Download I. Magnets

PSCI II – Chapter 8 – Magnetism and Its Uses (pages 224 – 255)
Section 1 – Magnetism
I.
Magnets
º The Greeks discovered a mineral that was a natural magnet.
º They found it in a region called Magnesia so they called it magnetic.
º Magnetism – refers to the properties and interactions of magnets.
Magnetic Force
º Have you ever played with magnets?
º What happened?
º Magnets exert a force on each other and other objects.
º The magnetic force is one between magnets that you can feel before the
magnets touch.
º Its strength increases as the magnets get closer and gets weaker as they
get farther apart.
Magnetic Field
º A magnet is surrounded by a magnetic field which exerts a magnetic force.
º This field can be represented by lines of force.
Magnetic Poles
º Defn. – where the magnetic force exerted by the magnet is the strongest.
º All have a north and a south pole.
º Magnetic field lines always connect the north and south poles.
PSCI II – Chapter 8 – Page 1 of 5
How Magnets Interact
º Magnets can either attract or repel.
º If you bring two north or south poles together you will feel them repel.
º If you bring a north and a south together you will feel them attract.
º When either of these happens a new magnetic field is formed.
A Compass Needle
º A magnet that is free to rotate when placed in a magnetic field.
º When you place a compass in a magnetic field the north pole of the needle
will align with the south pole of the magnet.
º Picture on page 228.
Earth’s Magnetic Field
º With out landmarks to guide you how would you know which way to go?
º The Earth acts like a big magnet so we could use a compass.
Earth’s Magnetic Poles
º Because we think that the Earth has an iron and nickel core and it’s rotating
we have magnetic fields.
º READ THIS SECTION
II.
Magnetic Materials
º Only a few metals are magnetic: iron, nickel, and cobalt.
º These are attracted to magnets or can be made into permanent magnets.
º Remember electrons have magnetic properties and in most elements they
cancel out.
º In the three listed above they don’t cancel and each atom behaves like a
mini magnet.
º You can have temporary magnets also.
PSCI II – Chapter 8 – Page 2 of 5
Magnetic Domains – A Model For Magnetism
º Because each magnetic atom exerts a force on a nearby atom they align
magnetically.
º Magnetic Domains – group of atoms having aligned magnetic poles.
º Each domain has billions of atoms yet we can’t see them.
º Because individual domains are aligned the overall domain acts like a
magnet.
Lining Up Domains
º If the domains are not aligned then the object is not magnetic because
they’ll all cancel out.
º If you place a magnet near the objects domain they will line up and form a
temporary magnet.
º Eventually they’ll go back and the object will no longer be magnetized.
º Objects with iron are the most common type of magnet.
Permanent Magnets
º These can be made by placing a piece of magnetic material in a strong
magnetic field.
º This causes the material to become magnetized to create a magnet.
º The strength of the magnetic field inside the material prevents the domains
from changing.
º Permanent magnets can lose their magnetism if heated or dropped enough.
Can a Pole Be Isolated?
º What happens if a magnet is broken in half?
º The poles in the atoms are aligned so now you have two magnets with
north and south poles.
Section 2 – Magnetism
I.
Current and Magnetism
º 1820 Hans Christian Oersted found electricity and magnetism to be related.
º While teaching he found that a current affected a compass.
Moving Charges and magnetic Fields
º Moving charges create a magnetic field.
º Forms a circular pattern (Figure 9 page 233)
º Direction of the magnetic field depends on the direction of the current (same
with the strength).
PSCI II – Chapter 8 – Page 3 of 5
II.
Electromagnets
º Defn – temporary magnet made by placing a piece of iron inside a current
carrying coil of wire.
º Figure 10.
º Inside a coil the magnetic field is strong so the iron is a strong magnet.
Properties of Electromagnets
º Temporary magnets.
º Strength increases by adding more turns (loops) or by increasing the
current.
º Behaves like any other magnet.
º Useful because properties can be controlled by changing the current.
º Converts electrical energy to mechanical energy.
Music to Your Ears – Stereo Speakers
º Read this section.
Galvanometers
º Defn – devices that use an electromagnet to measure electric current.
º Read this section.
III.
Electric Motors
º Defn – device that changes electrical energy into mechanical energy.
º Read this section.
º Figure 15 A-D shows how these work.
Section 3 – Producing Electric Current
I.
From Mechanical to Electrical Energy.
º 1831 Michael Faraday and Joseph Henry found that moving a loop of wire
through a magnetic field produces an electric current.
º Mechanical energy producing electrical energy.
º Electromagnetic induction – producing an electric current by moving a loop
of wire through a magnetic field or moving a magnet through a loop of wire.
Generators
º Defn – produces electric current by rotating a coil of wire in a magnetic field.
º Rotated by mechanical energy.
Using Electric Generators
º Used in cars (alternators) for lights and things.
PSCI II – Chapter 8 – Page 4 of 5
Generating Electricity for Your Home
º Our power comes from the power plant.
º Turbine – a large wheel that rotates when pushed by water, wind or steam.
II.
Direct and Alternating Current
º Is electricity from a battery the same as from a generator?
º Direct current (DC) – flows in one direction through a wire (batteries).
º Alternating current (AC) – reverses direction of the current flow in a regular
way (generators).
III.
Transformers
º Defn – device that increases or decreases the voltage of an alternating
current.
Stepping Up or Stepping Down.
º Figure 22 A and B page 245.
º Read the rest of the chapter.
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