R Ch 36 Magnetism pg 1 Text Qs pg 575 RQ 1, 2, 4, 5, 7- 9,12,13,19
Apr 01, 2015
R Ch 36 Magnetism pg 1
Text Qs pg 575
RQ 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.