pointed in the same direction.

R Ch 36.4 Magnetic Domains pg 12

• Permanent magnets are made by placing iron in a strong magnetic field and;

• 1) tapping the iron to help the domains align• 2) sliding a magnet along the iron to the domains

align• Permanent magnets are destroyed by• 1) heating 2) dropping the magnet because it

knocks the domains out of alignment

R Ch 36.5 Electric Currents & Magnetic Fields pg 13

• Electric current produces a magnetic field because it is the movement of charges.

• Looping wire around a piece of Iron will create an electromagnet that can be switched off and on.

R Ch 36.5 Electric Currents & Magnetic Fields pg 14

• MRI (magnetic resonance imaging) machines do this by using a strong magnetic field to flip all of your electrons up and then letting them flip back to their natural position.

• When your electrons flip back they move giving off a weak magnetic signal that the machine detects and uses to construct an image of your insides

R Ch 36.6 Magnetic Forces on Moving Charge Particles pg 15

• Moving charged Particles behave weirdly in a magnetic field;

• 1) static charge & static field = no interaction• 2) charge & field not moving relative to each other = no interaction

• 3) Charge & magnetic field moving at right angles to each other = creating a force perpendicular to the magnet motion & the charge motion

R Ch 36.6 Magnetic Forces on Moving Charge Particles pg 16

• This right angle motion is how TV sets bend the electrons that make the picture

R Ch 36.7 Magnetic Forces on Current Carrying Wires pg 17

• Wires also feel the force of a magnet field if the current is moving through the wire and the wire is at a right angle to the magnetic field.

• This is how motors, generators, speakers & microphones work

R Ch 36.8 Meters to Motors pg 18

• Voltmeter – measures volts• Ammeter – measures amps (current)• Electric motors changed the world by replacing

human & animal power.• Electric motors spin because a permanent magnet

puts a force on an electromagnet by switching the poles to keep like poles together so they always repel and keep moving

R Ch 36.9 The Earth’s Magnetic Field pg 19

• The earth has two North poles;• 1) Magnetic north (where a compass points) near

Hudson’s Bay in Canada. It moves a little every year.

• 2) True north is at the north pole. • In Chicago magnetic north & true north line up.• The magnetic field is not perfectly understood but

it caused by the magma moving around inside the earth

R Ch 36.9 The Earth’s Magnetic Field pg 20

• Even weirder and not understood;• 1) the earth’s magnetic field has reversed and the

poles switched places.• 2) the magnetic field has switched off for long

periods and compasses didn’t work

R Ch 36.9 The Earth’s Magnetic Field pg 21

• The average time of the magnetic reversals is every one million years and the time is about up so the next switch may be soon.

• The earth’s magnetic field has decreased 5% in the last 100 yrs. At this rate magnetic field will change in 2,000 yrs.