Chapter 21: Magnets and Solenoids
pp. 766- 772
Mr. Richter
Agenda
 Introduction to Magnetism
 Build an Electromagnet!
 Notes:
 Magnetism in Materials
 Ferromagnetic Materials
 Electromagnets
 Right Hand Rule
 Magnetic Forces and Fields
Objectives: We Will Be Able To…
 Learn how to build a simple electromagnet and change its
strength.
 Use the right-hand rule to locate an electromagnet’s poles.
 Explain the source of magnetism in materials.
Warm-Up:
 What do you know about magnets?
 Discuss at your table, and we’ll discuss as a class in a few
minutes.
Build an Electromagnet
Building an Electromagnet
 Pay close attention. We will build
an electromagnet in a few
minutes, but we have to be
careful. It can be dangerous.
 If you wrap wire around a nail
and connect it to a battery, you
have made an electromagnet.
 The wire can become VERY HOT.
Be careful.
Let’s Build Electromagnets
 If you do not handle these materials appropriately, I will ask
you to leave.
 At your table, you should have:
 A length of wire
 A nail
 A battery
 A rubber band
 A few paper clips
Let’s Build Electromagnets
 Is the nail magnetized? Try to attract the paper clips with
the nail.
 Wrap the wire NEATLY about 20 times around the nail.
Connect one end of the wire to the positive end one battery
and attach it with the rubber band.
 Carefully slide the other end of the wire under the band at
the negative end of the battery.
 Now try to pick up the paper clips. How many can you pick
up?
 Disconnect your wire after 30 seconds.
Make your magnet stronger!
 What can you do to make your magnet stronger?
1. Use two batteries.
2. Wrap more coils of wire.
 See if you can pick up more paper clips with your new
magnet.
 Remember to be safe and disconnect your wire after 30
seconds.
Magnetism in Materials
Where does magnetism come from?
Electromagnets (Where Magnetism
Comes From)
 Magnetism is created by moving charges.
 This means from moving electrons, either
 in electric current through coils of wire, or
 in individual atoms which have rotating electrons
The more electrons are rotating in the
same direction, the stronger the
magnetic field!
 On your feet!
Types of Materials (Vocab)
 Different materials allow electrons to move differently,
making them more or less magnetic.
 glass vs. iron for example
 There are three categories of materials based on their
magnetic properties (more details on following slides).
 Diamagnetic: almost no magnetic field at all.
 Paramagnetic: very small magnetic fields
 Ferromagnetic: very strong magnetic properties
Diamagnetic Materials
 In diamagnetic materials, the
electrons’ movements are random, in
all directions.
 Therefore, their magnetic fields
entirely cancel out, and the net
magnetic field is zero.
 If you hold a magnet up to a
diamagnetic material, nothing happens.
 Examples: diamond, lead
Paramagnetic Materials
 In paramagnetic materials, some of the
atoms are aligned, so there is a very weak
magnetic field.
 A strong magnet will align some of the
atoms, creating a weak field.
 But when the magnet is taken away, the
atoms go back to their random formation.
 Example: aluminum.
see p366
Ferromagnetic Materials
 Ferro- is the prefix for iron (Fe in the
periodic table)
 In ferromagnetic materials, atoms are NOT
random.
 They align into groups with electrons
rotating in the same direction, called
magnetic domains.
 They all face the same way.
 The magnetic fields of all the domains add
up, creating an overall strong net magnetic
field.
 Examples: iron, nickel, cobalt.
Ferromagnetic Materials
 Why isn’t all steel a magnet?
 Even though the atoms align in domains,
the domains can cancel out if they don’t
face in the same direction.
 For example: A paper clip will align its
domains while the magnet is close by,
but then will randomize again when the
magnet is removed.
Permanent Magnets
Permanent Magnets
 Permanent magnets are created when
magnetic domains become so well
aligned that they stay aligned after the
magnet has been removed.
 Like the nail after our exploration.
 Permanent magnets can only be made
from ferromagnetic materials
Permanent Magnets
 Hard magnets are ferromagnetic
materials that hold their magnetization
well.
 Soft magnets lose their magnetization
quickly. Like paper clips.
 Heat destroys magnets! The hotter a
material gets, the more randomly its
atoms moved.
 No more organized domains!
Electromagnets
Electromagnets
 Electromagnets are magnets created by
electric current flowing in wires.
 Reminder: all current flowing in wires
creates magnetic fields.
 A simple electromagnet is
 a coil of wire called a solenoid, tightly
wrapped around an iron core
 connected to a battery
Advantages of Electromagnets (over
permanent magnets)
 You can turn them on and off by switching the current on and
off.
 You can change the north and south poles by changing the
direction of the current.
 You can change the strength of the magnet by changing the
amount of current in the wire.
 You can make electromagnets much stronger than
permanent magnets.
Making Electromagnets Stronger
 The strength of an electromagnet can be increased by
either:
 Increasing the voltage of the power source (like adding a
battery)
 Wrapping more coils of wire around the core.
 Both of these increase the amount of current flowing around
the core.
 More electrons rotating in the same direction means a stronger
magnetic field.
The Right-Hand Rule (RHR)
Poles of an Electromagnet
 The north and south poles of the
magnet are at either end of the coil.
 Which pole is which? Use the Right
Hand Rule (RHR)!
 Right Hand Rule: If the fingers of your
right hand curl in the direction of the
current, your thumb points toward the
north pole.
Right-Hand Rule Practice
 Which picture below shows the correct north and south
poles of the electromagnet?
Right Hand Rule: Wires
 For straight wires (nonsolenoids) the RHR still
applies.
 If your right thumb points in
the direction of the current,
your fingers curl in the
direction of the magnetic field.
Magnets
Magnets
 A magnet is any material that can exert
a magnetic force on another magnet or
a magnetic material.
 A permanent magnet keeps its
magnetic properties even when not in
contact with another magnet.
Magnets
 Every magnet has two
different poles, called
north and south.
 Even if you cut a
magnet in half, it still
has two different poles.
Magnetic Forces
Magnetic Forces
 When two magnets are near each other, they exert forces on
each other.
 The direction of those forces depends on which poles face
each other.
 Like poles repel each other. Unlike poles attract.
Magnetic Forces
 Most insulators are transparent to
magnetic forces.
 This means that magnetic force can travel
through most insulators.
 Many metals, like aluminum, allow
magnetic forces to pass through, but may
change the force in some way.
 Iron and similar metals can actually block
magnetic force (more on this in 16.2).
Magnetic Fields
Magnetic Fields
 Magnetic forces are field
forces.
 This means that magnets
exert forces (attractive or
repulsive) spread out to
every point over an area.
 Physicists uses magnetic
field drawings to show the
forces a magnet could exert
in an area.
Magnetic Fields
 Magnetic field lines are
drawn to represent the
force that a North-only test
magnet would feel.
 Magnetic field lines always
point away from north poles
and toward south poles.
 WWNPD?
 What would a north pole
do?
Wrap-Up: Did we meet our objectives?
 Learn how to build a simple electromagnet and change its
strength.
 Use the right-hand rule to locate an electromagnet’s poles.
 Explain the source of magnetism in materials.
Homework
 p. 781 -782 #1-4, 7, 12, 13, 14, 23 Due Thursday