Magnetism UnitNotes 1

Grade 10 ST

Magnetic Behaviour

• After watching the demo, what conclusions can you make about what you saw?

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What is Magnetism?

• Magnetism is the force of attraction or repulsion in and around a material

• Magnetism is present in all materials but at such low levels that it is not easily detected

• Certain materials such as magnetite, iron, steel, nickel, cobalt exhibit magnetism at levels that are easily detectable

What is a Magnet?

• A magnet is any piece of material that has the property of attracting iron (or steel).

• Magnetite, also known as lodestone, is a naturally occurring rock that is a magnet.

• This natural magnet was first discovered in a region known as Magnesia in Greece and was named after the area in which it was discovered.

What is a Magnet? Cont’d

• Magnetism may be naturally present in a material or the material may be artificially magnetized by various methods.

• Magnets may be permanent or temporary.– After being magnetized, a permanent magnet will

retain the properties of magnetism indefinitely.– A temporary magnet is a magnet made of soft iron,

that is usually easy to magnetize; however, temporary magnets lose most of their magnetic properties when the magnetizing cause is discontinued.

Different Types of Magnetic Materials

Magnetic• A permanent magnet• Two magnetized objects can attract or repel

each other

Different Types of MagneticMaterials

Ferromagnetic• Materials which can be magnetized are called ferromagnetic

materials • They contain iron, nickel or cobalt• They are attracted to magnetic objects, they become temporarily

magnetized• Example: Iron

– Iron is made up of a set of regions called domains.– Each domain acts like a tiny magnet with it’s own north and south poles– When iron is not magnetized= domains are not aligned– When iron is magnetized domains are aligned

Different Types of MagneticMaterials

Non- magnetic• Non-magnetic objects do not react to the

presence of a magnet. They are not attracted or repelled!

Temporary vs. Permanent Magnets

Magnetic Properties• http://www.ndt-ed.org/EducationResources/HighSchool/Magnetism/magnetic

properties.htm

• After watching the above applet, explain what happens when a magnet is broken into smaller pieces

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Magnetic Properties

• Magnets can be cut into smaller and smaller pieces indefinitely

• All magnets have a north seeking and south seeking pole

• Each new piece makes a new magnet with it’s own north and south poles.

• The NORTH POLE of a magnet is the end that naturally seeks the earth’s magnetic pole near the geographic north pole

Magnetic Properties

• Magnets interact with each other through forces of attraction and repulsion

• In general, the following is true of magnetic poles– Opposite magnetic poles attract– Like magnetic poles repel each other

Question: Knowing this, is the North Pole a North Pole or a South Pole? _____________________________

Magnetic Properties Check

• If the north pole of a magnet is moved toward the south pole of another magnet, will the north pole be subjected to an attractive or a repulsive force? Explain your answer.

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Interesting Article

• http://www.scientificamerican.com/article.cfm?id=cattle-deer-sense-magnetic-field&print=true

Magnetic Fields

• In a magnetized object, the domains are all lined up in the same direction.

• Any magnetized object produces a magnetic field. The magnetic field is the area around the magnet where the magnetic force can be felt.

Magnetic Fields

• To draw the magnetic field, we use arrows.• Most magnets have 2 poles: north (N) and

south (S)• Two possible forces exist between

magnetized objects• Attraction• Repulsion

Magnetic Field around a bar magnet•Arrows are always pointing towards the south

Compasses

How do compasses work?• The north needle of a

compass always points in the same direction as the magnetic field.

Magnetic field around a straight wire

• When there is a current flowing through a wire, there is a magnetic field around the wire.

• The shape of the magnetic field is individual circles around the wire.

Magnetic Field around a Straight Wire

To determine the direction of the magnetic field lines we use the “Right Hand Rule”

• Place your right thumb along the direction of the current.

• Your fingers curl in the same direction as the magnetic field

Magnetic Field around a Straight Wire

Try it…

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Magnetic Field around a Straight Wire

• In a circuit • With Compass

Magnetic Field around Electromagnets

• When there is a current flowing through a coiled wire, there is a magnetic field that is created we call it an electromagnet.

• The shape of the magnetic field around the electromagnet is exactly the same as around a bar magnet

Magnetic Fields around Electromagnets

• To determine the location of N and S, we use the “Second Right Hand Rule”

• Place your fingers on your right hand so that they curl in the same direction as the current.• Your thumb indicates north

Magnetic Fields around Electromagnets

• Try it… • With Compass

Factors that affect the strength of an electromagnet

• To increase the strength of an electromagnet:– Increase current– More coils– Type and gauge of the conductor– The core material

Induction in an Electromagnet

• To generate a magnetic field:

Induction in an Electromagnet

• To generate an electric current: