Magnetism
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Magnetism. S. Bar Magnet. N. S. N. Magnetism. Since ancient times, certain materials, called magnets , have been known to have the property of attracting tiny pieces of metal. This attractive property is called magnetism. Iron filings. N. S. W. N. S. N. E. S. N. Bar magnet.

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Magnetism

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Magnetism

Magnetism


Magnetism

S

Bar Magnet

N

S

N

Magnetism

Since ancient times, certain materials, called magnets, have been known to have the property of attracting tiny pieces of metal. This attractive property is called magnetism.


Magnetic poles

Iron filings

N

S

W

N

S

N

E

S

N

Bar magnet

Compass

Magnetic Poles

The strength of a magnet is concentrated at the ends, called north and south “poles” of the magnet.

A suspended magnet: N-seeking end and S-seeking end are N and S poles.


Magnetism

Cutting a magnet in halfwill not isolate a singlenorth or south.  One magnet becomes two,then four, and so on.

This process will never end: it cannot be cut to reveal a single north or single south pole.

MAGNETS:

A magnet has polarity-it has a north and a south pole.


Magnetic attraction repulsion

N

S

S

N

N

S

S

N

N

S

Magnetic Attraction-Repulsion

Magnetic Forces: Like Poles Repel

Unlike Poles Attract


Magnetism

Magnetism is induced by aligning areas called domains within a magnetic field.


Magnetism

Magnetism originates in the motion of the electrons in iron atom.  

Spinning electrons act like tiny magnets.

Cancellation of this effect occurs in most materials.

Iron, nickel, cobalt are exceptions.


Magnetism

INDUCED MAGNETISM

Some metals can be turned into temporary magnets by bringing them close to a magnet


Magnetic field lines

N

S

Magnetic Field Lines

We can describe magnetic field lines by imagining a tiny compass placed at nearby points.

The direction of the magnetic field B at any point is the same as the direction indicated by this compass.

Field B is strongwhere lines are dense and weak where lines are sparse.


Field lines between magnets

N

N

N

S

Field Lines Between Magnets

Unlike poles

Attraction

Leave N and enter S

Repulsion

Like poles


Magnetism

B-field of bar magnet is similarto the Earth's magnetic field.B-field lines leave north face,enter at south face.


Origin of magnetic fields

E

+

+

^

B v

v

Origin of Magnetic Fields

Recall that the strength of an electric field E was defined as the electric force per unit charge.

Since no isolated magnetic pole has ever been found, we can’t define the magnetic field Bin terms of the magnetic force per unit north pole.

We will see instead that magnetic fields result from charges in motion—not from stationary charge or poles. This fact will be covered later.


Magnetism

Just like magnets produce magnetic fields,

current carrying wires can also produce magnetic fields.


Magnetism

We learned last time that a magnetic fields exert forces on moving charges

F = qvB

A magnetic field bending an electron beam


Magnetic field of a long wire

Iron filings

I

I

B

B

Magnetic Field of a Long Wire

When a current I passes through a long straight wire, the magnetic field B is circular as is shown by the pattern of iron filings below and has the indicated direction.

The right-hand thumb rule: Grasp wire with right hand; point thumb in direction of I. Fingers wrap wire in direction of the circular B-field.


Magnetism

ELECTROMAGNETIC INDUCTION

An electric field can produce a magnetic field. The reverse is also true: amagnetic fieldcan give rise to anelectric field.

A conductor that is caused to move relative to a magnetic field generates an electric field.


Magnetism

Moving the wire and keeping the magnet stationary is the same as moving the magnet and keeping the wire stationary.


Magnetism

Flux through coil changes because bar magnet is moved up and down.   

Michael Faraday  (1791-1867)

Michael Faraday discovered that when a conductor cuts magnetic flux lines, an emf(voltage)is produced.


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