THE PROPERTY OF A MATERIAL/OBJECT TO ATTRACT OTHER OBJECTS MADE OUT OF

IRON, MAGNETITE, OR STEEL

Magnetism

Magnets and Technology

Computers use them to read and write dataYour speakers and headphones use themCredit cards and ATM cards store data on

magnetized stripsJunkyards use them to lift scrap metalThey are used in many security systems

Two Categories

Permanent: Retains its magnetic properties all the time regardless

of whether it’s near another magnetic object.

Objects that can be magnetized: When brought close to another magnet i.e. a refrigerator magnet

There are no liquid magnets This is because magnetic effects come from the small

scale organization of atoms within a material. Disrupting this orderly arrangement (i.e. heating)

destroys magnetic effects.

Poles

All magnets contain two magnetic poles. They have an opposite polarity, called north and south

pole. If you cut a permanent magnet in half, each will have

a north and south pole.

You can separate + and – charges but not N and S poles of a magnet.

Attraction and Repulsion

When two magnets are close, they exert an attractive/repulsive force depending on how the poles are aligned.

Opposites attract!!! When north and south are put together, the magnets

will attract each other. North-north or south-south will repel.

The force between two magnets at shorter distances increase more rapidly causing the forces to become very strong and hard to separate.

Attraction and Repulsion (cont.)

Because magnets are dipoles (two poles), the strength decreases rapidly as they are separated.

Opposite magnetic forces on the two ends of abar magnet can create a torque (rotational motion) that causes the magnet to twist/rotate.

Force Field

An organization of energy in space that creates a force on any receptive matter that passes within its ‘influence’. Gravity is an example. The sun’s gravitational field causes forces on each

planetThe force between magnets acts through a

magnetic field. The poles create a magnetic field that extends outward

in the surrounding space. Other magnets ‘feel’ forces through their interaction

with this field. This is a non-contact force.

The Magnetic Field

It describes the strength and direction of magnetic forces around a magnet. A compass is a freely spinning magnet sensitive to

magnetic fields. Each location we place a compass tell us the direction

of the magnetic force. The red end typically is the magnetic north pole.

Magnetic Field Drawing

Uses lines and arrows to represent the force exerted on the north magnetic pole. They point into south poles and out of

north poles. The closer the field lines the stronger

the magnetic force. Field lines never cross Field lines always make closed loops

Our planet has a magnetic field approximately aligned with its north-south axis. Migratory birds sense this field and

use it (remember, The Core) Humans use a compass.

Compass

Why does a compass point north? Earth’s north geographic pole is a south magnetic

pole. People labeled the north magnetic pole of a magnet as

the pole that pointed north.

Earth’s magnetic field completely reverses itself every million years or so!

Diamagnetism, Paramagnetism and Ferromagnetism:

Diamagnetism: The effect of materials to slightly repel magnets of either

polarity. Silver, lead, and copper

Paramagnetism: Slightly attract magnets of either polarity Aluminum, Magnesium and Tungsten

These are substances that can become magnetized and experience strong magnetic forces in the presence of external magnets. Paperclips Iron, Cobalt, and nickle

Magnetic Domains

The property that makes iron ferromagnetic is that it only takes a small amount of energy for an iron atom to flip its magnetic axis.

Groups of Iron atoms form these domains in which atoms within a single domain have a similar magnetic alignment.

Magnetic Domains

When an external magnet is brought near iron, magnetic domains attracted to the magnet gain atoms and grow because the iron atoms easily change their magnetic orientation.

Domains repelled by external magnets easily lose atoms and shrink and vice versa

Ferromagnetic materials are crucial to many technologies such as motors and generators.

Each magnet has a north-seeking and south-seeking pole

Exert a magnetic force on the space around them = magnetic field

Lines closer together – stronger field

Where is the field strength the greatest on a magnet?

Nature of a Magnetic Field

Produced by the motion of an electric charge (current) Current carrying wire can produce a magnetic field

(macroscopic) Electrons around an atom produce a tiny current and

thus a magnetic field (microscopic)

SI unit for magnetic field is the TeslaDifferent examples of magnetic fields

Magnetic field (cont.)

Two e- spinning in same direction = stronger magnetic field

Two e- spinning in different direction = zero magnetic field

Common magnets are made of iron, nickel, and cobalt. The spin of their e- do not cancel => magnets

Magnetic Domains

Individual atoms align to form magnetic domains, a cluster of aligned atoms.

How can a magnet become weaker?

How does a magnet attract a piece of metal that is not magnetized?

Electric Currents and Magnetic Fields

A current-carrying wire will produce magnetic fields in a pattern of concentric circles

When wire is made into loops the magnetic field bunches up.

More loops = more magnetic field = electromagnet (made stronger by adding nail/ iron core)

Practical applications of electromagnets

Maglev trains – page 569, http://www.o-keating.com/hsr/maglev.htm

Stereo speaker, http://electronics.howstuffworks.com/speaker3.htm

Alarm systems

Junkyard crane

MRI, http://electronics.howstuffworks.com/mri2.htm

Magnetic Forces on moving charged particles

A charged particle must move relative to the magnetic field in order to be affected by the magnetic field

A charged particle that is moving perpendicular to a magnetic field will be forced upward

Force of a Magnetic Field on a moving e-

The force that a magnetic field places on a charged particle is … greatest when e- move perpendicular to field lines Zero when e- move parallel to field lines

Magnetic Forces on Current-carrying wires

If a moving charged particle is deflected by a magnetic field, so will a wire that contains moving e-

A wire will be forced … up if current is flowing one direction Down if current is flowing in opposite direction

We see this exhibited when using a compass under a wire.

Meters

Galvanometer-detects currentWhen calibrated it can measure current

(ammeter) or voltage (voltmeter)

Motors

By modifying a galvanometer you can make an electric motor

http://electronics.howstuffworks.com/motor1.htm

Investigation

When we moved a compass around the magnet during the experiment yesterday, what occurred?